1. No category

AS
tomic
pectroscopy
November/December 1998
Volume 19, No. 6
Special Issue on Sample Decomposition
In This Issue:
High Pressure Ashing
The High Pressure Asher: A High-Performance Sample Decomposition
System as an Alternative to Microwave-Assisted Digestion
R. Thomas White, Jr., Peter Kettisch, and Peter Kainrath............................................187
A Simple Closed-Vessel Nitric Acid Digestion Method
for a Polyethylene/Polypropylene Polymer Blend
Kerry D. Besecker, Charles B. Rhoades, Bradley T. Jones,
and Karen W. Barnes.......................................................................................................193
The Effect of Digestion Temperature on Matrix Decomposition
Using a High Pressure Asher
Meredith M. Daniel, James D. Batchelor,
Charles B. Rhoades, Jr., and Bradley T. Jones...............................................................198
Determination of Arsenic and Selenium in Foodstuffs:
Methods and Errors
P. Fecher and G. Ruhnke ................................................................................................204
Determination of Lead and Cadmium in Food Products by GFAAS
C. Blake and B. Bourqui ..................................................................................................207
Determination of Trace Element Contaminants in Food Matrices
Using a Robust, Routine Analytical Method for ICP-MS
P. Zbinden and D. Andrey ..............................................................................................214
Microwave Digestion
Interferences in ICP-OES by Organic Residue After
Microwave-Assisted Sample Digestion
G. Knapp, B. Maichin, and U. Baumgartner ..................................................................220
Microwave-Assisted Digestion of Plastic Scrap:
Basic Considerations and Chemical Approach
Miachel Zischka, Peter Kettisch, and Peter Kainrath ...................................................223
ASPND7 19(6) 187–228 (1998)
ISSN 0195-5373
AStomic
pectroscopy
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The High Pressure Asher: A High-Performance
Sample Decomposition System as an
Alternative to Microwave-Assisted Digestion
R. Thomas White, Jr., White’s Technical Resources LLC, Pfafftown, NC 27040 USA
Peter Kettisch, Anton Paar GmbH, Graz, Austria
Peter Kainrath, Bodenseewerk Perkin-Elmer GmbH, Postfach 10 17 61, Überlingen, Germany
INTRODUCTION
The two significant sample
preparation innovations that were
considered leading-edge technologies in the 1980’s are microwave
sample preparation systems and
the high pressure asher (HPA) system, designed by Knapp (1,2).
The HPA was a unique sample
preparation system, especially in
the early 80’s, because it offered
rapid and complete decomposition
of organic materials. Decomposition of samples occurred in closed
quartz vessels at desired reaction
temperatures up to 320°C under
a pressure of 130 bar (1920 psi)
(3–5). In comparison to other sample preparation techniques during
that time, the HPA was in a class
of instruments challenged by few.
Since that time, there have been
numerous methods developed and
documented in the literature using
HPA technology which encompasses
both a broad range of sample sizes
and difficult-to-prepare matrices
(1,7,8).
•High temperature for rapid,
more complete reactions. Chemical rates roughly double for
each 10°C temperature increase.
To achieve temperatures above
the normal boiling points of acids,
it is necessary to work under pressure, i.e., in a closed vessel. The
higher the pressure capability of
a digestion vessel, the higher the
possible temperature and the
usable sample amount. Ideal materials for digestion vessels are quartz
glass, some fluor polymers (PTFE,
PFA), and glassy carbon. In gastight, closed vessels, the danger
of contamination or analyte loss is
quite low, and smaller amounts of
reagents are required; briefly: systematic errors are minimized (2).
The HPA Concept
Reaction vessels
In the HPA-S High Pressure
Asher™ system (Perkin-Elmer/Paar)
This paper discusses high pressure digestion and points out the
differences of these procedures in
comparison to microwave-assisted
digestion.
The basic principle for a successful wet chemical digestion
has always depended on the following influences:
•Appropriate consideration of
chemistry using small volumes
of high-purity acids or acid
mixtures.
•Resistant reaction vessels made
of temperature-stable and pressure-resistant pure materials.
Fig. 1. The HPA-S digestion principle.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
187
(Figure 1), various vessels made of
quartz glass or glassy carbon may
need to be used. Both materials are
characterized by high-temperature
resistance and purity. They are gastight, pressure-tight, and easy to
handle without elaborate tools.
The original HPA quartz reaction
vessels were available in 10-, 30-,
and 70-mL sizes. A 70-mL vessel
could accommodate a 1.0-g sample
of ground plant tissue requiring
5 mL concentrated HNO3 and 1 mL
HCl for complete sample decomposition. The vessels designed for
the new HPA-S come in sizes and
materials to accommodate most
any sample as shown in Table I.
For decompositions requiring HF,
glassy carbon vessels (20 mL) are
used instead of quartz for sample
preparation.
