1. science
2. chemistry

AS
tomic
pectroscopy
March/April 1998
Volume 19, No. 2
In This Issue:
Issue
Special
paration
e
r
P
e
l
p
Sam
using tion
ve Diges
a
w
o
r
c
i
M
Standardization of Sample Preparation for Trace Element
Determination Through Microwave-Enhanced Chemistry
H.M. Skip Kingston .......................................................................................27
A Streamlined Approach to the Determination of Trace Elements in Foods
Karen W. Barnes............................................................................................31
A Streamlined Flame Atomic Absorption Method for Animal Feed Analysis
Cynthia P. Bosnak and Karen W. Barnes ....................................................40
The Determination of Minerals in Multivitamin Samples Using
Microwave Digestion and ICP-OES Analysis
Paul D. Krampitz and Karen W. Barnes ......................................................43
Digestion and Preparation of Organic and Biological Microsamples for
Ultratrace Elemental Analysis
Ingeborg Müller.............................................................................................45
A Simple Closed-Vessel Nitric Acid Digestion Method for Cosmetic Samples
Kerry D. Besecker, Charles B. Rhoades, Jr., Bradley T. Jones,
and Karen W. Barnes ....................................................................................48
Closed-Vessel Nitric Acid Microwave Digestion of Polymers
Kerry D. Besecker, Charles B. Rhoades, Jr., Bradley T. Jones,
and Karen W. Barnes ....................................................................................55
The Analysis of Coal Tar Pitch by ICP Optical Emission
Spectrometry After Digestion in a Microwave Oven System
Maryanne Thomsen and Peter Kainrath.....................................................60
Digestion and Characterization of Ceramic Materials and Noble Metals
S. Mann, D. Geilenberg, J.A.C. Broekaert, P. Kainrath,
and D. Weber .................................................................................................62
• Announcements
ASPND7 19(2) 27–66 (1998)
ISSN 0195-5373
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pectroscopy
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Standardization of Sample Preparation
for Trace Element Determination Through
Microwave-Enhanced Chemistry
H.M. Skip Kingston
Duquesne University, Department of Chemistry and the
Environmental Science and Management Program
Pittsburgh, PA 15282 USA
INTRODUCTION
The development of standard
sample preparation protocols by
microwave have simplified and
unified trace element determinations in thousands of analytical laboratories. The rational is one that
was articulated by Berzelius over
170 years ago when he instructed,
“seek to find the method of analysis
that depends least on the skill of
the operating chemist...” (1).
Berzelius was seeking a method
that is robust, dependable, and
reproducible from one laboratory
and instrument to another. Control,
transferability, reproducibility,
and robustness are the traits that
permit a method to become a standard instrumental procedure and
protocol. Once the sample preparation method is instrumented, the
apparatus takes part in the control
and transfer of the procedure and
enables instrumental assistance
with many steps in the process.
After a decade of development
and evaluation, microwave sample
preparation has emerged as the
method of choice for spectroscopic
trace element determination.
Standard Microwave Trace
Element Methods
The efficiency and robustness
of microwave sample preparation
for trace element determination is
exemplified by the number of
international and multiple standard
agencies that now depend on
microwave procedures. Table I
lists over a dozen examples of
microwave trace element standards
from a variety of standards agencies
and countries (2,3).
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
This standardization is exemplified by the first total decomposition
method for trace element determination for U.S. Environmental Protection Agency (EPA) Resource
Conservation and Recovery Act
(RCRA) Method 3052 (4,5). It is
the inherent robustness of the
method that permits it to handle all
26 RCRA elements simultaneously
in one sample preparation. Before
Method 3052 was developed, several analytical procedures were
required to thoroughly evaluate
a sample for its elemental components. These procedures, some
taking many hours, have been
replaced by a single microwave
procedure producing equivalent
results for all 26 elements simultaneously. Many more elements
than the validated 26 EPA elements
are possible from a single sample
preparation using Method 3052
for general applications.
Generic Digestion Protocol
Method 3052
In the United States, perhaps
the most useful standard method
is also the most flexible. Method
3052 was developed to handle
such a diversity of matrix types
and elements that it provides an
excellent starting point for industrial development of process control and quality assurance elemental
analysis produres. This method is
also being adopted and applied
internationally.
Method 3052 is applicable for
the general analysis of at least
80–90% of industrial and common
samples. Table II lists some of the
matrix types and elements that
27
have been validated in Method
3052 (5). The method is adaptive
and provides optimization procedures for specific matrix types
and elemental chemistries (Table
III). The original closed-vessel
microwave protocol that became
EPA Method 3052 was based on
earlier work at the National Institute of Standard and Technology
(NIST) for elemental certification
of Standard Reference Materials
(SRMs) (5).
Procedure
A summary of the steps and
a brief overview provide some of
the key aspects of Method 3052.
A 0.2-g to 2.0-g sample, depending
on the reactivity and the potential
for the production of gaseous byproducts during digestion, is transferred into a microwave digestion
vessel (5). Subsequently, 9 mL of
sub-boiled distilled concentrated
nitric acid and various quantities
of sub-boiled distilled concentrated
hydrofluoric and hydrochloric acid
are added to each vessel. The
choice of digestion reagents is
dependent on many factors including the matrix, analytes of interest,
and the detection technique. The
vessels are capped, sealed, and
heated simultaneously for a total
of 15 minutes. The first stage
involves heating of the samples
to at least 180±5°C in 5.5 minutes.
The temperature is then maintained
at 180±5°C for 9.5 minutes. The
heating profile may be altered for
reactive matrices or excessively
slow decomposing components.
Figure 1 shows a typical digestion
temperature and pressure profile
of a soil.
TABLE I
Standard Microwave Sample Preparation Methods (2,3)
Dissolution
Type
Matrix
Leach
Water
Organization
ASTM
Method
D4309-91
Analytes
Al, Cd, Cr, Cu, Fe,
Mn, Ni, Pb, Zn
As, Cd, Cu, Mg, Mn,
Ni, Pb, Zn
Ag, As, Ba, Be, Cd,
Cr, Hg, Pb, Sb, Tl
ASTM
D5258-92
Leach
Sediment, Soil
ASTM
D5513-94
Total
ASTM
U.S. EPA
E1645-94
3015
Leach
Leach
U.S. EPA
3031
Total
U.S. EPA
3050B
Leach
Industrial Furnace
Feedstreams, Coal,
Coke, Cement, Raw
Feed Materials, Waste
Derived Fuels
Paint
Pb
Water, Wastewater
Al, Ag, As, Ba, Be,
Ca, Cd, Co, Cu, Cr,
Fe, K, Mg, Mn, Mo,
Na, Ni, Pb, Sb, Se,
Tl, V, Zn
Oil
Ag, As, Ba, Be, Cd,
Co, Cr, Cu, Mo, Ni,
Pb, Sb, Se, Tl, V, Zn
Sediment,
Al, As, Ba, Be, Ca, Cd,
Sludge, Soil
Co, Cr, Cu, Fe, K, Mg,
Mn, Mo, Na, Ni, Pb,
Se, Tl, V, Zn
Oil
Al, Ag, As, B, Ba, Be,
Sediment
Ca, Cd, Co, Cu, Cr,
Sludge
Fe, Hg, K, Mg, Mn,
Soil
Mo, Na, Ni, Pb, Sb,
Se, Sr, Tl, V, Zn
Fly Ash ,
Al, Ag, As, B, Ba,
Oil,
Be, Ca, Cd, Co,
Sediment,
Cu, Cr, Fe, Hg, K,
Sludge,
Mg, Mn, Mo, Na,
Soil
Ni, Pb, Sb, Se, Sr,
Tl, V, Zn
U.S. EPA
3051
Leach
U.S. EPA
Total
U.S. EPA
3052
can be
used as a
screen for
TCLP
1311
(sec 1.2)
EMMC
Leach
Oil,
Sediment,
Sludge,
Soil
U.S. EPA
NPDES
Leach
Standard
3030K
Methods for the
Examination of
Water and
Wastewater
Leach
Domestic and
Industrial
Wastewater
Water
Republic of
China
NIEA
C303.01T
Total
Fish, Shellfish
France
Kjeldahl
N V 03-100
Total
Milk, Meat Products,
Animal Food, Starch
and Starchy Foods
28
Al, Ag, As, B, Ba, Be,
Ca, Cd, Co, Cu, Cr,
Fe, Hg, K, Mg, Mn,
Mo, Na, Ni, Pb, Sb,
Se, Sr, Tl, V, Zn
Al, As, Ba, Cd, Cr, Cu,
Fe, Mn, Ni, Pb, Sb,
Se, Zn
Ag, Al, As, Au, Ba, Be,
Bi, Ca, Cd, Ce, Co,
Cr, Cu, Hg, Ir, K, Li,
Mg, Mn, Mo, Na, Ni,
Os, Pb, Pd, Pt, Rh, Sb,
Se, Si, Sn, Sr, Th, Ti,
Tl, V, Zn
Al, As, Ba, Be, Ca, Cd,
Co, Cr, Cu, Fe, K, Mg,
Mn, Mo, Na, Ni, Os,
Pb, Se, Th, V, Zn
N
After heating, the sample is
filtered and diluted for analysis.
The chemistry can be adjusted to
optimize the sample matrix compatibility with the instrument.
To maintain clean chemistry conditions of low and controlled analytical blank conditions, the operating
steps involving reagent addition,
capping microwave vessel, and
post-digestion procedures should
be performed in clean environments. The use of microwave
sample preparation for ultratrace
elemental analysis is discussed
extensively in the new microwave
reference text from the American
Chemical Society and includes
protocols and procedures to
enhance the reduction of the
analytical blank (5).
Synergy of Clean Chemistry
and Microwave Dissolution
Improve Quality Control in
Trace Element Determination
The improvements in efficiency
and reproducibility due to microwave-based sample preparation also
reduces significant sources of error
that were previously obscured by
lengthy sample preparation procedures. The analytical blank plays
an important role in the chemical
determination of trace metals and
are a primary source of error in
many instances. Trace element
determination depends as much
on the control of the analytical
blank as it is does on the accuracy
and precision of the instrument
used. The inability to control sample contamination that is external
to the sample, or those contributions of the analyte coming from
sources other than the sample,
are frequently the limiting factor
in trace (ppm/ppb) and ultratrace
(ppb/ppt) determinations. The
major sources of analytical blank
contributions are:
• atmosphere in which the
sample preparation and analysis
are conducted,
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
Type
Matrices
Detection
Control
Al, Ag, As, B, Ba,
Be, Ca, Cd, Co,
Cu, Cr, Fe, Hg, K,
Mg, Mn, Mo, Na,
Ni, Pb, Sb, Se, Sr,
Tl, V, Zn,
other elements
provided
validation
achieved
FAAS
ET-AAS
ICP-AES
ICP-MS
other analytical
techniques
provided
validation
achieved
Temperature
Feedback
Control and
provisions
for calibration
control
Pressure (atm)
Microwave
Soil
Total Digestion Oil
Sludge
Biological
Foods
Glass
Metals
Fly Ash
Sediment
other matrices
Analytes
Temperature (°C)
TABLE II
Overview of EPA Method 3052: Microwave-Assisted
Acid Digestion of Siliceous and Organically-Based Matrices
Fig. 1. A typical microwave digestion profile
for temperature and pressure with a soil (silicate-based material) in a high-pressure vessel.
TABLE III
Digestion Parameters Used in the Analysis of
Several Matrices by EPA Method 3052
Matrix
HNO3
HF
• purity of the reagents used in sample
preparation,
HCl
Soil
NIST SRM 2710
9
Highly Contaminated Montana Soil
NIST SRM 2711
9
Moderately Contaminated Montana Soil
3
0 –2a
3
0 – 2a
Sediment
NIST SRM 2704
Buffalo River Sediment
Biological
9
3
0 – 2a
NIST SRM 1566a
Oyster Tissue
NIST SRM 1577a
Bovine Liver
9
0
0 – 1a
9
0
0 – 1a
9
0
0 – 1a
9
0
0 – 1a
9
0
0 – 1a
9
0.5
0 – 1a
Botanical
NIST SRM 1515
Apple Leaves
NIST SRM 1547
Peach Leaves
NIST SRM 1567a
Wheat Flour
NIST SRM 1572
Citrus Leaves
Waste Oil
NIST SRM 1084a
Wear-Metals in Lubricating Oil
a
9
0.5
0 – 2a
HCl is added to stabilize elements such as Ag and Sb.
If elements that need to be stabilized with HCl are not being analyzed, then HCl was
not used.
29
• materials used in the digestion or
extraction vessels that come in contact
with the sample,
• analyst’s technique and skill in preparing the samples and in performing the
analysis.
The sources of each of these potential
contaminants and the solutions to control
them are presented elsewhere along with
an extensive discussion of the synergy
between clean chemistry and microwave
sample preparation for trace element
determination (5). The development of
microwave sample preparation, coupled
with clean chemistry techniques, has
broken some of the traditional barriers to
blank limitations, and permits measurements at lower concentration levels.
Microwave sample preparation has
improved analytical blank control in synergistic ways by lowering the blank level,
increasing blank precision, and improving
quality control.
Microwave sample preparation reduces
blank contribution from environmental
exposure, reagent use, and losses from
evaporation, and in addition reduces sample preparation times. It also offers a
more reproducible method for duplicating ultratrace determination, both within
a laboratory or between laboratories. The
required overall skill level of the analyst
is less critical in deciding the fate
of an analysis. This certainly shows
that we have reached the point of
providing a method of analysis, as
suggested by Berzelius in 1814,
that “depends least on the skill of
the operating chemist....” Because
of its efficiency, capability, and
reproducible contribution to the
overall sample preparation and
analysis process, microwave
sample preparation is emerging
as the appropriate tool of choice
in controlling the quality of sample
preparation in elemental analysis
as well as being one of the most
efficient methods.
CONCLUSION
Until the last decade, chemists
accepted the mismatch in technology levels between analysis and
detection instrumentation. In reality, sample preparation should be
done at the same level of instrumental
sophistication as the analysis.
Microwave sample preparation
provides practical and efficient
approaches for multielement determinations by using the advanced
technologies in ICP-MS, ICP-AES,
ET-AAS, and AA instrumentation.
Time compatibility is an important
issue, because inefficiencies exist in
an analytical scheme where one
portion is usually waiting for product from another. Plasma-based
instruments require rapid, efficient,
and reliable sample preparation
technology to complement their
efficiency and capability. Increasingly, microwave sample preparation is becoming the standard
technique for sample preparation
in trace and ultratrace determinations. Used in conjunction with
clean chemistry techniques,
microwave sample preparation is
a natural complement that
improves the overall analytical
process. Below the nanogram per
gram range, other traditional sample preparation techniques offer
few advantages. In contrast,
microwave closed-vessel and controlled-atmosphere open-vessel
are being effectively used at lower
concentration ranges for trace and
ultratrace analysis. As trace analysis
is more frequently extended to the
ultratrace region by more sensitive
and efficient instrumentation, sample preparation is keeping pace
with appropriate technological
advances that complement and
enhance detection capabilities.
30
REFERENCES
1.
J.J. Berzelius, “Lehrbuch der
Chemie,” F. Wöhler, Arnoldsche
Buchhandlung: Dresden, Vol. 4,
2nd part, p. 74 (1831).
2.
P.J. Walter, S.J. Chalk, and H.M.
Kingston, Chapter 2: Overview of
Microwave Assisted Sample Preparation In Microwave Enhanced
Chemistry; H. M. Kingston, S.
Haswell, Eds.; American Chemical
Society: Washington, D.C. (1997).
3.
H.M. Kingston, and Peter J. Walter,
“The Art and Science of Microwave
Sample Preparation for Trace and
Ultra-trace Elemental Analysis” , in
Inductively Coupled Plasma Mass
Spectrometry: From A to Z, Akbar
Montaser (Ed.), VCH-Wiley, 1998.
4.
SW-846 EPA Method 3052:
Microwave assisted acid digestion
of siliceous and organically based
matrices, In Test Methods for Evaluating Solid Waste, 3rd edition,
3rd update; U.S. EPA: Washington,
DC (1995).
5.
H.M. Kingston, P.J. Walter,
S.J. Chalk, E. Lorentzen, and
D. Link, Chapter 3: Environmental
Microwave Sample Preparation
Fundamentals, Methods, and Applications In Microwave Enhanced
Chemistry: Fundamentals, Sample
Preparation, and Applications;
H.M. Kingston and S. Haswell, Eds.;
American Chemical Society: Washington, D.C. (1997).