The quartz vessels are graduated,
allowing dilution within the digestion vessel without transfer of
Vessel
volume
90 mL
TABLE I
Reaction Vessels for the HPA-S System
Material
No. of Weight of
Typical
vesselsa
sample
applications
maximum
Quartz
5
Investigation of foodstuffs,
petrochemistry, environmental
analysis
50 mL
Quartz
7
0.8 g
Standard vessel for multiple
applications
15 mL
Quartz
14
0.2 g
Medical, pharmaceutical,
or 21
biological, forensic microsamples
20 mL Glassy carbon
6
0.2 g
For reactions with HF: geology,
materials research
a Maximum number of vessels in heating block per digestion process.
liquids or danger of contamination.
This permits work using the “onepot-principle,” whereby all steps
can be performed in one vessel,
fulfilling another requirement for
trace metal determination (6).
Vessel seal
Lids made from the same material as the vessels and a strip of analytically-pure PTFE tape complete
the vessel assembly. A wrapping
aid has been developed for the
HPA-S system that makes vessel
sealing very reproducible. A specially produced low-blank PTFE
tape is used to seal the vessel. The
first step to seal a vessel for the
HPA-S requires that a strip of PTFE
tape be placed over the opening of
the vessel to provide a seal between
the quartz lid and vessel. A 3-mm
hole is punched into the PTFE tape
with a suitable tool. This prevents
the formation of condensation
between the lid and sealing film
and provides, if necessary, a means
for reaction gases to escape without
loss of sample. Reaction gases will
normally vent from the sample vessels during the slow depressurization of the autoclave at the
completion of a digestion program
when internal vessel pressure
exceeds the (decreasing) external
pressure. A quartz lid is put on
top of the vessel and a weighted
plunger on the wrapping aid tool
1.5 g
holds the lid in position as two
or three coils of the PTFE tape are
wound around vessel neck and lid.
This method results in an inexpensive, safe, disposable seal.
The unit is electrically heated to
reach a programmed temperature.
At reaction temperatures of 320ºC
maximum, the digestion reaction
produces pressure within the vessels, equivalent to the sum of the
vapor pressure of the acid(s) and
the partial pressures of the reaction
gases. The nitrogen pressure of the
autoclave keeps the vessels hermet-
Fig. 2. Heating blocks.
188
ically sealed and prevents their rupture for any required time, even at
maximum temperature.
By a simple exchange of the heating block unit and the vessels, the
instrument can be fitted ideally to
different application tasks and to the
required vessel volumes. The sample
size determines the vessel volume
required as shown in Table I.
The analyst using the HPA-S
system should expect:
• Minimum acid requirements
• Minimum organic residue
• Complete sample mineralization
• No loss of elements
• Minimal contamination
• Automated sample preparation
• Maximum operator safety
Heating block inserts
The heating blocks very closely
embrace the bottom part of the vessels for good heat transfer in the
range of the digestion solution.
There is a strong vertical temperature gradient in the vessel, which
favors mixing of the sample and
reagent by convection (Figure 2).
Vol. 19(6), Nov./Dec. 1998
The sealed vessels in the heating
block unit are placed into the autoclave of the HPA-S system. The autoclave is closed with a pressure-tight
cover within a few seconds by
means of a quick-fit bayonet seal. It is
very user-friendly and safe from user
error and spontaneous reactions.
The high safety standard of the
entire instrument is verified by the
“GS” seal of approval (Geprüfte
Sicherheit” = Certified Safety),
awarded by the German TÜV
agency (Technical Surveillance
Agency). The HPA-S fulfills all
requirements of the instrument
safety law and of the German regulatory requirements for autoclaves.
During the digestion process,
the vessel is placed in the heating
block, located within a heatable
pressurized container (autoclave)
filled with nitrogen to 100 bar. As
the autoclave is pressurized, the
pressure on the exterior of the
vessel and lid forms a tight seal to
protect the sample from potential
sources of contamination during
sample decomposition.
Heating
Heating of the autoclave takes
place electrically with an input of
1600 W. This input is sufficient for
heating the unit from ambient temperature to 250°C within 20 minutes. A total of approximately
40 minutes is required to attain
300°C; the maximum temperature
is 320°C (Figure 3). Typically,
microwave digestion systems cannot
attain temperatures high enough to
reduce residual carbon to very low
levels because of pressure
limitations.
Investigations have shown that
the 300–320°C temperature allows
the organic substances to achieve
solutions that are practically free
from residual matrix for voltammetry (7), ICP-MS, cold vapor, and
hydride AAS. Complete sample mineralization is important for interference-free measurements and to
avoid systematic errors (8). Due to
the high temperature, it is possible
to digest organic samples with pure
nitric acid; thus, the use of perchloric acid is not required (Figure 3).
Digestions are uniform due to
even radial temperature distribution
in the heating blocks of the HPA-S,
achieved by the concentric arrangement of the heating source in the
round pressure chamber. Highest
uniformity of the temperature from
one vessel to the other and from
one run to the next is an essential
condition for good analytical reproducibility.