A Streamlined Approach to the Determination
of Trace Elements in Foods
Karen W. Barnes
The Perkin-Elmer Corporation
761 Main Avenue, Norwalk, CT 06859 USA
INTRODUCTION
In the past, trace metal determination in foods was performed
primarily to ensure product authenticity, quality, or safety. Nutrient
labeling was required on a food
product only if health claims were
made or implied on the food label,
or nutrients had been added (1).
Regulatory agencies typically performed metal or mineral determination to analyze nutrient content, to
confirm or establish product identity, to determine compliance with
current trade and food labeling
laws, and to assure product safety.
In an attempt to assist consumers
in maintaining healthy dietary practices, the United States Congress
enacted the Nutrition Labeling and
Education Act of 1990 (NLEA) on
November 8, 1990. NLEA amended
provisions of the Federal Food,
Drug, and Cosmetic Act and
requires full nutritional labeling
for most U.S.Food and Drug Administration (FDA) regulated packaged
food products. Very few products
are exempt from NLEA. The complete regulations have been published (2) and a May 8, 1994, compliance deadline was established.
NLEA mandates labeling of 14
mandatory nutrients including
calcium, iron, and sodium. Thirtyfour other nutrients, including nine
additional metals, may be labeled
voluntarily. The mandatory and
voluntary nutrients for NLEA are
listed in Table I. It has been
predicted that the labeling of the
voluntary nutrients will become
mandatory (3). It has been
estimated that 17,000 food companies in the United States are
affected by NLEA, and that labels
for 196,000 to 257,000 products
required modification. Industry
costs to implement NLEA were
projected at $2 billion (4).
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
ABSTRACT
The Nutrition Labeling Education Act of 1990 (NLEA)
mandated significant reform of
food labels and required extensive chemical testing and analytical method development to
implement. Many minerals and
nutrient metals were targeted for
labeling. Part of the analytical
challenge to be met through
compliance with NLEA results
from the large numbers of samples to be tested and widely
divergent levels of the elements
of interest within the samples.
Ideally, any new methodology
developed for food analysis
should be generic and applicable
to many elements and matrices
simultaneously.
This work presents one
streamlined approach for sample
preparation and determination of
trace metals that is applicable to
all food types as categorized by
the Food Matrix Triangle. Inductively coupled plasma optical
emission spectrometry (ICP-OES)
with an axially-viewed plasma
was used to determine the metals. Sample preparation time has
traditionally been the primary
limitation to sample throughput
and this was minimized using
microwave digestion. Multiple
dilution and analysis iterative
steps were not necessary due to
the wide linear dynamic range of
the ICP-OES. Trace and macro
levels of nutrients were determined simultaneously in all samples. Quality assurance (QA) was
performed through the analysis
of replicate samples, laboratoryfortified samples and blanks, and
standard reference materials
(SRM). Acceptable precision,
spike recoveries, and agreement
with certified and U.S. Department of Agriculture values
were attained.
31
The determination of metals and
minerals in foods is challenging due
to the wide range of concentrations
present, which may vary from ppb
(ng/g) to percent levels. The situation is further complicated by naturally occurring seasonal and varietal
differences. Many official methods
are analyte- and matrix-specific.
A review of current validated
Association of Official Analytical
Chemists International (AOAC)
methodology for minerals and
metals in foods (5) revealed that
many single-element methods are
currently in use. These methods
employ the techniques of colorimetry, UV/Visible spectrophotometry,
and flame and graphite furnace
atomic absorption spectroscopy.
Many of the methods have sample
throughput constraints and
relatively narrow linear dynamic
ranges, and others require the use
of solvents banned by the Montreal
Protocol on Substances that
Deplete the Ozone Layer (6).
A recent collaborative study was
performed for metals in foodstuffs
by dry ashing followed by atomic
absorption spectrometry (7);
however, NLEA elements were
not specifically targeted. Neither
the methodology from the collaborative study nor the current official
methods can be used to simultaneously determine all the elements
specified in NLEA. Time constraints
make multiple sample dilutions
and independent analysis impractical and limit the utility of some validated methods. AOAC recognized
the need for improved methodology and developed a tool to help
analysts develop rugged, generic
methods. The Food Matrix Triangle
(8) categorizes foods into nine
sectors based on relative protein,
fat, and carbohydrate content.
The Methods Committee of AOAC
proposed that a method could be
TABLE I
The Nutrition Labeling Education Act of 1990
Food Labeling Requirements
Mandatory
Nutrients
Calories
Calories from fat
Total Fat
Saturated Fat
Cholesterol
Sodium
Total
Carbohydrates
Total Sugars
Dietary Fiber
Protein
Vitamin A
Vitamin C
Calcium
Iron
Voluntary Nutrients
Calories from Saturated Fat
Calories from Unsaturated Fat
Calories from Carbohydrates
Calories from Protein
Unsaturated Fat
Polyunsaturated Fat
Monounsaturated Fat
Vitamin K
Thiamin
Riboflavin
Niacin
Vitamin B6
Folate
Vitamin B12
Copper
Manganese
Fluoride
Chromium
Molybdenum
Chloride
Sugar Alcohols
Soluble Fiber
Insoluble Fiber
Protein as %
Potassium
Vitamin D
Vitamin E
Biotin
Pantothenic Acid
Phosphorus
Magnesium
Zinc
Iodine
Selenium
Reprinted from The Referee,
Volume 17, Number 7,
pages 06-07, 1993.
Copyright 1993 by
AOAC International
Fig. 1. Food Matrix Triangle.
tested for all food matrices by analyzing eighteen types of samples,
two from each sector of the food
triangle. The food triangle,
presented in Figure 1, is reprinted
with permission from The Referee,
Volume 17, Number 7, pages
06–07, 1993. Copyright 1993 by
AOAC International. A recent (9)
article by the members of Official
Methods Board of AOAC discusses
the difficulty of such an approach
and further states that the food triangle should be used to categorize
similar types of foods to be tested
using a single method. Vast differences in composition of foods make
a streamlined approach unlikely in
their opinion. This work presents
a sample preparation and analysis
protocol that does indeed prove
useful for all sectors of the food
matrix triangle.
32
The FDA regulates foods based
upon tolerances of label claim for
different classes of nutrients; a 20%
tolerance is allowed for naturally
occurring nutrients (10), and state
regulatory agencies will probably
adopt similar guidelines for monitoring compliance. Although no
AOAC food methods currently
employ inductively coupled plasma
optical emission spectrometry
(ICP-OES), it is a well-established
multielement technique, and the
analysis of foods has been a natural
application (11). Due to the wide
variations of mineral levels in foods,
a multielement technique must
be able to simultaneously monitor
trace levels of elements in the presence of macro levels of other elements. ICP-OES features multielement capability, wide linear
dynamic range, high analytical
sensitivity, and high sample throughput. All these are attributes that
will prove invaluable to analysts
striving to meet the challenges
posed by NLEA.
Extensive sample preparation
of foods before elemental analysis
is common and, particularly when
ICP-OES is employed, often proves
to be more time-consuming than
the actual analysis. Methods involving hot plate digestion in open
vessels or dry ashing followed by
acid dissolution are commonly
employed, but are time-consuming
and prone to contamination and
evaporative losses. Microwave
digestion has been shown to be
an acceptable alternative and has
been successfully applied to food
analyses (12). Advantages to
microwave digestion result from
the nature of the process, i.e.,
because the system is sealed,
contamination and evaporative
losses, and chemical consumption
are reduced. In general, microwave
procedures are fast due to the
increased heating that occurs
from the microwave interaction
with the reagents and samples.
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
This paper presents a microwave
digestion and ICP-OES analytical
protocol to determine metals and
minerals in food. At least two foods
were chosen from each sector of
the Food Matrix Triangle. Although
this study primarily targets NLEA
elements, the procedures discussed
here have been successfully used
to determine numerous target elements in many different foods
(13–22), plants, agricultural products (23), and biological matrices
(24). With the careful selection
of metallic analytes, many different
analytical questions may be
answered. For example, adulteration and misbranding may be identified from metal profiles because
an adulterated product that has had
a valuable constituent abstracted
will often have a different metal
profile than an authentic product
(23). The geographical origin of
a food or plant may be determined
by characterizing a metal fingerprint in a product and comparing it
with the fingerprint from a known,
authentic sample of the product.
Related cases of food tampering
have been linked by comparing
metal profiles. Metal determination
is appropriate for establishing food
safety as well. Contamination from
lead solders or aluminum from
breached can linings may be identified by ICP-OES analysis. Typically,
the suspect products are measured
simultaneously with known,
authentic products for comparison.
EXPERIMENTAL
Reagents and Standard Solutions
Micro laboratory cleaner (International Products Corporation,
Trenton, NJ USA)
HNO3-double subboiling distilled
in quartz (GFS, Cleveland, OH USA)
PE Pure single-element standards
(PE XPRESS, Perkin-Elmer, Norwalk,
CT USA)
ICP multielement standard solution IV (Merck, Darmstadt, Germany)
Deionized water, 18 MΩ, (Continental Water Systems, San Antonio,
TX USA)
Standard Reference Materials:
SRM 1548 Total Diet, SRM 8413
Corn, SRM 1549 Milk Powder, SRM
1570 Spinach, SRM 1566a Oyster
Tissue, and SRM 1577a Bovine Liver
(National Institute of Standards and
Technology, Gaithersburg, MD USA)
Certified Reference Material:
CRM Tort-2 (National Research
Council Canada, Ottawa, Canada)
Instrumentation
Optima 3000™ DV inductively
coupled plasma optical emission
spectrometer (Perkin-Elmer, Norwalk, CT USA), equipped with an
AS-91 autosampler, Multiwave
microwave digestion system, and
quartz digestion vessels rated to
110 bar.
The food grater was designed
for home use and the high density
polyethylene bottles were
NALGENE® or equivalent (Nalge
Nunc International, Rochester,
NY USA).
ICP-OES Analyses
The ICP-OES analyses were performed on two different PerkinElmer Optima 3000 DV ICP-OES
systems, equipped with a standard
torch, Scott-type spray chamber,
and a GemTip™ cross-flow nebulizer. A Perkin-Elmer AS-91 autosampler was used exclusively. All the
standard reference materials (SRM)
and some of the spike determinations were performed at the PerkinElmer site in Überlingen, Germany.
The remaining samples were analyzed in the Perkin-Elmer ICP-OES
research laboratory in Wilton, CT
USA. The Optima 3000 DV is a
simultaneous ICP-OES instrument
with an echelle polychromator and
a segmented array charge-coupled
detector (SCD). Because measurement of back-ground and analyte
emissions occurs simultaneously,
accurate correction of transient
background fluctuations for multi33
ple lines for each element of interest is possible. Also, due to the
simultaneous measurement, no
reduction in sample throughput
occurs from making measurements
at multiple wavelengths of an element. Therefore, where possible,
multiple emission lines were measured simultaneously for each element to verify analytical results.
Plasma conditions used for this
work are listed in Table II, and
wavelength selection and background correction points are listed
in Table III. No attempt was made
to optimize source conditions for
specific analytes, rinse or read
delays, or to maximize sample
throughput. Calibration was performed with a 10-ppm multielement standard. This is clearly
not optimal considering the high
level of mineral nutrients, but
unfortunately this was the only
material feasible for use at the time.
For a more detailed study, multiple
standards should have been used.
Due to the wide linear dynamic
range of an ICP-OES instrument,
the results were acceptable, except
for K where the disparity between
sample and standard is reflected in
the relatively large standard deviations. This can be improved using
TABLE II: ICP-OES
Instrumental Conditions
Parameter
Optima 3000 DV
RF Power
1450 W
Nebulizer Flow
0.7 L/min
Auxiliary Flow
0.6 L/min
Plasma Flow
15.0 L/min
Sample Flow
1.5 mL/min
Plasma Height
15 mm
Plasma Viewing
Axial
Processing Mode
Area
Auto Integration
10–50 sec
Read Delay
75 sec
Rinse Delay
120 sec
Replicates
3
Wavelengths
Multiple
Background
Manual selection
of points
TABLE III
ICP-OES Analytical Parameters
Analyte
Al
B
Ca
Ca
Cu
Fe
Fe
K
Mg
Mg
Mg
Mn
Na
Pb
Zn
Zn
Zn
Wavelength (nm)
396.152
182.527
315.887
317.933
324.754
259.940
273.955
766.491
279.079
279.549
285.208
257.610
589.592
216.999
202.547
206.197
213.858
a higher concentrated standard.
Results do overlap within 3 standard deviations (RSD) with the
reported confidence intervals.
Microwave System
A Perkin-Elmer Multiwave
microwave digestion system with
1000 W power with temperature
and pressure monitoring and control was used to digest all samples.
Digestion was performed using
high-pressure quartz vessels tested
to 140 bar. Method development
was simplified with the Multiwave
system because temperature and
pressure are measured and controlled in each vessel, features
which make the system extremely
safe and easy to use. Because pressure is monitored in all vessels
simultaneously, different types of
foods may be digested in a single
run without the risk of seal or vessel rupture. Groupings will be
shown below.
Sample Selection and Preparation
The need for procedures to
control the analytical blank and
ensure sample integrity for success-
Lower Bcg Point
Upper Bcg Point
–0.030
–0.015
–0.047
–0.023
–0.029
–0.023
–0.020
–0.099
–0.024
–0.026
–0.026
–0.016
–0.053
–0.016
–0.019
–0.019
0.020
0.034
0.018
0.030
0.029
0.021
0.024
0.030
0.126
0.024
0.026
0.026
0.025
0.064
0.014
0.019
0.019
–0.018
ful trace metal analysis has been
stressed (25–26) and is an advantage inherent in microwave-assisted
chemistry. To minimize the blank,
all labware, except the high density
polyethylene (HDPE) bottles used
to store the diluted samples, was
washed with Micro laboratory
cleaner, rinsed with 18 MΩ, deionized H2O, soaked in 2% (v/v) ultrapure HNO3 and rinsed with
deionized H2O. To minimize the
potential for sample contamination,
disposable powder-free gloves
were worn for the entire sampling
process and were dipped in 2% acid
and rinsed with deionized H2O.
At least two foods from each
sector of the Food Matrix Triangle
were digested and analyzed. The
food selections along with protein,
fat, and carbohydrate compositions
are listed in Table IV. SRMs were
run as foods from sectors of the
triangle where possible. For other
foods where SRM materials were
not available, spike recoveries were
determined for quality assurance.
Multiple samples of the reference
materials were digested simultane-
34
ously with the non-certified samples. Two or three subsamples
were digested for each sample.
Standard deviations reported are
based on the means of all samples.
Standards, blanks, spiked blanks,
and wash solutions were matrixmatched using acids digested in the
microwave as part of the sample
preparation procedure. A concentrated 1000-mg/L multielement
stock standard was used to prepare
working standards and to fortify
(spike) the blanks and samples. The
samples and blanks were fortified
to approximately 2 mg/L, and
spiked samples and blanks were
subjected to digestion concurrently
with the samples to monitor any
process-related losses.
Almonds were broken by hand
into smaller pieces. The chocolate
and macadamia nuts were processed to improve sample homogeneity using a manual food grater
that was washed with Micro
cleaner and rinsed with deionized
H2O. Tuna fish salad (100 g) was
prepared from the USDA recipe
(27) in Handbook 8–15 as follows:
53.8% light tuna in oil, 14.7% pickle
relish, 14.1% salad dressing, 10.2%
onion, and 7.2% celery. The tuna
fish salad was processed in a miniature electric food processor
designed for home use. The food
processor was washed with Micro
detergent and rinsed with deionized H2O before use. The samples
were chopped for approximately
two minutes each until no further
reduction in particle size was apparent to the eye. A thick, inhomogeneous slurry resulted that was
transferred into HDPE bottles for
subsampling. The slurry was well
shaken before sampling from the
HDPE bottle. Acid was not used to
clean the grater or food processor
because of the increased contamination that would result from partial dissolution of the metal blades.
Wolnik et al. (26) reported using
a food processor with all plastic
parts presumably to prevent sample
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
TABLE IV
Composition of Food Samples
Sector
Food
1
Heavy Cream
Macadamia Nuts
2
Chocolate
Dry Sweetened Coconut
3
Almonds
Creamy Peanut Butter
4
Part skim Mozzarella Cheese
Eggs
5
NIST SRM Corn
Banana Juice
6
NIST SRM Total Diet
Vegetable Chicken Baby Food
7
NIST SRM Spinach
NIST SRM Nonfat Dry Milk
8
NIST SRM Oyster Tissue
USDA Recipea Tuna Salad
9
NIST SRM Bovine Liver
NRCC CRM Tort
% Protein
% Fat
0–33
5
8
0–33
5
3
0–33
20
30
33–67
58
51
0–33
13
4
0–33
21
20
33–67
41
41
33–67
62
46
67–100
69
93
67–100
88
79
33–67
36
41
33–67
59
53
33–67
36
45
0–33
5
2
0–33
21
11
0–33
4
1
0–33
13
27
0–33
13
4
% Carbohydrate
0–33
7
13
33–67
59
55
0–33
21
17
0–33
7
4
67–100
81
94
33–67
58
69
33–67
55
58
0–33
25
27
0–33
18
3
Group 5: Heavy cream and part
skim mozzarella cheese
Group 6: Homogenized raw eggs
with USDA recipe tuna
salad
Group 7: Grated milk chocolate
with macadamia nuts
Group 8: Sweetened dried coconut
with peanut butter
Group 9: Almonds
Group 10: Milk powder
All samples were prepared following the protocol:
Step 1: Weigh food into clean dry
quartz vessel
Step 2: Add 5mL concentrated
ultrapure HNO3
Step 3: Cap vessels using the sealforming tool to widen the
lip seals
Step 4: Digest with preprogrammed
Coffee Bean sample procedure, resident on the Multiwave computer
Step 5: Open vessels and dilute to
50–100 mL with DI H2O
The Coffee Bean sample program,
typically used for biological materials, is listed in Table V. Sample
weights used are listed in Table VI.
RESULTS AND DISCUSSION
a
USDA Tuna Salad Recipe:
Light tuna in oil 53.8 g
Onion
10.2 g
Relish
Celery
14.7 g
7.2 g
Salad dressing 14.1 g
contamination during grinding.
Unfortunately this was not an
option for this study. Although the
metal blades were considered a possible source of contamination, the
analytical results indicate that no
gross contamination occurred.
possible in microwave systems that
only measure one control vessel. In
that case, the samples digested
must be very homogeneous or
seals and vessels may rupture.
Foods digested together in this
study were:
Because pressure is measured in
all vessels simultaneously in the
Multiwave system, different sample
types may be digested together.
Also inhomogeneous samples such
as the tuna salad were digested
safely with no venting. This is not
Group 1: Spinach
Group 2: Corn powder and
banana puree
Group 3: Vegetable chicken
baby food, typical diet,
and Tort-2
Group 4: Oyster tissue and
beef liver
35
General
For ease of comparison, the
results obtained for this work are
presented with available authentic
data. All concentrations for this
work are expressed in mg/L (ppm).
The mean value obtained for the
three subsamples of each SRM is
reported in the experimental section and the reported uncertainty
is one standard deviation of the
measurement. P, Se, Cr, and Mo
were not measured in the samples.
Wolnik et al. (26) reported that the
ICP-OES detection limits for Pb, Cd,
and Se are not sufficient for routine
determination of background levels
TABLE V
Coffee Bean Digestion Protocol
Stage
1
2
3
4
Power (W)
Time (min: sec)
100
600
1000
0
5:00
5:00
10:00
15:00
in crops and that the Mo levels are
near the detection limit for ICP-OES.
Although the levels of Se, Cr, and
Mo in the foods were below the
detection limits for this work due to
the dilution used, the determination
of these elements is certainly possible with plasma optimization and
the implementation of preconcentration techniques, and has been
repeatedly demonstrated. Sample
throughput for the study was
impressive, even though throughput optimization was not
performed. Sample throughput
could be improved by optimizing
read and rinse delays, and by
increasing the sample introduction
rate and the rate at which the sample is introduced into the plasma.
For laboratories with large sample
loads, optimization of sample
throughput will be necessary.
Considering that FDA will regulate compliance with NLEA based
upon a ±20% criteria, the protocols
employed to prevent outside contamination may be excessive and
time-consuming; however, each
process discussed is appropriate
for monitoring metals for any of
the reasons discussed in the introduction. In which case, the ±20%
criteria may not be appropriate and
contamination could be an important consideration.
SRM Materials
SRM materials were available for
Sectors 5–9 of the food matrix triangle and their use is certainly the
best method to evaluate the utility
and accuracy of a method. A review
of the data is presented in Tables
VII–XIII. The results are the means
TABLE VI
Sample Weights Digested
Power (W)
600
600
1000
0
Fan
Food
1
1
1
3
and one standard deviation of the
three subsamples of each material,
and are acceptable for all reported
elements. As discussed above,
a more concentrated standard for
K would have improved the results.
For Ca, the results could have been
improved by using a different analytical wavelength with a wider linear dynamic range. However, as
with K, the mean ±3 sigma do fall
within the reported confidence
interval. Good agreement was seen
between multiple wavelengths for
the same analytes.
CEM Preparation
The results shown in Table XIV
compare banana puree previously
analyzed using an Optima 3000 XL
ICP-OES and a CEM MDS™ 2100
microwave digestion system (CEM
a
Sample
weight (g)
Heavy Cream
0.7
Macadamia Nuts
0.5
Chocolate
0.6
Dry Sweetened Coconut
0.5
Almonds
0.6
Peanut Butter
0.3
Part Skim Mozzarella
Cheese
0.6
Eggs
0.7
NIST SRM Corn
0.5
Banana Juice
6.0
NIST SRM Total Diet
0.6
Vegetable Chicken
Baby Food
1.0
NIST SRM Spinach
0.6
NIST SRM Nonfat
Dry Milk
0.5
NIST SRM Oyster Tissue
0.5
USDA Recipea Tuna Salad
0.5
NIST SRM Bovine Liver
0.5
NRCC CRM Tort
0.4
See Table IV footnote.
TABLE VII
NIST SRM 8413 Corn (Sector 5) in µg/g
Analyte
Ca 315
Ca 317
Cu 324
Fe 238
Mg 285
Mn 257
Zn 206
Mean
62.2
62.4
2.43
28.8
1060
5.24
21.4
Std. Dev.
1.1
1.2
1.5
0.08
32
0.45
2.4
Certified
42 ± 5
42 ± 5
3.0 ± 0.6
23 ± 5
990 ± 82
4.0 ± 0.03
15.7 ± 1.4
TABLE VIII
NIST SRM 1548 Total Diet (Sector 6) in µg/g
Analyte
Al 396
Ca 315
Ca 317
Mg 285
Zn 206
Mean
74.6
1910
1910
563
29.6
36
Std. Dev.
4.7
18
11
6.8
1.3
Certified
66.7 ± 0.03
1960 ± 5
1970 ± 6
589 ± 2
24.05 ± 0.05
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
TABLE IX
NIST SRM 1549 Milk Powder (Sector 7) in µg/g
Analyte
Ca 317
Mg 279.5
Mn 257
Na 589
Zn 213
Mean
12900
1210
0.44
5080
46.5
Std Dev.
450
46
0.04
130
1.2
TABLE XIII
NRCC CRM Tort-2 (Sector 9)
Certified
13000 ± 500
1200 ± 300
0.26 ± 0.06
4970 ± 100
46.1 ± 2.2
13400
13800
13
549
39100
175
54.3
Analyte Mean
Std
Certified
3.5
13500 ± 300
5.6
13500 ± 300
0.02
12.2 ± 2
0.05
550 ± 20
< 0.01
35600 ± 300
0.01
165 ± 6
0.07
50 ± 2
Mean
1690
1670
57.2
463
1040
1010
11.3
825
781
Std Dev.
85
110
1.6
27
35
94
0.42
110
93
Cu 324
Fe 238
Mn 257
Zn 206
92.6
93.4
12.8
183
Std. Dev.
1.8
1.5
0.01
8
Certified
106 ± 10
105 ± 13
13.6 ± 1.2
180 ± 6
Banana Juice
TABLE XI
NIST SRM 1566a Oyster Tissue (Sector 8) in µg/g
Analyte
Ca 315
Ca 317
Cu 324
Fe 238
Mg 279.5
Mg 285
Mn 257
Zn 202
Zn 206
Mean
TABLE XIV
Multiwave System Results vs. Juice Digested
with CEM MDS 2100
TABLE X
NIST SRM 1570 Spinach (Sector 7) in µg/g
Dev.
Ca 315
Ca 317
Cu 324
Fe 238
K 766
Mn 257
Zn 202
Analyte
Certified
1960 ± 190
1960 ± 190
66.3 ± 4.3
539 ± 15
1180 ± 170
1180 ± 170
12.3 ± 1.5
830 ± 57
830 ± 57
Analyte
Mean
Std. Dev.
MDS 2100
Ca 315
Ca 317
Cu 324
Fe 238
K 766
Mg 279
Mg 285
Mn 257
Zn 206
21
21
0.09
0.87
1000
102
102
0.88
0.67
1.7
1.7
0.06
0.01
40
7
10
< 0.01
0.01
17
17
0.39
0.67
1090
98
98
0.8
0.69
Corporation, Matthews, NC USA) for the analysis and
sample preparation steps, respectively. The puree sample had been frozen for about two years.
Correlation was excellent considering that different
ICP-OES instruments were used. The Multiwave
microwave preparation was far simpler than that
required for the CEM MDS 2100 microwave digestion
system (presented in References 13, 15, 20, and 21)
and had the added advantage that H2SO4 and H2O2
were not required. This would also permit the
determination of S if desired.
TABLE XII
NIST SRM 1577a Bovine Liver (Sector 9) in µg/g
Spike Recoveries
The results are shown in Figure 2. Quality assurance
for the sectors where SRM materials were not available
was performed using fortified laboratory samples.
Other elements were chosen for this part of the study
to demonstrate the utility of the ICP-OES for the determination of multiple analytes. All results recovered
within the FDA established criteria of ±20%. Elements
that are missing from some of the samples were not
present in the spiking solution that was available for
that portion of the experiment. As previously reported
by Wolnik et. al. (26), best results were found in samples with the greatest homogeneity but were acceptable even for the coarsely ground samples.
Analyte
Mean
Std Dev.
Certified
Ca 315
112
9.2
120 ± 7
Ca 317
111
9.9
120 ± 7
Cu 324
178
21
158 ± 7
Fe 238
240
25
194 ± 20
K 766
8400
540
9960 ± 70
Mg 279.5
568
49
600 ± 15
Mg 285
552
31
600 ± 15
Mn 257
10.1
1.1
9.9 ± 0.08
Na 589
2050
340
2430 ± 130
Zn 206
126
15
123 ± 8
*Value is not certified but given for information only.
37
% Recovery
C N Ch Co
P M E B T
C N Ch Co
P M E
T
C N Ch Co A P M E B T
C N Ch Co A P M E B T
C N Ch Co A P M E
T
Fig. 2. Spike recoveries for sectors 1–4, 6, 8.
CONCLUSION
This work demonstrates that a
single digestion and analytical protocol is effective for the determination of metals and minerals in foods
from all sectors of the food triangle
using ICP-OES with microwave
digestion. This work indicates that
ICP-OES is an effective tool for the
analyst attempting to meet the challenges imposed by NLEA.
Microwave digestion was shown to
be a simple, safe, and effective sample preparation tool allowing
acceptable precision, spike recoveries, and agreement with SRM certified values. Remarkable agreement
was seen between different types of
axial Optima instruments and different Optima dual-view instruments,
and comparable results were seen
between different laboratory locations. Trace levels of elements were
determined simultaneously with
macro levels of nutrient metals and
minerals. Sample preparation time,
which was the primary limitation to
sample throughput, was minimized
due to the wide linear dynamic
range and useful analytical range of
the ICP-OES and due to the features
of the Multiwave system. All nutrients were determined simultaneously in all samples without
multiple dilution/analysis iterations.
38
Although no attempts were made to
optimize sample throughput, actual
instrumental sample throughput
was impressive relative to current
AOAC validated methods. If rigorous optimization procedures are
implemented, then higher sample
throughputs will be possible and
will be acceptable for analysts routinely making NLEA or other determinations. Sample homogeneity
and freedom from environmental
contamination were shown to be
important to analytical precision.
This work also suggests that other
elements and minerals in food
matrices may be determined effectively using ICP-OES.
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
REFERENCES
1. The Labeling of FDA Regulated
Foods, a Shortcourse presented by
FDA, IFT, AIB Orlando, FL USA
(2/1988).
18. K.W. Barnes, E. Debrah, and Z. Li,
ICP-OES Application Study No. 72,
The Perkin-Elmer Corporation
(1994).
2. Fed. Regist. 58, 3, 632-690, 20662964 (1/6/93).
19. K.W. Barnes and E. Debrah, ICPLOES Application Study No. 73, The
Perkin-Elmer Corporation (1994).
3. N. Millier-Ilhi, Appl. Spectrosc. 47,
14A (1993).
4. Instrument and Business Outlook 1,
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Study No. 78, The Perkin-Elmer
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Corporation (1996).
22. K. O’Hanlon and K.W. Barnes, ICPOES Application Study No. 81, The
Perkin-Elmer Corporation (1996).
23. K.W. Barnes, unpublished work,
Bureau of Food Laboratory, Florida
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24. S.R. Koirtyohann and D.A. Yates,
ICP-OES Application Study No. 62,
The Perkin-Elmer Corporation
(1993).
25. Microwave-Enhanced Chemistry:
Fundamentals, Sample Preparation,
and Applications, H.M. Skip
Kingston, and S.J. Haswell (eds.),
American Chemical Society 274
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M.W. Meyer, R.D. Satzger, E. Bonnin, and C.M. Gaston, J. Agric.
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39
A Streamlined Flame Atomic Absorption Method
for Animal Feed Analysis
Cynthia P. Bosnak and Karen W. Barnes
The Perkin-Elmer Corporation
761 Main Avenue, Norwalk, CT 06859 USA
INTRODUCTION
Current methods for animal feed
analysis are very time-consuming.
Many feed methods, for example
Association of Official Analytical
Chemists International (AOAC)
Method 968.08, Minerals in Animal
Feed and Pet Food, involve an ash
step for several hours in the muffle
furnace to destroy the organic matter, followed by acid digestion
using a hot plate. For the determination of some elements, the animal feed samples must be considerably diluted before atomic
absorption analysis. Also, before
the determination of calcium, a
releasing agent, such as lanthanum,
must be added for the control of
interferences.
Microwave digestion has been
used to greatly reduce the amount
of time needed for sample preparation. A microwave digestion system
with temperature and pressure
control of each digestion vessel
provides simplified method development and allows for different
sample matrices to be digested with
the same digestion program. Digestion of the feed samples is complete
using a 20-minute microwave heating program. The samples are then
automatically cooled in 15 minutes
and are ready for analysis.
An automatic dilutor was used
to prepare the atomic absorption
calibration standards and for diluting the feed samples to bring the
concentrations into the working
range. The diluter was also used
to automatically add the reagents
for control of interferences.
The AA Winlab™ software
(Perkin-Elmer, Norwalk, CT USA)
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
ABSTRACT
A faster digestion protocol
using nitric acid and microwave
digestion was used for the preparation of animal feed samples.
Flame atomic absorption using
on-line dilution and the
automated addition of lanthanum
solution provided a much easier
and quicker analysis of the feed
samples.
was used to automatically provide
quality control parameters such
as checking the correlation coefficients and precision. Using these
quality control parameters allows
for unattended instrument operation. The software also automatically converted the results to
percent solid weight values.
Commercially available domestic
cat, dog, and rabbit food, as well
as animal feed supplements, check
samples from the Association of
American Feed Control Officials,
Inc. (AAFCO), were used for this
study.
EXPERIMENTAL
Instrumentation
A Perkin-Elmer (Norwalk, CT
USA) AAnalyst™ 100 flame atomic
absorption spectrometer equipped
with an AutoPrep 50™ automatic
dilutor and an AS-91 autosampler
using AA Winlab software were
used for the animal feed analysis.
Perkin-Elmer hollow cathode lamps
were used for the determination of
each element. The instrumental
conditions for the analysis of the
sample and for the determination of
each element are listed in Tables I
and II, respectively.
40
A Perkin-Elmer Multiwave
microwave digestion system was
used to digest all of the animal feed
samples. The Multiwave system
default program for Coffee Bean
digestion was used for all samples
and is shown in Table III.
TABLE I
Instrumental Parameters
Signal type
Signal measurement
Flame
Read time
Read delay
Replicates
AA
Time average
Air-acetylene
5 sec
15 sec
3
Reagents and Standard
Solutions
Nitric acid: Merck, Pro Analysi
grade.
Hydrochloric acid: Merck, Pro
Analysi grade.
0.5% lanthanum oxide solution:
Weigh out 58.6 g of La2O3 (Merck)
into a 1-L flask and add 50 mL of
deionized water. Mix well. Slowly
add 250 mL of concentrated HCl
to the flask and mix well until
dissolved. Dilute to volume with
H2O.
100 mg/L multielement standard:
Pipette 20 mL of the concentrated
ICP multi-element standard (Merck),
22 elements, (Solution IV, concentration 1000 ug/mL, Lot No.
60090886), and dilute to 200 mL
with deionized water containing
5 mL of concentrated HNO3.
Diluent: Deionized water
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
Parameters
Wavelength, nm
Slit, nm
Calibration
equation
Std. 1, mg/L
Std. 2, mg/L
Std. 3, mg/L
Std. 4, mg/L
Standards
preparation
Stock standard, mg/L
Dilution mode
Diluent
Ca
TABLE II
Element-Specific Parameters
Cu
Fe
Cr
Zn
Se
422.7
0.7
Linear, Calc
Intercept
1.00
5.00
10.00
—
System
324.7
0.7
Linear,Calc
Intercept
1.25
2.50
3.75
5.00
System
248.3
0.2
Linear, Calc
Intercept
1.25
2.50
3.75
5.00
System
357.9
0.7
Linear, Calc
Intercept
5.00
—
—
—
User
213.9
0.7
Non-linear
1.00
2.50
5.00
—
System
196.0
2.0
Linear, Calc
Intercept
10.00
—
—
—
User
100.0
On-line
1% La
100.0
None
—-
100.0
On-line
H2O
—
None
—
100.0
On-line
H2O
—
None
—
Sample Preparation
Dry Food
After using a mortar and pestle
to grind the dry food samples for
homogenous sampling, accurately
weigh 0.5–0.6 g of dry animal feed
into the microwave digestion vessel. Add 5 mL of concentrated
HNO3. Close the vessel and place
in the microwave oven.
Wet Food
Accurately weigh 2.0–2.5 g
of the wet animal food into the
TFM microwave digestion vessel.
Add 5 mL of concentrated HNO3.
Close the vessel and place in the
microwave oven.
TABLE III
Microwave Program
Power Time Power
Step (W) (mm:ss) (W) Fan
1
2
3
4
100
600
1000
0
5:00
5:00
10:00
15:00
600
600
1000
0
1
1
1
3
Each sample was digested and
analyzed in duplicate using the Coffee Bean digestion protocol that is
resident on the on-board computer
of the Multiwave system. The Coffee Bean protocol is presented in
Table III.
Mixed sample types were
digested together in the same rotor.
This is possible because the Multiwave system monitors the temperature and pressure in each vessel.
Pressure is maintained at the maximum working pressure for each
vessel type: 30 bar for TFM vessels
and 75 bar for quartz vessels. If
a sample begins to overpressure,
the microwave power, which is
unpulsed, is reduced in 1 W increments so that the maximum working pressure is maintained.
After digestion and cooling,
quantitatively transfer the digestion
vessel contents directly into 50-mL
polypropylene autosampler vessels.
Use deionized water to thoroughly
rinse out the digestion cap and vessel. Bring samples to a final volume
of 50 mL with deionized water.
Procedure
Prior to analysis, the nebulizer
and burner system were optimized
using a 5-mg/L Cu standard. In addition, before the determination of
each element, the flame gas flows
were optimized.
41
When using the AutoPrep 50
system for on-line dilutions, the
nebulizer flow must be calibrated
with the flame atomic absorption
system. Using a 10-mL graduated
cylinder, the nebulizer flow was
measured, the uptake rate
calculated, and this value was
stored in the software.
For the determination of Ca,
a lanthanum solution was used as
the diluent solution so that it was
automatically added for the control
of interferences. For all other elements, deionized water was used
for dilution.
The AutoPrep system automatically prepared the working calibration standards from the 100-mg/L
stock standard. After the calibration
curve was generated, each sample
was analyzed and dilutions made if
the sample value was over the highest calibration standard. The sample
was diluted until the value was
within the working range or until
a maximum dilution factor of 200
was obtained. An automatic rinse
using deionized water was
performed after each set of three
replicates in order to eliminate
carryover between samples.
RESULTS AND DISCUSSION
CONCLUSION
The results of the analyses are
shown in Table IV. There generally
is good agreement between the
measured and the expected values.
Although the results for the calcium
determination were acceptable, it
would be preferable to increase
the lanthanum solution to a level
of 1.5–2.0% to allow for the tolerance of higher levels of matrix
interferences.
The digestion protocol followed
was satisfactory for all feed types.
Digestion was complete in 35 minutes. The atomic absorption analysis was simplified by the use of the
AutoPrep 50 diluter. The standards
were automatically prepared from
a stock standard solution and overrange samples were diluted to bring
the results within the working
range. Duplicate samples provided
good precision and the results for
check samples were acceptable.
TABLE IV
Results of the Analysis of the Samples
Calcium
Sample
AAFCO 9630 –
Broiler starter
AAFCO 9631 –
Turkey Grower
AAFCO 9632 –
Pig Starter,
Medicated
AAFCO 9722 –
Swine Grower,
Medicated
AAFCO 9723 –
Beef Concentrate,
Medicated
AAFCO 9725 –
Lamb Ration,
Medicated
Commercial Dry
Cat Food – FSD
Commercial Dry
Cat Food – NLC
Commercial Wet
Cat Food – FSDW
Commercial Dry
Dog Food – CSD
Commercial Wet
Cat Food – MDW
Rabbit Food
Measured
Value (%)
Copper
Iron
Measured
Value (%)
Zinc
Expected
Value (%)
Measured
Value (%)
Expected
Value (%)
Expected
Value (%)
Measured
Value (%)
Expected
Value (%)
1.09
1.10
0.015
0.015
0.035
0.036
0.014
0.014
0.95
0.97
0.013
0.014
0.018
0.019
0.016
0.017
7.04
7.04
0.24
0.23
0.35
0.37
0.15
0.16
0.87
0.70
0.0028
0.0028
0.023
0.025
0.0110
0.011
3.51
3.35
0.030
0.026
0.094
—
0.083
0.090
1.17
1.14
0.0023
0.0020
0.025
0.027
0.014
0.013
0.81
0.5 – 0.86
0.0018
—
0.027
—
0.022
—
1.2
>1.0
0.0026
—
0.029
—
0.017
—
0.3
—
0.0005
—
0.004
—
0.002
—
0.72
>0.6
0.0014
—
0.016
—
0.012
—
0.77
—
0.0006
—
0.030
—
0.006
—
1.0
—
0.0076
—
0.020
—
0.010
—
42
The Determination of Minerals in Multivitamin Samples
Using Microwave Digestion and ICP-OES Analysis
Paul D. Krampitz and Karen W. Barnes
The Perkin-Elmer Corporation, 761 Main Avenue, Norwalk, CT 06859 USA
INTRODUCTION
In the past, the U.S. Food & Drug
Administration (FDA) considered
dietary supplements to be foods
and therefore subject to the Nutrition Labeling Education Act. The
rigor of this ruling was relaxed with
the implementation of the Dietary
Supplement Health and Education
Act of 1994. In this scheme, dietary
supplements including vitamins,
mineral, herbal or botanical, and
amino acids are not subject to regulations as a food additive or drug.
However, if the substance is
intended to cure, prevent, treat,
or ameliorate a disease, then it
would be subject to drug laws.
Materials are adulterated if they
contain poisonous or harmful substances or have inadequate safety
information. Labels must show serving size and types and amounts of
nutrients per serving.
The determination of metals and
minerals in vitamins is challenging
due to the wide range of concentrations present, which may vary from
ppb (ng/g) to percent levels. Due
to the wide variations of mineral
levels, a multielement technique
must be able to simultaneously
monitor trace levels of elements
in the presence of macro levels
of other elements. Inductively
coupled plasma optical emission
spectrometry (ICP-OES) features
multielement capability, wide linear
dynamic range, high analytical sensitivity, and high sample throughput. All these are attributes that
will prove invaluable to analysts
striving to meet the challenges
posed by determinations in nutritional supplements.
Extensive sample preparation of
foods before elemental analysis is
common and, particularly when
ICP-OES is employed, often proves
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
ABSTRACT
Samples of multivitamin
tablets were digested using conventional open-vessel digestion.
The elements Ca, Mg, Zn, Mn,
and Cr were determined using
a sequential ICP-OES. Recoveries
were low compared to the label
claim, so the work was repeated
using a microwave digestion system. The results were comparable to label claims and spike
recoveries were acceptable for
the samples.
to be more time-consuming than
the actual analysis. Methods involving hot plate digestion in open
vessels or dry ashing followed by
acid dissolution are commonly
employed, but are time-consuming
and prone to contamination and
evaporative losses. Microwave
digestion has been shown to be an
acceptable alternative and has been
successfully applied to food analysis
(1). Advantages to microwave digestion result from the nature of the
process, i.e., because the system
is sealed, and contamination,
evaporative losses, and chemical
consumption are reduced. In general, microwave procedures are fast
due to the increased heating that
occurs from the microwave interaction with the reagents and samples.
This paper presents a microwave
digestion and ICP-OES analytical
protocol to determine metals and
minerals in nutraceuticals
EXPERIMENTAL
Procedure
Samples of vitamin tablets were
obtained for analysis. The tablets
were crushed and then ground into
a fine powder with a mortar and
pestle. Approximately 5 g of the
43
sample was accurately weighed and
transferred to 500-mL beakers and
mixed with 50 mL of deionized
water (DI). Then 25 mL each of
HNO3 and HCl was added to the
beaker. The samples were warmed
on a hot plate for 30 minutes and
then filtered into 500-mL volumetric flasks and diluted to volume
with DI. Complete digestion was
not attained and therefore the analytical results were not compatible
with label claims, but were about
40% low.
Instrumentation
The decomposition was performed again using microwave
digestion. The Perkin-Elmer Multiwave microwave digestion system
(Perkin-Elmer, Norwalk, CT USA)
was used in its standard configuration with quartz vessels. The operating conditions for the Multiwave
system were as follows:
Step 1:
100 W for 10 minutes
Step 2:
300 W for 10 minutes
Step 3:
1000 W for 15 minutes
Step 4:
0 W for 15 minutes
The pressure was monitored for
all vessels and the power modified
to maintain the maximum operating
conditions according to reactivity
of the sample. This feature ensures
complete and safe digestion for
nearly any sample type.
ICP-OES determinations were
performed on the Plasma 400
instrument (Perkin Elmer, Norwalk,
CT USA). The Plasma 400 is a
sequential ICP-OES that is operated
in the radial configuration. Calibration was performed using multielement aqueous standards
(Perkin-Elmer, Norwalk, CT USA).
For the determination of Ca and Mg,
the samples were diluted 10-fold.
RESULTS
All results were corrected for
dilution and are reported in Table I.
The results have also been converted to amount per serving, so
that the levels could be referenced
to the label supplied. The equation
for conversion was as follows:
A = B x C/D x E x F x G
Where:
TABLE I
ICP-OES Results for Samples Digested in the Multiwave System
Analyte
Sample 1
Sample 2
Label Claim
Recovery
Zn 206
Mn 257
Cr 267
Mg 279
Ca 315
(ppm)
(ppm)
(ppm)
(%)
47
20
190
489
542
48.5
19
183
470
545
25
20
200
500
500
188
100
95
98
108
A = Amount of analyte per serving
B = Concentration of analyte
in ppm (µg/mL)
C = Dilution volume of
solution (mL)
D = Sample weight (g)
E = Weight of sample (g)/1 tablet
F = Number of tablets/serving
G = 1 mg/1000 µg
CONCLUSION
REFERENCES
The results were in very good
agreement with label values. Spike
recoveries ranged from 95 to 114%.
Because of the problems with Zn,
the sample was reanalyzed using
multiple wavelengths and was run
against four different standards, all
with comparable results. This suggests that there was an error in the
formulation. The Multiwave system
did an excellent job of consistently
putting the sample into solution.
The system used one program for
the entire digestion and was very
simple to use.
1. S.K. Chang, P. Rayas-Duarte, E.A.
Holm, and C. McDonald, Anal.
Chem. 65, 12, 334R (1993).
44
Digestion and Preparation of Organic and Biological
Microsamples for Ultratrace Elemental Analysis
Ingeborg Müller
Schering AG, Allgemeine Physicochemie
Berlin, Germany
INTRODUCTION
In the the development of pharmaceutical drugs, smaller and
smaller quantities are available
for analytical and pharmacological
studies. For the analytical investigation, this also means smaller quantities are available for chemical
analysis. In the case of the biochemical
production of proteins and
peptides, the typical amount of sample is normally in the range of nmol
and pmol. The analytical chemist
must therefore develop methods
of analysis to meet this challenge.
To achieve the ultimate detection
limits, the dilution factor in
the sample preparation step has
to be low. For this study, a VG
PlasmaQuad ICP-MS (Thermo Jarrell
Ash, Franklin, MA USA) was used
for the determination of trace
elements. The detection limits of
the system ranged from 0.1–10 ppb
in the measured solution. With a
dilution factor of 1000 after digestion, this represents a limit of
determination of 0.1–10 ppm
in the drug substance.
INSTRUMENTATION
Digestion
All digestions were performed
using the Perkin-Elmer Multiwave
microwave digestion system
(Perkin-Elmer Norwalk, CT USA).
A 20-mL quartz vessel with a modified Ti-pressure cap allows for
the decompositions used with
this system.
The advantage of the modified
vessel is that less acid is needed
for digestion of the sample. With
700 µL of nitric acid, 50–100 mg
of organic or biological material can
ABSTRACT
In pharmaceutical research,
smaller and smaller quantities of
samples are available. Analytical
methods must therefore be
adapted to micro- and nanotechnology. This paper describes the
sample digestion and preparation
steps for organic and biological
microsamples.
The digestion was performed
with nitric acid in a microwave
system using 20–mL vessels. The
acid was evaporated in a nitrogen
stream and the residue dissolved
in diluted nitric acid. The
elements were determined by
ICP-MS. The method was verified
by the recovery rates of multielement standards and certified
reference materials. It was found
that the loss of some elements is
a limiting factor of the evaporation step.
be digested. Clear, colorless solutions without residues were
obtained. Table I shows the Multiwave microwave program which
can be used to decompose nearly
all organic and biological materials
analyzed in the laboratory.
After digestion, the residue
was diluted with ultrapure water to
50–100 g, resulting in an acid concentration of 0.1–0.2 mol/L nitric
AS
While generally at least 100-mg
amounts of a substance are required
for digestion of a sample in triplicate, the actual total amount available is no more than 10–15 mg.
This means that only 5 mg is available for a single digestion which
then is diluted to 5 g (to give a dilution factor of 1000). In order to
achieve the necessary sensitivity for
subsequent determination, the acid
is evaporated after digestion and
the remainder dissolved in 0.1
mol/L HNO3.
Evaporation
Figure 1 shows a microwaveheated evaporation device which
eliminates possible sources of contamination and requires less time for
the evaporation process, contrary
to a conventionally heated device.
A turntable from a CEM MDS™
2000 microwave digestion system
(CEM Corporation, Matthews, NC
USA) was adapted for our purposes
in-house. The openings for the
external gas tubes were modified
for use in the evaporation step with
nitrogen. The six vessels were positioned on the turntable. Every vessel was sealed with a cap with
double bore-holes. One bore-hole
TABLE I
Digestion Conditions for Organic and Biological Materials
Materials
Liver
Acids
700 µL HNO3
700 µL H2O2
700 µL HNO3
700 µL H2O2
Steroids
Proteins
Atomic Spectroscopy
Vol. 19(2), March/April 1998
acid. This is the acid concentration
routinely used in the our
laboratory.
700 µL HNO3
45
Multiwave microwave program
100 W
800 W
0W
5 min
15 min
15 min
500 W
800 W
0W
Contamination in the final solutions, most probably due to dust in
the laboratory, was detected for Cu
(0.1–0.4 ppb) and Zn (0.5–1 ppb).
All other elements were determined
by ICP-MS below their detection
limits.
Next, the recovery rate of our
multielement standard was tested.
Figure 2 shows the recovery rates
of 37 elements in the mass range
7 to 238 amu.
Fig. 1 . Microwave-heated evaporation device.
was used for the nitrogen stream.
Through the other bore-hole, the
acid condensed in the big vessel
in the middle of the turntable
which was filled with approximately 100 mL of water. All parts
of the evaporation device were
made from PTFE. All solutions
were evaporated to dryness within
30 minutes. Table II lists the energy
program used for the evaporation
process.
When the residues are evaporated
to dryness, the microwaves can still
couple to the water in the vessel in
the middle of the turntable.
Materials
Liver
Steroids
Procedure
First, the analytical blanks of
the evaporation device were tested.
Only suprapure acid cleaned by
subboiling distillation or ultrapure
nitric acid (Merck) was used. After
the digestion step, the PTFE lip
seals of the pressure cap were
rinsed with 500 µL of water and
added to the digestion acid. The
total solution was evaporated to
dryness and then dissolved in 5 mL
0.1 mol/L nitric acid.
TABLE II
Evaporation Conditions for the Acid Solutions
Reagents
CEM MDS 2000 program
700 µL HNO3
700 µL H2O2
500µL H2O
700 µL HNO3
700 µL H2O2
500 µL H2O
Proteins
EXPERIMENTAL
700 µL HNO3
500 µL H2O
2 min
10 min
1 min
5 min
10 min
46
70 %
25 %
90 %
35 %
20 %
The poor recovery for Cu and Zn
was caused by the contamination
from dust in the laboratory as mentioned above. The elements Ti, Nb,
Sn, and Sb showed losses which
were also seen with a matrix in the
digestion solution. After evaporation of the NIST 1643d certified
water sample, the Sb was lost completely. These findings were also
confirmed when the organic or biological materials were spiked before
digestion. It was assumed that these
elements were volatilized as fluorides. Fluorine could be present
only in ng-quantities in the materials analyzed. For the formation of
a volatile form of those elements it
might have been sufficient. According to the Duquesne University
SamplePrep Webpage (1), Sb and
Sn are insoluble in HNO3, and Nb
and Ti both form insoluble oxide
coatings. Although this would not
explain poor recoveries for Nb and
Ti, obviously mismatched acid
chemistry was prevailing. The
water in the big absorption vessel
in the middle of the turntable,
where the condensed acid is collected, was analyzed. The whole
spectrum of our multielement standard was found there. This shows
that evaporation is not conducive
to good spike recovery.
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
Fig. 2. Percentage recovery (n=3) of 37 elements in the mass range 7 to 238 amu.
RESULTS AND DISCUSSION
Based on the results of this
study, the evaporation of the
digestion residues may create more
problems than advantages for multielement determination. It is important to individually check every
element to be determined for eventual evaporation losses. A single
internal standard such as indium,
often advised as a reference, cannot
be assumed to be representative of
the behavior of all other elements.
For the analysis of small quantities of material there may be a solution to the problem. By means of
the MCN 6000 microconcentric
nebulizer (CETAC Technologies,
Omaha, NB USA) it is possible to
introduce concentrated acids with
an uptake rate of only 50 µL/min.
This permits the digestion solution,
diluted to 5 mL, to be measured
directly without preliminary evaporation. With a resultant acid concentration of 2 mol/L HNO3, the
required dilution factor of 1000 can
be achieved.
small-volume ultrapure quartz vessels, the advantages of microwave
decomposition can be realized
even for minute amounts of samples. Advantages, of course, include
the increased sample throughput
as well as the prevention of blank
contamination, volatilization, and
evaporative losses, and the elimination of cross-contamination.
REFERENCES
CONCLUSION
1.
The Multiwave microwave digestion system is a useful instrument
for the preparation of microsamples. Due to the availability of
47
http://nexus.chemistry.duq.edu/
samplepre/index.html
A Simple Closed-Vessel Nitric Acid Digestion
Method for Cosmetic Samples
Kerry D. Besecker, Charles B. Rhoades, Jr., and Bradley T. Jones
Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109 USA
and
Karen W. Barnes
The Perkin-Elmer Corporation, 761 Main Avenue, Norwalk, CT 06859 USA
INTRODUCTION
The major constituents of cosmetic samples typically include talc,
zinc oxide, magnesium carbonate,
titanium dioxide, and various pigments. Some of these components
are nearly insoluble, even in relatively harsh acid mixtures. Trace
metal determinations performed
on such samples therefore must
include a rigorous sample preparation step. In most cases this
includes a microwave digestion
procedure (1). For example,
a 0.5-g sample of face powder is
transferred to a closed-vessel acid
digestion bomb. Five mL of concentrated nitric acid is added to the
vessel, followed by 5 mL of concentrated sulfuric acid. Microwave
power is applied for a length of
time (15–30 min). The sample is
allowed to cool, and then 7.0 mL
of hydrofluoric acid is added.
A second heating stage is applied,
the sample is allowed to cool,
diluted with water, and analyzed
by inductively coupled plasma
(ICP) or atomic absorpiton spectroscopy (AAS) techniques.
A similar method for the determination of Zn in dyes and cosmetics has also been reported (2).
Samples were digested using an
HNO3–H2SO4–HClO4 (70:7:23)
mixture. This use of this mixture
can be dangerous. The same
authors determined Cu, Pb, Zn, and
Cd in cosmetics (3). The same acid
mixture was used for the digestion.
The digestate was then evaporated
to dryness, and ashed at 550°C for
five hours. The ash was dissolved in
an HCl-HNO3 mixture, and photooxidized prior to analysis by AAS.
ABSTRACT
A simple, closed-vessel
microwave digestion method
has been developed for cosmetic
samples. A 0.15-g sample of lipstick, face powder, or foundation
was digested in 3 mL of nitric
acid in a high pressure (75 bar)
closed-vessel digestion bomb.
The digestate was filtered to
remove any silicate residue, and
analyzed by inductively coupled
plasma optical emission spectrometry (ICP-OES). Twentyseven elements were determined
in the three sample types. The
detection limits were in the low
parts-per-billion range, and precision was better than 5% relative
standard deviation for most metals. The accuracy of the method
was determined by performing
spike recoveries for seven test
elements. Recoveries were in the
95–105% range for all elements.
A standard reference material
(estuarine sediment), having
a digestion residue similar to
that observed for the cosmetics,
gave percent recoveries in the
95–110% range. The digestion
residues were analyzed by wavelength dispersive x-ray fluorescence spectrometry. The
digestion technique eliminated
the need for the harsh acid mixtures (including H2SO4 and HF)
that are routinely used for cosmetic samples.
Equally complicated acid mixtures
have been employed in the sample
preparation procedures reported
for lipstick (4), eyeshadow (5),
sunscreens (6), and other cosmetics
(7).
Inductively coupled plasma optical emission spectrometry (ICP-
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
48
OES) methods for trace metal determination are advantageous
due to their low detection limits
and fast analysis times (8–11). The
effectiveness of ICP-OES methods
is further increased when analyzing
samples with minimal dissolved
solids and residual carbon. Closedvessel microwave digestion technology maximizes sample decomposition through rapid heating at
elevated pressures (12–16). The
decreased time for sample decomposition coupled with the ability to
control reaction parameters makes
microwave digestion an excellent
means of sample preparation for
ICP-OES determination.
Digestion systems capable of
operating at elevated pressures
enable the decomposition of samples without time-consuming predigestion steps. The process of
developing microwave digestion
procedures is streamlined through
real-time feedback of reaction
parameters such as temperature,
pressure, and power. Customized
programs can be developed for
specific sample matrices utilizing
multi-stage programs and power
ramping capabilities. Additionally,
closed-vessel digestion systems
that utilize Teflon® and/or quartz
vessels reduce the risk of sample
contamination (17).
A further advantage of highpressure closed-vessel systems is
the ability to decompose the sample
matrix with a minimal amount of
acid. Due to the high pressures
which increase the boiling points,
the digestion may also be
accomplished with a single acid
instead of acid mixtures. Limiting
the volume and types of acids used
in sample preparation reduces the
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
dilution of the analytes in the final
solution and reduces the risk of
contamination or matrix interferences for axial ICP-OES.
Obviously, the goal of previous
efforts has been the development
of a method for the complete dissolution of the cosmetic sample. Such
a technique would ensure that trace
amounts of metals were not
released from any inorganic matrix
and therefore not detectable during
the analysis of the analytical solution. The aim of the present work
was to develop a simple sample
preparation procedure for the
analysis of cosmetic samples by
ICP-OES. The procedure requires
only nitric acid and a high-pressure
closed-vessel microwave digestion
system. The technique is therefore
easier than those previously
reported, much safer since harsh
acid mixtures are avoided, and less
prone to sample contamination.
In cases where the sample was not
completely dissolved, the digestate
was filtered, and the residue analyzed by wavelength dispersive
x-ray fluorescence spectrometry
to determine the composition of
the residue. The accuracy of the
technique is demonstrated by the
analysis of a standard reference
materials, and by recovery data
observed for spiked real samples.
EXPERIMENTAL
Instrumentation
Perkin-Elmer Optima 3000™ DV
(dual view) ICP-OES (18) (PerkinElmer, Norwalk, CT USA). Table I
lists the operating parameters. The
samples were atomized with a
GemTip™ cross-flow nebulizer
assembly. The axial view was used.
A Rainin Dynamax® peristaltic
pump, Model RP-1 (Rainin Instrumental Co., Inc.,Woburn, MA USA),
was used with a pump speed of
31.19 RPM together with a PerkinElmer AS-90 autosampler. Table II
shows the elements, wavelengths,
background correction points,
points per peak, and the processing
TABLE I
ICP-OES Operating Parameters
Parameter
Setting
RF power
1360 W
Auxiliary Ar gas flow
0.5 L/min
Nebulizer flow
0.70 L/min
Plasma flow
15 L/min
Sample flow rate
1.60 ml/min
Wash time
30 sec
Sample read delay time
50 sec
Processing mode
Area
Background
Manual
selection of
points
Replicate measurements
3
modes for this analysis.
A Perkin-Elmer Multiwave
microwave digestion system (19)
(Perkin-Elmer, Norwalk, CT USA)
was used. The system contains a
six-position rotor that monitors
pressure and temperature in all
six vessels and monitors pressure
by way of hydraulic arms sitting
atop each vessel. These arms are
centrally connected and the vessel
with the highest pressure controls
the system. The pressure is
recorded by an optical transmission.
The microwave power output
ranges from 0–1000 W at 2450
MHz, and the cooling fan operates
at 0–100 m3/hr. The cooling fan
protects the oven’s interior from
corrosive fumes and minimizes the
cool-down period after the completion of a digestion program. Quartz
vessels (50 mL) were used for the
digestions. As the carousel rotates
during the sample digestion, an
infrared (IR) temperature detector
measures each sample’s temperature sequentially every two
seconds. The temperature is
detected through bottom of the
IR transparent quartz vessel.
A Model PW-1404 Wavelength
Dispersive X-Ray Fluorescence
Spectrometer (WXRF) (Phillips
Electronic Instruments) (20)
49
equipped with a scandium x-ray
tube was used. The instrument has
3 kW of power and five diffraction
crystals together with a flowthrough and a scintillation detector.
Reagents and Standard Solutions
Nitric Acid
Fisher Scientific (Pittsburgh, PA
USA) Optima Purity.
Calibration Standards
Prepared from various elemental
concentrations of mixed SPEX Certiprep (Metuchen, NJ USA), Custom
Multi Element ICP-grade Standards,
by dilution to give a final concentration of 12% HNO3 solution.
SRM 1646 Estuarine Sediment
Standard reference material from
National Institute of Standards and
Technology (Gaithersburg, MD
USA)
Preparation of Sample
Approximately 0.15 g of the various cosmetic samples were accurately weighed into clean, dry
microwave quartz vessels. Then,
3.0 mL HNO3 was added the 0.15-g
samples using an Optifix® Basic
Dispenser (EM Science, Gibbstown,
NJ USA). A seal-forming tool was
used to expand the lip-seals on the
lids that were placed in each vessel.
The vessels were placed in a bomb
jacket with caps screwed on handtight. The vessels were placed in
the carousel and the protective
shield was placed around the
carousel and tightened. The carousel
was placed in the Multiwave microwave system and the optimized
digestion program was employed
for the sample type being analyzed.
The final programs for the
different sample types are shown
in Table III. Upon completion of
the microwave step, the carousel
was removed from the microwave
system and then the vessels were
removed. Under a fume hood, the
screw caps of each bomb jacket
were slowly unscrewed, allowing
the nitrogen oxides to escape
slowly. The lids and the quartz
TABLE II
Emission Wavelengths, Background Correction Wavelengths Relative to Emission Wavelengths,
Points per Peak, and Processing Mode for Each Element Determined
Element
Emission
Wavelength
(nm)
Background
Correction
Relative to
Process
Mode
Points
per
Peak
Element
Emission
Wavelength
(nm)
Background
Correction
Relative to
Process
Mode
Points
per
Peak
Ag
328.068
-0.036
Area
1
Mo
202.030
-0.022
Area
2
Al
396.152
+0.050
Area
2
Na
589.592
+0.062
-0.060
Area
Area
2
As
188.979
+0.018
-0.012
Area
2
Ni
231.604
-0.025
Area
1
Au
242.795
-0.020
Area
1
P
177.428
+0.020
-0.020
Area
1
B
249.773
-0.030
Area
2
Pb
220.353
-0.025
Area
2
Ba
233.527
-0.030
+0.030
Area
1
Pd
340.458
-0.040
Area
1
Be
313.042
-0.050
Area
1
Pt
265.945
-0.025
Area
1
Bi
223.061
-0.020
Area
1
Rb
780.040
+0.119
-0.079
Area
3
Ca
317.933
-0.030
+0.030
Area
1
S
180.669
+0.015
-0.020
Area
2
Ca
396.847
-0.043
Area
2
Sb
217.581
+0.025
-0.018
Area
2
Cd
226.502
+0.030
Area
2
Sc
361.384
+0.035
-0.035
Area
1
Co
228.616
+0.025
Area
2
Se
196.026
+0.023
-0.015
Area
1
Cr
205.552
-0.023
Area
2
Si
288.158
+0.028
-0.027
Area
1
Cu
324.754
+0.033
Area
2
Sn
189.933
+0.020
Area
1
Eu
381.967
-0.047
Area
1
Sr
407.771
-0.038
Area
1
Fe
259.940
-0.035
Area
1
Te
214.281
+0.060
Area
1
K
766.491
-0.140
+0.129
Area
2
Ti
334.941
+0.040
Area
1
La
379.478
-0.041
+0.047
Area
1
Tl
276.787
-0.017
Area
Li
670.781
-0.110
+0.102
Area
2
V
292.402
-0.030
Area
1
Mg
279.079
-0.033
+0.030
Area
1
Yb
369.419
-0.040
Area
1
Mg
279.553
-0.040
Area
1
Zn
213.856
-0.021
Area
2
Mn
257.610
+0.026
Area
2
Zr
343.823
-0.030
Area
1
50
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
TABLE III
Multiwave Programs for Each of the Cosmetic Sample Types
Step 1
Step 2
Step 3
Step 4
Power
W
200
450
550
0
Lipstick
Time
Power
(mm:ss) (W)
15:00
400
20:00
450
15:00
550
15:00
0
vessels were removed from the
bomb jackets. A small amount of
18MΩ deionized, distilled water
(dd H2O) was added to each vessel
to facilitate the removal of any dissolved gases. The samples were
quantitatively transferred into 25-ml
volumetric flasks and diluted with
dd H2O. The solutions were filtered
with NALGENE® 0.45 µm, 115-mL
disposable filters (Nalge Nunc International, Rochester, NY USA) to
remove any particles that could disrupt the nebulizer flow. The solutions were then transferred to
autosampler vials for analysis.
The residue that remained on
filters was analyzed using the WXRF
spectrometer. Filters were cut with
ceramic scissors (Jensen Equipment
Company, Inc., Pewaukee, WI USA)
to 30 mm diameter to match the
size of the x-ray sample cups. Filters
were placed between a thin polypropylene film and a piece of
lucite. The x-ray sample cup used
was a 30-mm disposable x-ray
spectroscopic sample cup made
by Chemplex Industries. A qualitative scan was performed on
each sample which required
approximately 20 min.
Method Development
The Multiwave program used for
digestion of the individual samples
involved optimization of several
parameters: time, sample size, and
volume of acid. These factors contributed to the amount of pressure
inside of the vessel. The system
controls the pressure at 74 bar,
Fan
speed
1
1
1
3
Power
(W)
200
400
550
0
Powder
Time
Power
(mm:ss)
(W)
30:00
400
10:00
400
30:00
550
15:00
0
Fan
speed
1
1
1
3
but the vessels hold up to 110 bar
before the seals are broken.
In developing the programs for
the digestion of cosmetics, a conservative approach was used to
prevent any unwanted pressure
increase. The first precaution
observed was the amount of sample
used in the digestion. First attempts
used 0.05–0.10 g of sample. For the
final program and analysis, 0.15 g
of sample was used. The volume of
acid used in the digestion was an
important factor to consider for
the dilution step after digestion.
The ICP-OES instrument analyzes
the samples optimally when the
acid content for ICP-OES determinations is approximately 10–20%.
Most important is matrix matching
of samples, blanks, and analytical
standards. Minimal dilutions were
required to measure elements that
were present at very low levels.
With these factors in mind, the optimum amount of acid was the smallest amount that resulted in
complete digestion. The final program for the cosmetics used 3 mL
of nitric acid.
One of the factors affecting trace
metal determinations is contamination. Tremendous care was taken
to ensure the cleanliness of the
vessels, volumetric flasks, filters,
and sample vials. Each of the quartz
vessels was cleaned with HF (5%
by volume) followed by rinsing
with dd H2O. The flasks and vials
were acid-washed (10% HNO3)
and rinsed with dd H2O. The same
cleaning procedure was followed
51
Power
W
200
450
550
0
Foundation
Time
Power
mm:ss
W
5:00
350
10:00
450
15:00
550
15:00
0
Fan
speed
1
1
1
3
between each sample run. All samples and materials were handled
with powder-free acid-resistant
gloves.
A residue remained in the vessel
after digestion in the microwave.
This residue was believed to be
mostly silicates and titanium dioxide, which require HF to be
digested. Due to the use of quartz
vessels, HF was not used in the
digestion procedure. The residue
was examined using the WXRF
spectrometer (21) to confirm
composition. The samples were
scanned at all wavelengths and
studied qualitatively. Table IV
shows the results of this x-ray
study for each type of cosmetic.
In all three samples, the residue
contained silicon, titanium, and
iron. The lipstick contained potassium, the powder contained magnesium and zinc, and the foundation
contained magnesium. Polymeric
vessels which will permit the use
of HF digestions are available from
The Perkin-Elmer Corporation but
were not used for this study.
TABLE IV
Elements Present in
Filtered Residue of Digested
Cosmetic Samples Using
WXRF Spectroscopy
Cosmetic
Elements
Lipstick
Powder
Foundation
Silicon, Titanium, Iron,
Potassium
Silicon, Titanium, Iron,
Magnesium, Zinc
Silicon, Titanium, Iron,
Magnesium
TABLE V
Detection Limits, Concentration Levels in ppm, and Relative Standard Deviations
for the Cosmetic Samples
Powder
Element
Det. limit
(µg/g)
Ag
Al
As
Au
B
Ba
Be
Bi
Ca
Cd
Co
Cr
Cu
Eu
Fe
K
La
Li
Mg
Mn
Mo
Na
Ni
P
Pb
Pd
Pt
S
Sb
Sc
Se
Si
Sn
Sr
Te
Ti
Tl
V
Zn
Zr
0.004
0.025
0.054
0.004
0.010
0.001
0.0005
0.016
0.005
0.004
0.006
0.002
0.002
0.001
0.001
0.037
0.004
0.001
0.0006
0.0003
0.011
0.006
0.005
0.076
0.022
0.008
0.016
0.106
0.032
0.0002
0.051
0.019
0.007
0.0002
0.048
0.001
0.063
0.002
0.001
0.002
Concn.
(µg/g)
Foundation
RSD
Concn.
(µg/g)
435
Lipstick
RSD
15800
9.7
2.0%
30%
4.4
106
0.7
15%
1.2%
2.2%
5.8
0.7
2170
0.7
1.9%
2.5%
178
1.2
1.9
1.7%
1.9%
1.5%
2.0
5.0
1.0%
2.2%
3650
786
2.1
10.7
11500
21.5
0.9%
4.6%
1.1%
4.0%
1.2%
1.6%
9880
53.8
1.0%
5.5%
34.1
1200
14.6
1.4%
0.8%
0.6%
22.4
313
25.4
334
1.7
353
5.8
1.5%
3.9%
3.4%
8.1%
860
1.1
60.9
1.5%
3.0%
7.4%
460
1.5
44.9
4.2
216
2.3%
79.5
14.6%
0.7
0.8%
15.3
1.0%
20.1
35.7
11.9%
3.3
11100
6.9
3.2%
1.0%
5.7%
52
1.1%
Concn.
(µg/g)
12%
34%
14600
RSD
2.5%
11.3
4810
0.7
6.9%
0.3%
11%
162
1.5
2.0
2.1
0.6
0.4%
2.1%
1.5%
0.9%
4.2%
9960
6670
1240
2.5%
1.7%
0.9%
0.3%
0.04%
1.1%
11%
0.9%
6.5%
2.9%
2.1
1.1%
1.4%
73.6
0.5%
39.3
5.2%
48.8
1.8%
0.5
10.4
2.2
4.0%
1.4%
3.9%
2.6
12.0
0.4
1.8%
0.0%
2.7%
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
RESULTS
The detection limits (µg/g),
concentration levels (µg/g), and
relative standard deviations (RSD)
for the cosmetic samples are
reported in Table V. Concentration
levels were below the detection
limit for that specific element
where no value is reported. Due to
incomplete digestion of refractory
elements, the actual concentration
values of the elements determined
by WXRF analysis are higher than
what is reported. The data in Table
V are the means of four samples.
where each sample solution was
analyzed three times by ICP-OES.
Blank subtraction was performed
automatically by the computer software. Rubidium (30 µg/g) and ytterbium (5 µg/g) were used as internal
standards. The samples were analyzed with and without dilution to
determine the maximum number
of elements. The elements at higher
concentrations were determined
after dilution by a factor of 20 with
12% (v/v) Optima grade nitric acid.
This enabled the concentration levels to be in the appropriate linear
range of each element’s calibration
curve.
Table VI shows spike recovery
values for each of the samples and
Estuarine Sediment (SRM 1646).
This SRM was chosen because,
upon digestion with HNO3, it also
contained a residue similar to the
cosmetics. The SRM values are
reported in Table VII. The values
for elements present at high levels
are not shown because these are
the same elements present in the
residue.
CONCLUSION
Sample preparation can be very
time-consuming in elemental determinations. The lenght of time taken
to perform an analysis has been
shortened with the advancements
of ICP-OES. With a reduction in
sample preparation time, sample
TABLE VI
Spike Recovery of Cosmetic Samples and SRM 1646
Ba
Ca
K
Li
Mg
Na
Sr
Element
Powder
Foundation
Lipstick
SRM
99.6%
94.0%
99.2%
96.1%
89.0%
97.9%
96.8%
95.9%
96.7%
99.9%
101.8%
97.7%
95.3%
95.6%
96.0%
101.5%
101.7%
99.7%
97.2%
97.4%
100.8%
102.3%
97.6%
92.4%
101.2%
101.9%
106.0%
101.5%
TABLE VII
Results of Analysis of SRM 1646
Experimental (µg/g)
NIST value (µg/g)
As
Be
Co
Cu
Li
Ni
P
Pb
Zn
12.3 ± 3.9
1.36± 0.05
11.5 ±0.3
18.0±0.3
44.3 ±1.0
27.3±0.3
533±15.3
28.5±1.1
132±0.5
11.6 ± 1.3
(1.5)
10.5±1.3
18± 3
(49)
32 ±3
540±50
28.2±1.8
138±6
% Recovery
106%
90.0%
110%
100%
90.4%
85.3%
98.7%
101%
95.4%
( ) = non-certified value.
throughput increases. Using these
methods for specific cosmetics, the
complete sample preparation
process ranged from 1.25–2.25 hr.
The second advantage of these
methods was that the harsh acid
mixtures were eliminated and only
nitric acid was required. It also
eliminated the time normally
required for preparing the acid
mixtures, reduced possible sources
of contamination, and avoided
matrix-induced interferences
possible with axial-view ICP-OES.
Additionally, HNO3 is the preferred
acid in ICP-OES analysis.
Using the Multiwave system to
digest samples for elemental analysis has definite advantages when
compared to traditional sample
preparation procedures. The use
53
of closed quartz vessels provides
for an extremely clean environment
and reduces sources of contamination. In addition, the quartz vessels
enable rapid heating at elevated
pressures which eliminates predigestion procedures. The Multiwave system also monitors the
pressure and temperature of the
vessels, and if any of the vessels
reach the pressure cut-off point,
the microwave energy radiated is
reduced. These features together
with the continuous temperature
measurements aided in the development of this method.
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
ACKNOWLEDGMENT
REFERENCES
Special thanks are expressed
to Jannell Rowe of R.J. Reynolds
Tobacco Company. The authors
also gratefully acknowledge the
financial support from The PerkinElmer Corporation and a grant
from the NSF-GOALI program
(CHE-9710218).
1.
Application Note MB-5,
Revision 9-88, CEM Corporation,
Matthews, NC.
2.
J. Maslowska, and E. Legedz, Rocz.
Panstw. Zakl. Hig. 33, 149
(1982).
3.
J. Maslowska, and E. Legedz, Rocz.
Panstw. Zakl. Hig. 35, 431
(1984).
4.
M. Okamata, M. Konda, I.
Matsumoto, and Y. Miya, J. Soc.
Cosmet. Chem. 22, 589 (1971).
5.
E.S. Gladney, At. Absorpt. Newsl.
16, 144 (1977).
6.
J.T. Mason, J. Pharm. Sci. 69,
101 (1980).
7.
I.V. Kubrakova, T.F. Kudinova,
E.B. Stavnivenko, and N.M.
Kuzmin, J. of Anal. Chem. 52,
522 (Jun. 1997).
8.
9.
Modern Methods for Trace
Element Determination, C. Vandecasteele and C.B. Block (ed.),
John Wiley and Sons (1993).
John W. Milburn, At. Spectrosc.
17(1), 9 (1996).
10. Jo Rita Jordan, Referee 7
(June 1995).
11. Juan C. Ivaldi and Julian F. Tyson,
Spectrochim Acta Part B 50,
1207 (1995).
54
12. Introduction to Microwave Sample
Preparation: Theory and Practice,
H.M. Kingston and Lois B. Jassie
(ed.), American Chemical Society,
1988.
13. Methods of Decomposition in
Inorganic Analysis, Z. Sulcek
and P. Povondra, CRC Press, Inc.
(1989).
14. R.T. White, Jr. and G.E. Douthit,
J. Assoc. Off. Anal. Chem. 68,
766 (1985).
15. H.M. Kingston and L.B. Jassie,
Anal. Chem. 58, 2534 (1986).
16. D. Chakraborti, M. Burguera, and
J.L. Burguera, Fresenius’ J. Anal
Chem. 347, 233 (1993).
17. Charles B. Rhoades, J. Anal. Atom.
Spec. 11, 751 (1996).
18. Perkin Elmer ICP-Emission Spectrometry Optima 3000 Hardware
Guide, 1993.
19. Perkin Elmer Multiwave Preliminary User Manual, 1996.
20. Phillips PW1404 Automatic
Sequential Spectrometer Service
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21. Principles and Practice of X-Ray
Spectrometric Analysis, 2nd Ed., E.
P. Bertin, Plenum Press (1975)
Closed-Vessel Nitric Acid Microwave Digestion
of Polymers
Kerry D. Besecker, Charles B. Rhoades, Jr., and Bradley T. Jones
Department of Chemistry, Wake Forest University, Winston-Salem, NC 27109 USA
and
Karen W. Barnes
The Perkin-Elmer Corporation, 761 Main Avenue, Norwalk, CT 06859 USA
INTRODUCTION
Polymers that have high strength
are typically resistant to elevated
temperatures. Wet digestion methods for trace metal determinations
performed on such samples therefore must include a rigorous sample
preparation step. In most cases, this
includes a microwave digestion procedure (1–3). For example, one
procedure specifies to transfer
a 0.5-g sample of polyethylene to
a closed-vessel acid digestion bomb.
Three mL of concentrated nitric
acid is added to the vessel, followed
by an additional 8 mL concentrated
sulfuric acid. Microwave power is
applied for a 15-min period. The
sample is allowed to cool, and then
an additional 5 mL of nitric acid is
added. A second heating stage is
applied, the sample allowed to
cool, diluted with water, and analyzed by inductively coupled
plasma (ICP) or atomic absorption
spectrometry (AAS) techniques.
A similar method for the digestion of poly(vinyl chloride) has also
been reported (3,4). The samples
were digested using H2O2 which
can be a dangerous procedure.
Polymeric printed circuit boards
have been analyzed for trace metals
on the surface by immersing the
board in a mixture of hot HNO3 and
HCl, followed by analysis by plasma
optical emission spectrometry
(OES) (3).
ICP-OES methods for trace
metal determination are advantageous due to their ability to achieve
low detection limits and fast analysis times (3,5–7). The efficacy of
ICP-OES methods is further
increased when analyzing samples
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
ABSTRACT
A simple, closed-vessel
microwave digestion method has
been developed for several polymer samples. A 0.15-g sample of
high density polyethylene, polystyrene, polyethylene/polypropylene blend, and polyethylene
precursor was digested in 4 mL
nitric acid in a high-pressure
(75 bar) closed-vessel digestion
bomb. The digestate was filtered
and analyzed by inductively coupled plasma optical emission
spectrometry (ICP-OES). Depending on the polymer, 1–18
elements were determined in the
three sample types. Detection
limits were in the low parts-perbillion range, and precision was
better than 5% relative standard
deviation for most metals. The
accuracy of the method was
determined by performing
spike recoveries for 1–15 test
elements. Recoveries were in the
90–100% range for all elements.
The digestion technique eliminated the need for harsh acid
mixtures (including H2SO4 and
HF) that are routinely used for
polymer samples.
with minimal dissolved solids and
residual carbon. Closed-vessel
microwave technology maximizes
sample decomposition through
rapid heating at elevated pressures
(8–12). The decreased time for sample digestion coupled with the ability to control reaction parameters
makes microwave digestion an
excellent means of sample preparation for ICP-OES determinations.
Digestion systems capable of
operating at elevated pressures
enable the decomposition of samples without time-consuming pre-
55
digestion steps. Closed-vessel systems, utilizing Teflon® and/or
quartz vessels, reduce the risk of
sample contamination (13).
A further advantage of highpressure closed-vessel systems is
the ability to decompose the sample matrix with a minimal amount
of acid. Assuming that high enough
pressures are available, the digestion may also be accomplished
with a single acid instead of a mixture of acids. Limiting the amount
and types of acids used in the sample preparation reduces the dilution of the analytes in the final
solution (to keep final acid concentrations reasonable), reduces the
risk of contamination, and may
reduce the possibility of matrix
interferences in axial ICP-OES
measurements.
Obviously, the goal of previous
efforts has been to develop a
method for the complete dissolution of the polymer sample of interest. Such a technique would ensure
that trace amounts of metals were
released from the matrix and thus
would be detectable during the
analysis of the analytical solution.
The aim of the present work was
to develop a simple sample preparation procedure for the analysis
of several polymer samples by ICPOES. The procedure requires only
nitric acid and a closed-vessel
microwave digestion system. The
technique is therefore simpler
than those previously reported;
safer, since harsh acid mixtures
are avoided; and less prone to sample contamination. The accuracy
of the technique is demonstrated
by recovery data obtained for
spiked real samples.
EXPERIMENTAL
Instrumentation
A Perkin-Elmer Optima 3000™
DV (dual-view) inductively coupled
plasma optical emission spectrometer (Perkin-Elmer, Norwalk, CT
USA) was used in the axial mode
(14). Table I lists the operating
parameters. The samples were
atomized with a GemTip™ crossflow nebulizer assembly. A Rainin
Dynamax® peristaltic pump, Model
RP-1 (Rainin Instrumental Co., Inc.,
Woburn, MA USA), was used with
a pump speed of 31.19 RPM. A
Perkin-Elmer AS-90 autosampler
(Perkin-Elmer, Norwalk, CT USA)
was also used. Table II shows the
elements, wavelengths, background
correction points, points per peak,
and the processing modes for this
analysis.
A Perkin-Elmer Multiwave
microwave digestion system (15)
(Perkin-Elmer, Norwalk, CT USA)
was used for sample digestion,
which contains a 6-way rotor that
monitors pressure and temperature
in all six vessels. The pressure is
measured simultaneously in all
vessels by means of a hydaulic system whereby the vessel with the
highest pressure controls the system. The pressure is recorded by an
TABLE I
ICP-OES Operating Parameters
Parameter
Setting
RF Power
1360 W
Auxiliary Ar gas flow
0.5 L/min
Nebulizer flow
0.70 L/min
Plasma flow
15 L/min
Sample flow rate
1.60 mL/min
Wash time
30 sec
Sample read
50 sec
delay time
Processing mode
Area
Background
Manual
selection of
points
Replicate measurements
3
optical transmission. The IR temperature detector measures each
sample’s temperature sequentially
every two seconds as the carousel
rotates during the sample digestion.
The microwave unpulsed power
output ranges from 0–1000 W at
2450 MHz, and the cooling fan
operates at 0–100 m3/hr. The
cooling fan protects the oven’s
interior from corrosive fumes and
minimizes the cool-down period
after the completion of a digestion
program. Quartz vessels with
a 50-mL volume were used for
the digestions.
Reagents and Standards
Nitric acid was Optima grade
from Fisher Scientific (Pittsburgh,
PA USA).
Calibration standards were
prepared from various elemental
concentrations of mixed SPEX Certiprep (Metuchen, NJ USA) Custom
Multi Element ICP-grade standards
by dilution to give a final concentration of 16% HNO3 solution.
Preparation of Samples
The various polymer samples
were accurately weighed (0.15 g)
into clean, dry quartz microwave
digestion vessels. To the 0.15-g
samples, 4.0 mL HNO3 was added
using an Optifix® Basic (EM
Science, Gibbstown, NJ USA) dispenser. A seal-forming tool was
used to expand the seals for the
vessels. The vessels were then
placed in a bomb jacket with caps
screwed on hand-tight. The vessels
were placed in the rotor and the
protective shield placed around the
rotor and tightened. The rotor was
then placed in the Multiwave system and the optimized digestion
program was employed for the sample to be analyzed.
The final programs for the different sample types are shown in Figure 1. Upon completion of the
microwave digestion, the rotor was
removed from the microwave oven.
56
Under a fume hood, the screw caps
of each bomb jacket were slowly
unscrewed, allowing the nitrogen
oxides to escape slowly. A small
amount of 18 Mohm, deionized distilled water (dd H2O) was added to
each vessel to facilitate the removal
of any dissolved gases. The samples
were quantitatively transferred into
25-mL volumetric flasks and diluted
to volume with dd H2O. The solutions were filtered with Nalgene®
(Nalge Nunc International,
Rochester, NY USA) 0.45-µm, 115mL disposable filters to remove any
particles that could disrupt the nebulizer flow. The solutions were then
transferred to ICP-OES sample vials
for analysis.
Method Development
The Multiwave programs used
for digestion of the individual polymer samples involved optimization
of several parameters: time, power,
sample size, and acid volume. All
of these factors contributed to the
amount of pressure inside the vessels. The Multiwave system controls
the pressure at 74 bar.
In developing the programs
for the digestion of the polymers,
careful attention was given to the
prevention of unwanted pressure
increases. The first precaution
observed was the amount of the
polymer used in the digestion.
First attempts used 0.05–0.10 g
of polymer. For the final program
and analysis, 0.15–0.16 g of polymer
was used. To measure the elements
present at very low levels, minimal
dilutions were required. With this in
mind, the smallest volume of nitric
acid needed for complete digestion
of the polymer samples was 4.0 mL.
Contamination is a major problem
in trace metal determinations.
Tremendous care was taken to
ensure the cleanliness of the vessels,
volumetric flasks, filters, and sample
vials. Each of the vessels was
cleaned by performing a blank
HNO3 digestion in the Multiwave
system. Each vessel was filled with
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
TABLE II
Emission Wavelengths, Background Correction Wavelengths Relative to
Emission Wavelengths, Points per Peak, and Processing Mode for Each Element Determined
Element
Emission
wavelength
(nm)
Background
correction
Process
Mode
Points
per
peak
Element
Emission
wavelength
(nm)
Background
correction
Process
Mode
Points
per
peak
Ag
328.068
–0.036
Area
1
Mo
202.030
–0.022
Area
2
Al
396.152
+0.050
Area
2
Na
589.592
+0.062
–0.060
Area
2
As
188.979
+0.018
–0.012
Area
2
Ni
231.604
–0.025
Area
1
Au
242.795
–0.020
Area
1
P
177.428
+0.020
–0.020
Area
1
B
249.773
–0.030
Area
2
Pb
220.353
–0.025
Area
2
Ba
233.527
–0.030
+0.030
Area
1
Pd
340.458
–0.040
Area
1
Be
313.042
–0.050
Area
1
Pt
265.945
–0.025
Area
1
Bi
223.061
–0.020
Area
1
Rb
780.040
+0.119
–0.079
Area
3
Ca
317.933
–0.030
+0.030
Area
1
S
180.669
0.020
+0.015
Area
2
Ca
396.847
–0.043
Area
2
Sb
217.581
–0.018
+0.025
Area
2
Cd
226.502
+0.030
Area
2
Sc
361.384
–0.035
+0.035
Area
1
Co
228.616
+0.025
Area
2
Se
196.026
–0.015
+0.023
Area
1
Cr
205.552
–0.023
Area
2
Si
288.158
–0.027
+0.028
Area
1
Cu
324.754
+0.033
Area
2
Sn
189.933
+0.020
Area
1
Eu
381.967
–0.047
Area
1
Sr
407.771
–0.038
Area
1
Fe
259.940
–0.035
Area
1
Te
214.281
+0.060
Area
1
K
766.491
–0.140
+0.129
Area
2
Ti
334.941
+0.040
Area
1
La
379.478
–0.041
+0.047
Area
1
Tl
276.787
–0.017
Area
1
Li
670.781
–0.110
+0.102
Area
2
V
292.402
–0.030
Area
1
Mg
279.079
–0.033
+0.030
Area
1
Yb
369.419
–0.040
Area
1
Mg
279.553
–0.040
Area
1
Zn
213.856
–0.021
Area
2
Mn
257.610
+0.026
Area
2
Zr
343.823
–0.030
Area
1
57
Power (W)
Ag
Al
Ba
Cd
Ca
Co
Cr
Cu
Fe
Mg
Ni
P
Pb
S
Zn
Power (W)
Power (W)
Power (W)
TABLE III
Spike Recoveries of
Polymer Sample (polyethylene/
polypropylene blend)
Batch A
Batch B
Element % Recovered % Recovered
Figure 1. Microwave Digestion Programs for each Polymer.
5 mL HNO3 and placed in the Multiwave system for five minutes at
1000 W. The cleaning of the vessels
was completed by rinsing with dd
H2O. The flasks and vials were acidwashed (10% HNO3) and rinsed
with dd H2O. The same cleaning
procedure was followed between
each sample run. All samples and
materials were handled with powder-free acid-resistant gloves.
RESULTS AND DISCUSSION
The spike recoveries for the samples are listed in Tables III–V using
several different SPEX Custom
Multi-Element ICP-grade Standards.
Sample preparation can be very
time-consuming in the elemental
analysis process. However, the time
required to perform an analysis has
been shortened with the advancements of ICP-OES and with a reduction in sample preparation time,
thus increasing sample throughput.
Using the method described, the
complete sample preparation
process for the polymer sample,
polyethylene/polypropylene blend,
ranged from 1.0 to 1.5 hr, while
previous digestion methods
93.8
91.2
96.2
93.5
94.6
93.2
89.8
95.4
90.3
97.9
91.6
95.0
87.0
102.2
92.3
94.8
94.3
93.6
94.6
94.5
93.8
89.3
95.9
90.0
95.5
91.5
94.6
84.8
100.9
93.2
TABLE IV
Spike Recoveries of Polymer
Sample (polyethylene)
Batch A
Batch B
Element %Recovered %Recovered
Ca
Mg
P
Ti
95.2
98.0
98.8
92.0
92.6
94.5
90.0
88.1
TABLE V
Spike Recoveries of Polymer Sample (polystyrene)
Batch A
Batch B
Batch C
Element
% Recovered
% Recovered
% Recovered
Zn
93.9
58
93.8
93.5
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
required approximately 3.0 hr (16).
This shows a tremendous reduction
in sample preparation time with the
Multiwave system. Another advantage of the method described is the
ability to use nitric acid instead of
the harsh acid mixtures. These
acids require time for preparation
and are sources of contamination.
In addition, matrix-induced interferences possible with axial-view ICPOES are also avoided.
CONCLUSION
The Multiwave system used for
the digestion of samples for elemental analysis has definite advantages
over traditional sample preparation
procedures. The quartz closedvessels provide an extremely clean
environment, reduce sources of
contamination, and enable rapid
heating at elevated pressures,
thus eliminating pre-digestion
procedures. The Multiwave system
also monitors the pressure and the
temperature of the vessels. If any
of the vessels reach the pressure
cut-off point, the microwave
energy is reduced, thus the decompositions are achieved with maximum safety for the user.
ACKNOWLEDGMENT
The authors gratefully acknowledge financial support from The
Perkin-Elmer Corporation and a
grant from the NSF-GOALI program
(CHE-9710218).
REFERENCES
8.
Introduction to Microwave Sample Preparation: Theory and Practice, H.M. Kingston and Lois B.
Jassie (ed.), American Chemical
Society (1988).
9.
Z. Sulcek and P. Povondra, Methods of Decomposition in
Inorganic Analysis, CRC Press,
Inc. (1989).
1.
Application Note OP-6, Revision
10-88, CEM Corporation,
Matthews, NC USA.
10. R.T. White, Jr. and G.E. Douthit,
J. Assoc. Off. Anal. Chem. 68, 766
(1985).
2.
Peter J. Fordham, John W.
Gramshaw, Laurence Castle, Helen
M. Crews, Diana Thompson, Susan
J. Parry, and Ed McCurdy, J. Anal.
At. Spectrom. 10, 303 (1995).
11. H.M. Kingston and L.B. Jassie,
Anal. Chem. 58, 2534 (1986).
C. Vandecasteele and C.B. Bloc,
Modern Methods for Trace Element Determination, John Wiley
and Sons (1993).
13. Charles B. Rhoades, J. Anal. At.
Spectrom. 11, 751 (1996).
3.
4.
V.C. Rao Peddy and J. Valsamma
Koshy, Analyst 117, 27 (1992).
5. John W. Milburn, At. Spectrosc.
17(1), 9 (1996).
6.
Jo Rita Jordan, Referee, 7, (June
1995).
7.
Juan C. Ivaldi and Julian F. Tyson,
Spectrochim. Acta Part B, 50, 1207
(1995).
59
12. D. Chakraborti, M. Burguera, and
J.L. Burguera, Fresenius’ J. Anal
Chem. 347, 233 (1993).
14. Perkin-Elmer ICP-Emission Spectrometry Optima 3000 Hardware
Guide (1993).
15. Perkin-Elmer Multiwave Preliminary User Manual (1996).
16. Kerry D. Besecker, Charles B.
Rhoades, Jr., Karen W. Barnes,
and Bradley T. Jones, unpublished
results.
The Analysis of Coal Tar Pitch
by ICP Optical Emission Spectrometry
After Digestion in a Microwave Oven System
Maryanne Thomsen
Perkin Elmer European ICP Support Group, Hansa Allee 195, D-40549 Duesseldorf, Germany
and
Peter Kainrath
Bodenseewerk Perkin Elmer GmbH, P.O. Box 10 17 61, D-88647 Überlingen, Germany
INTRODUCTION
Tar and pitch are materials
used in modern organic chemistry
as base products for a number of
heterocyclic compounds in addition to many other industrial uses.
Pitch can be used for electrodes in
the production of aluminum, for
example. For such applications, the
product must be specified and the
specification adhered to at all times.
The samples can be viscous liquids
or black solids. They are difficult
samples to analyze for their inorganic components because of their
complex chemical structure. Usually the analysis of the major
elements is sufficient; however,
when the material is for industrial
use, a full and accurate analysis is
often required.
coupled plasma optical emission
spectrometry (ICP-OES). The
method of destruction and of measurement is described in full and
some results for a reference coal tar
pitch sample are presented.
SAMPLE PREPARATION
Microwave-assisted digestion
procedures are becoming more
popular as they considerably
reduce the amount of time spent
preparing the sample for the spectrometric analysis. Due to the
nature of the heating process in
microwave-assisted digestion, several precautions have to be considered before digesting a sample of
high reactivity like tar and pitch.
The following considerations have
to be made:
Theoretically, the samples could
be dissolved in an organic solvent
and aspirated into the atomic spectrometer. In practice, the high viscosity and complex nature of the
samples means that they are not
very soluble and the results with
this method are not of the required
standard. The samples must therefore be digested in acid prior to
analysis. This step takes considerably longer than the ICP analysis
and can become the bottleneck in
the laboratory. It is also very laborintensive unless a technique like
microwave digestion is employed
to automate the procedure. The
samples also contain a very great
deal of carbon, making their complete digestion more difficult.
• What happens to the sample
if it is exposed to microwave
radiation?
A method has been developed
where samples of tar and pitch can
be digested easily and analyzed in
little over an hour by inductively
• Maximum temperature and pressure tolerance of the equipment.
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
• What is the maximum sample
weight the digestion system
can handle?
• Are any spontaneous reactions
expected?
In addition to the reaction conditions, the instrumental parameters
have to be checked as well, such as:
• Ramping up the microwave
power to ensure controlled
chemical reactions.
• Safety measures in case of spontaneous or vigorous chemical
reactions.
• Available vessel materials of the
required degree of cleanliness.
60
In the work presented here,
approximately 200 mg of the sample was accurately weighed into the
quartz vessels of the Perkin-Elmer
Multiwave microwave digestion
system (Perkin-Elmer, Norwalk, CT
USA). To each sample, 5 mL of
concentrated (65%) HNO3 (Merck®
Suprapur) was added. For each run
of the programs, at least one of the
vessels contained acid only for a
sample blank. The samples were
digested using the program listed
in Table I.
Phase 1 ramps the unpulsed
power from 100 W to 600 W over
a period of 30 minutes. Phase 2
maintains this power level for an
additional 10 minutes. Phase 3 is
a cooling phase where the microwave power is turned off and the
fan speed increased to effect rapid
cooling of the sample. Once the
system has reached the maximum
operating pressure of 75 bar (1125
psi), the microwave power is
reduced to the level necessary to
maintain a continuous operating
pressure of 72 bar (1080 psi),
a level at which the maximum
digestion temperature is reached
(see Figure 1).The whole program
takes 55 minutes, after which the
vessels can be opened, and the
clear, colorless digest can be
washed into a 25-mL flask and
diluted to the mark with water.
TABLE I
Microwave Digestion Program
Phase Power Time Power Fan
(min) (W) (W) speed
1
2
3
100
600
0
30
10
15
600
600
0
1
1
3
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
The final acid concentration in
the prepared samples is then
20% HNO3.
MEASUREMENT PARAMETERS
The samples were analyzed using
a Perkin-Elmer Optima 3000™
radial inductively coupled plasma
optical emission spectrometer
(ICP-OES). The ICP system was
calibrated with the Merck IV multielement standard at 10 mg/L and
some standards were prepared
from single-element solutions. The
calibration was checked with the
PE Pure Atomic Spectroscopy Calibration Verification standard. All
standards were prepared in a
matrix of 20% HNO3.
The compromise plasma conditions for the Optima 3000 ICP-OES,
equipped with a GemTip™ crossflow nebulizer and Ryton™ spray
chamber, were used without any
attempt to optimize the system.
The instrumental parameters are
listed in Table II.
RESULTS
The results were measured in
mg/L in solution and converted to
mg/Kg in the solid sample by entering the sample weight taken and
final volume into the Optima software. The results obtained for a
Domtar certified reference material
are listed in Table III.
The values obtained agree well
and are within two standard deviations of the certified values in all
cases. No systematic bias is
observed indicating that there are
no significant matrix effects.
For each analyte element, a sensitive emission line free from spectral interferences was available.
Therefore, no overlap corrections
were necessary. Simple one- or twopoint background correction was
sufficient in every case.
The very wide working range is
a feature of ICP analysis and trace
Fig. 1. Multiwave Digestion Procedure.
element (mg/Kg) and major
element (g/Kg) concentrations can
be determined on the same sample
solution without the need for dilutions.
CONCLUSION
The solutions are clear after the
digestion procedure and are simply
made up to the final volume with
deionized water and analyzed on
the ICP.
The high pressure of the
microwave oven and gentle ramping of the power help to digest the
sample without any losses and to
remove most of the carbon in the
sample. The residual carbon levels
are minimal as can be seen from the
clearness of the solutions. Partially
digested solutions tend to give a
yellow colored solution. Also, a
high carbon content can lead to
unwanted background peaks in the
ICP and to density and surface tension effects. This causes the solution to aspirate at a different rate to
the standards and thus introduces
error into the analysis.
The process of sample decomposition followed by ICP analysis takes
little over an hour and both the
microwave system and the ICP-OES
can be left to work unattended.
61
TABLE II
Instrumental Parameters
Plasma gas
Auxiliary gas
Nebulizer gas
Power
Viewing height
Sample uptake
15 L/min
0.5 L/min
0.9 L/min
1200 W
10 mm above
the load coil
1 mL/min
Table III
Results for Domtar Certified
Reference Coal Tar Pitch
Sample, Measured Values
Compared with
Certified Values in mg/Kg
Certified
Found
Element (mg/Kg)
(mg/Kg)
Al
Ca
Cr
Fe
Na
Ni
P
Pb
S
V
Zn
245(7)
95(18)
0.94(0.07)
208(13)
286(24)
2.6(0.3)
10(2)
91(6)
4900(300)
1.33(0.07)
91(12)
243.8
93.2
0.979
213
318
2.75
10.5
108
5187
1.5
91.1
( )=The values in parentheses are the
uncertainties.
Digestion and Characterization of
Ceramic Materials and Noble Metals
S. Mann*, D. Geilenberg*, J.A.C. Broekaert*, P. Kainrath**, and D. Weber*
*FB Chemie, Analytische Chemie, University of Dortmund, Otto-Hahn-Strasse 6,
D-44227 Dortmund, Germany
**Bodenseewerk Perkin Elmer GmbH, PO Box 10 17 61, D-88647 Überlingen, Germany
INTRODUCTION
In the synthesis and industrial
production of ceramics, oxide
materials, and noble metal alloys,
the analytical characterization of
main, minor, and trace elements is
very important before their final
utilization as high-purity materials
or semiconductors (1). The characterization is necessary for either
the determination of the exact
stoichiometry or of impurities
that influence the properties of the
materials. The characterization of
ceramics, oxide materials, and
noble metal alloys was found to be
difficult because of the high resistance of these materials to thermal
and chemical attack, even to the
attack of concentrated acids.
ABSTRACT
This paper discusses the
digestion and characterization
of ceramic materials and noble
metal alloys, and the advantages
and limitations of several digestion methods. For the ceramic
materials, three commercial pressure digestion systems were used
with conventional, microwaveassisted heating, and decomposition via alkali fusion. Several
noble metals and their alloys
were digested in a microwave
system and alternatively in a highpressure, high-temperature asher.
The sample weight and especially
the gas phase decomposition
were optimized. The different
digestion systems and some analytical results are presented.
Solid state analyses or nondestructive methods, such as X-ray
fluorescence, glow discharge optical emission spectrometry, glow
discharge mass spectrometry, etc.,
are either not sensitive enough or
cannot be used because there are
no reference materials available.
For this reason, wet chemical digestion methods are predominantly
used for routine analysis. In the
case of fusion or digestion with
acids in open systems, the possibility of contamination or loss of
highly volatile elements, depending
on the samples or the elements of
interest, must be considered.
closed vessels is the best method
for the complete mineralization
of inorganic as well as organic
compounds. Temperatures of
200–320oC, which are necessary
for the complete mineralization,
can only be reached at pressures
of 30–120 bar in closed vessels.
Microwave-assisted pressure
digestion is an efficient alternative
to the conventional, heated decompositions.
INSTRUMENTATION
Microwave Digestion
The concept of the Perkin-Elmer
Multiwave microwave digestion system (Perkin-Elmer, Norwalk, CT
USA) is described in detail by Kainrath et al. (8). Figure 1 (left section)
shows the power temperature
curve obtained from the decomposition of the ceramic samples.
Acid digestion in an open system
as well as the digestion in a closed
system (pressure digestion with
conventional heating) is in most
cases very time-consuming. In several publications (2–7) it has been
shown that digestion at high temperature and high pressure in
Fig. 1. Microwave power and temperature for a digestion procedure of a ceramic
material.
AS
Atomic Spectroscopy
Vol. 19(2), March/April 1998
62
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
It can also be seen at which time
the pressure limit of 35 bar or the
maximum temperature (200°C as
in this case) was reached in one
of the vessels. The temperature is
measured during the digestion procedure in all vessels and is shown
in the graph of Figure 1 (right section). For this example, only three
vessels were used. The vessels in
positions 1 and 3 contained the
sample and the vessel in position 5
contained a blank solution. In the
two vessels containing the sample,
nearly the same temperature and
therefore the same pressure was
reached. The advantage of this form
of presentation and control is that
differences in sample constitutions,
sample weights, and the composition of the acid mixtures as well as
leakage of a vessel can be observed.
High Pressure Asher Digestion
For some very specific applications, e.g., the platinum group
elements and their alloys, the temperatures and digestion times
reached by microwave-assisted
pressure digestion are not sufficient.
In these cases, the conventional
heated HPA-S High Pressure
Asher™ (Perkin-Elmer, Norwalk,
CT USA) can be used. The quartz
vessels of the high pressure asher,
filled with sample and acid, are
closed with a quartz lid, which is
fixed by a quality-controlled PTFE
tape wrapped tightly around the lid
and the top of the vessel. The vessels are placed inside an autoclave.
The autoclave is closed and filled
with nitrogen. The nitrogen pressure counteracts the reaction pressure inside the quartz vessels.
Especially for the platinumgroup elements and their alloys,
the “gas phase decomposition”
was developed. A small vessel
with solid potassium chlorate is
suspended above the sample solution within the digestion vessel
which, as a result of heating, forms
elemental chlorine with chloric
acid. The elemental chlorine reacts
with the sample to be digested.
This method is described in detail
by Knapp et al. (9).
consumption were varied. The
digestion conditions for pressure
digestions with concentrated acids
are shown in Table II.
APPLICATIONS
For salt fusion, 0.5 g of an
equimolar mixture of Na2CO3 and
K2CO3 (both pro Analyse grade,
Merck, Darmstadt, Germany) and
50 mg of the sample were mixed
and heated in a platinum crucible
with a Bunsen burner for 10 minutes. For silicon-containing samples,
the cooled melt was dissolved in
either 5 mL nitric acid or a mixture
of 3 mL nitric acid and 2 mL hydrofluoric acid.
Digestions of several coarsely
powdered ceramic materials were
performed by using the Multiwave
Microwave digestion system, the
conventional heated HPA-S High
Pressure Asher digestion system,
and DAB III, the classical digestion
by salt fusion (see Tables I–IV). For
optimization of the digestion parameters, sample weights, acid mixtures, temperatures, and time
TABLE I
Instrumental Parameters
Power
Number
Material Volume (mL)
(W)
of vessels
of vessels
of vessels
Instrument
Multiwave
Microwave
Digestion System
1000
(unpulsed power
control)
6
TFM /
quartz
100 /
50 (20)
HPA-S
High Pressure
Asher
4 x 400
(conventional
heating)
5–21
quartz /
glassy carbon
15–90 /
20
DAB III
2000
(conventional
heating)
2–4
PTFE
250
Sample
TABLE II
Pressure Digestions with High Concentrated Acids
Digestion Sample
HFa
HClb HNO3c H2SO4d Time
method weight (mg) (mL)
(mL)
(mL)
(mL)
BN
DAB III
Multiwave
50
30–250
2
3
6
–
4
2
–
2
17 h
33 min
Si3N4
DAB III
Multiwave
30
30
2
3
6
–
4
2
–
2
15–20 h
33 min
Si–B–N–C
(10)
DAB III
Multiwave
50
30
2
3
6
4
2
–
2
15 h
33 min
ZrO2
Multiwave
200
–
–
2
2
33 min
TiO2
Multiwave
50
1
–
1
2
33 min
a38%,
b37%,
c
d95–97%,
p.a., J. T. Baker
65%, puriss., Riedel de Haën
63
puriss., Riedel de Haën
puriss., Riedel de Haën
TABLE III
Digestion of Platinum Group Elements with HPA-S System
Weight
Reagents
Temperature program Notes
Sample
(mg)
(°C) (min) (°C)
Pt, Pd, Os
100
12 mL HCl (37%)
4 mL HNO3 (67%)
0.7 g KClO3
250
180
250
70-mL vessel
Pt - Ir 10
100
12 mL HCl (37%)
4 mL HNO3 (67%)
0.7 g KClO3
280
180
280
90-mL vessel
modified
heating block
Rhodium
Ruthenium
100
12 mL HCl (37%)
0.7 g KClO3
280
180
280
70-mL vessel
Iridium
100
15 mL HCl (37%)
0.7 g KClO3
300
180
300
70-mL vessel
TABLE IV
Comparison of Several Digestion Methods for Nitride Ceramics
Sample
Digestion system
Si (%)
B (%)
BNa
Si3N4b
Si-B-N-C
a
b
DAB III (n=11)
Multiwave (n=6)
Salt fusion (n=9)
46 ± 2
46.0 ± 0.4
42.3 ± 0.8
DAB III (n=10)
Multiwave (n=8)
Salt fusion (n=8)
60 ± 1
59 ± 1
60 ± 2
DAB III (n=10)
Multiwave (n=7)
Salt fusion (n=12)
31.3 ± 0.8
33.4 ± 0.6
33 ± 2
13.6 ± 0.2
13.1 ± 0.2
13.4 ± 0.3
Manufacturer’s specification: 42.5% B.
Manufacturer’s specification: 60.06% Si (corresponding to the stoichiometry).
Table III shows the conditions
used for the digestion of the platinum group elements and their
alloys. The advantages of the
conventional heated HPA-S High
Pressure Asher are the higher temperatures that can be reached and
set very exactly in contrast to the
microwave-assisted system, and the
long digestion times that are possible. The modified heating block
used for a Pt–Ir 10 alloy with a
smaller contact surface between
heating block and digestion vessel
inhibits the deposit of sample material at the rim of the fluid surface.
RESULTS
All materials were completely
digested using the methods
described. The solutions were
clear and colorless. Principally, the
biggest advantage of microwaveassisted pressure digestion with the
Multiwave microwave system is the
much shorter digestion time
required in comparison to the conventional heated pressure digestion,
thus resulting in a considerable
time-savings.
The reproducibility of the analytical results is somewhat better for
digestions with the Multiwave
64
microwave system than with the
other methods. While the noble
metals Au, Ag, and Pt can be easily
digested by microwave-assisted
pressure digestion, Ir and its alloys
cannot be dissolved in microwave
systems. The gas phase decomposition in the conventional, heated
high pressure system is a very successful method for the digestion of
these compounds without contamination and losses. The HPA-S high
pressure, high temperature asher
can be successfully used for ceramics and noble metal alloys up to
sample weights of 120 mg because
of the ability to reach high temperatures and long digestion times. For
higher sample weights, even this
method may fail. The gas phase
decomposition with elemental
chlorine as the reacting compound
enables the digestion of highly inert
materials.
CONCLUSION
Sample preparation using the
Multiwave microwave high pressure digestion system provides high
quality digestates for inorganic
materials, because of the high temperatures that can be reached in
this system, corresponding to a
working pressure of 35 bar in TFM
vessels. Another advantage of this
system is that one can judge the
quality of the digestions because
of the ability to measure the temperature in each vessel. The sample
throughput and the low amount
of acid required make this method
very attractive for routine analysis.
High pressure, high temperature
digestion offers an alternative to
effect long digestion times at high
temperatures or very slow heating
procedures for highly reactive samples. With this method, even samples that normally can be dissolved
only with conventional fusion or
with acid mixtures unsuitable for
atomic spectroscopy are accessible.
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
REFERENCES
1. J.A.C. Broekaert, T. Graule, H. Jenett, G. Tölg, and P. Tschöpel, Fresenius’ Z. Anal.
Chem. 332, 825 (1989).
2. G. Knapp, Mikrochim. Acta 2, 445 (1991).
3. E. Sucman, M. Sucmanova, O. Celechovska, and S. Zima, Colloqium
Atomspektrometrische Spurenanalytik 6, 617 (1991).
4. P. Schramel, S. Hasse, and G. Knapp, Fres. J. Anal. Chem. 326, 142 (1987).
5. G. Knapp, B. Maichin, and F. Panholzer, Colloquium Atomspektrometrische
Spurenanalytik 6, 571 (1991).
6. J. Borszeki, P. Halmos, E. Gegus, and P. Karpati, Talanta 41, 1089 (1994).
7. F. Panholzer, G. Knapp, P. Kettisch, and A. Schalk, Colloquium
Atomspektrometrische Spurenanalytik 6, 633 (1991).
8. P. Kainrath, P. Kettisch, A. Schalk, and M. Zischka, LaborPraxis 11, 34 (1995)
9. G. Knapp and P. Kettisch, Colloquium Analytische Atomspektrometrie 1993, 963.
10. H.P. Baldus, O. Wagner, and M. Jansen, Mater. Res Soc. Symp. Proc. 271, 821
(1992)
11. S. Mann, D. Geilenberg, J.A.C. Broekaert ,and M. Jansen, J. Anal. At. Spectrom. 12,
975 (1997)
65
Learn MORE about Modern Sample
Preparation from the Experts in Trace Analysis
Modern Sample
Preparation
Sample Preparation for AAS,
ICP-OES and ICP-MS
Seminar and Technical
Workshop
Monday, June 1 –
Friday, June 5, 1998
University of Massachusetts,
Amherst, MA
This workshop, held in the classrooms and laboratories of the
Chemistry Department of the University of Massachusetts, will
specifically address the interests of chemists and technicians
working in the field of sample preparation and atomic/mass
spectroscopy. The course will include classroom lectures presented by experienced analysts from universities (R. Barnes, S.
Kingston, G. Knapp) and industry (R.T. White). The workshop
will include hands-on laboratory with a variety of digestion and
sample handling systems as well as state-of-the-art analytical
instrumentation (AAS, ICP-OES, and ICP-MS). The goal of the
course is to teach the skills necessary to develop strategies and
methodology for successful handling, preparation and analysis
of a variety of samples. Attendees are encouraged to provide
their own samples and solve existing problems from their field
of work. All sessions are guided by faculty and staff of the University of Massachusetts Chemistry Department.
Course Fee: $1500
Early Registration Discount: $1350 for
registration received by April 30, 1998
Course Fee includes: Course text, lab workbooks,
lab supplies, course souvenir, coffee breaks,
transportation to and from hotel, and graduation dinner.
Lab coats, gloves and safety glasses will be supplied.
Continuing Education Credits are available.
For more information, or to receive the full course agenda
or registration form contact:
A training course organized by the
University of Massachusetts
Division of Continuing Education
and Perkin-Elmer
Dr. Ramon M. Barnes
Department of Chemistry
Lederle Graduate Research Center
University of Massachusetts
Box 34510
Amherst, MA 01003-4510
Tel: 413-545-2294, Fax: 413-545-3757
AStomic
pectroscopy
Vol. 19(2), Mar./Apr. 1998
3RD EUROPEAN FURNACE SYMPOSIUM
June 14–18, 1998
Centre of Post-Graduate and Management Studies of the Charles University,
the Czech Technical University and the Prague School of Economics
Prague, Czech Republic
GOALS OF THE SYMPOSIUM:
The symposium program will comprise four days of presentations, posters, and discussions. It will focus on
recent research related to various aspects of electrothermal atomization in atomic spectroscopy. Once of its main
aims is to stimulate discussion between spectroscopists from the East and West. The main topics are:
• Furnace techniques and materials
• Spatially resolved spectroscopy in graphite atomizers
• Calibration techniques
• Solid sampling techniques
• Mechanism and kinetics of reactions
• Absolute analysis
• Multielement determinations
• Analytical applications
To register, request further information, or to offer a paper/poster, contact:
Dr. Bohumil Docekal
Institute of Analytical Chemistry, Czech Academy of Sciences
Veveri 97, CZ-61142 Brno, Czech Republic
e-mail: Docekal@iach.cz • Tel: +420/5/7268-246 • Fax: +420/5/41212113
THE 6TH INTERNATIONAL CONFERENCE ON PLASMA SOURCE MASS SPECTROMETRY
September 13–18, 1998
University of Durham, England
PROPOSED SESSIONS:
Fundamentals –Plasma dynamics, diagnostics, mass analyser operation, quadrupole, sector, ion trap
Instrumentation – Low resolution, high resolution, alternate plasmas, collision cells, electrospray,
other analyzers and ionizers
Applications – Geological, environmental, plume and effluent tracking, water, soil, radionucleide analyses
Novel and Extended Applications – Developments in hardware and methods that extend the conventional
range of applications
Speciation – Life sciences, environmental biochemical, metalloproteins
The future of plasma source mass spectrometry – Emerging technologies and applications
To register, request further information, or to offer a paper/poster, contact:
Dr. Grenville Holland, Conference Secretary
Department of Geological Sciences, Science Laboratories
South Road, Durham City, DH1 3LE, England
Fax: +(0) 191 374 2510
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