Temperature programs
The built-in program controller
permits storage of several temperature programs of any given length
with up to 16 segments each
(ramps, holding times). In practical
application it has been shown that
three to four simple programs are
usually sufficient for achieving
optimum digestion with any type
of sample. The choice of heating
temperature and maximum temperature depends on the reaction
behavior of the sample, the sample
weight, and the vessel volume.
Fig. 3. Residual carbon content.
189
When using different vessel
volumes, it is merely necessary
to proportionally convert the
sample weights as needed.
Temperature profiles suggested
for organic samples (Figure 4)
almost always begin with a moderate heating ramp in the range of
80 to 140°C. In a second program
step, the sample and intermediate
products are digested at high end
temperatures. Inorganic inert samples on the other hand often may
be digested with full heating power
in a one-step program.
Applications for using the
HPA-S system
Table II lists only the most characteristic of applications for use
with the HPA-S system together
with their respective sample
amounts, vessel size, maximum
temperature, and total digestion
time. Sample amounts are always
given as dry weights.
The complete methods collection for the HPA-S system is available (9). It includes information
on the recommended reagents and
their amounts, exact temperature
programs, expected digestion
results, and tips regarding
appropriate sample preparation
and after-treatment procedures
of a wide variety of other sample
materials from all fields of analytical
chemistry.
The following examples show
the use of the HPA-S system in
selected fields of elemental trace
analysis.
Food SRMs
Samples (Table III):
0.35 g NIST SRM 1586
Rice Flour
Fig. 4. Typical reaction process.
TABLE II. HPA-S Digestion Parameters
Sample weight
1.2 g
1.2 g
1.0 g
0.9 g
0.8 g
0.8 g
0.5 g
0.5 g
0.5 g
0.4 g
0.4 g
0.4 g
0.4 g
0.3 g
0.3 g
0.3 g
0.3 g
0.2 g
0.2 g
0,2 g
0.2 g
0.1 g
0.1 g
0.1 g
0.1 g
0.2 g
0.2 g
0.1 g
0.1 g
0.1 g
GC = Glassy carbon.
Sample
Vessel
maximum
Time
total
Time
Wheat grains
Vitamin pills
Tobacco leaves
Soya lecithin
Filter dusta
Slaga
Lubricating oil
Cr-Ni steel
Wood
Bovine liver
Chocolate
Soila
Sewage sludgea
River sedimenta
Hair
Muscle tissue
PVC
PP
Coal
Paint pigment
Automotive catalyst
Ferrochromium
Rh, Ru, Ir
Whole blood
Pharm. agents
Ni-Nb alloy
River sediment
(full digestion)
TiO2, ZrO2
Various rocks
Bauxite
90 mL
90 mL
90 mL
90 mL
90 mL
90 mL
90 mL
90 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
50 mL
15 mL
15 mL
20 mL GC
260ºC
280ºC
260ºC
260ºC
320ºC
300ºC
300ºC
220ºC
250ºC
260ºC
290ºC
280ºC
240ºC
220ºC
250ºC
250ºC
300ºC
320ºC
280ºC
250ºC
280ºC
280ºC
280ºC
280ºC
320ºC
220ºC
1.5 h
2h
1.5 h
1.5 h
2h
2h
2.5 h
1.5 h
2h
1.5 h
2h
3h
2h
2h
2h
2.5 h
2.5 h
4h
2h
3h
3h
3h
3h
2h
3h
2h
20 mL GC
20 mL GC
20 mL GC
20 mL GC
260ºC
240ºC
220ºC
260ºC
3h
3h
2h
6h
a
= Leaching by aqua regia.
190
0.35 g NIST SRM 1577
Bovine Liver
0.35 g BCR SRM 151
Skim Milk Powder
Vessels: 50-mL quartz
Reagents: 2 mL HNO3
Temperature: 270°C for
2 hours
These materials were analyzed
for Pb and Cd, which are the two
elements most frequently
determined in food analysis. The
standard reference materials (SRMs)
were selected, so that the Pb and
Cd contents were determined over
a range of two to three orders of
magnitude from very low element
concentrations (10).
HNO3 / HCl - Extract of Sewage
Sludge
Sample (Table IV):
0.3 g sewage sludge, dry
(SRM BCR 144 and 146)
Vessels: 50-mL quartz
Reagents: 4 mL HNO3 +
1 mL HCl
Temperature: 220°C for
2 hours
Even though it is not really a typical HPA-S application, this example
Vol. 19(6), Nov./Dec. 1998
TABLE III
Pb and Cd Determination in Food Samples
from the field of environmental
analysis shows the good element
recovery rates.
(values given are 3 sigma)
Reference material
Rice Flour
NBS-SRM 1586
Bovine Liver
NBS-SRM 1577
Skim Milk Powder
BCR-RM No. 151
Pb (ng/g)
Measured
Certified
54 ± 10
45 ± 10
Cd (ng/g)
Measured
Certified
30 ± 5
29 ± 4
380 ± 40
340 ± 80
300 ± 40
270 ± 40
2060 ± 100
2002 ± 26
110 ± 10
101 ± 8
Vessels: 20-mL glassy carbon
TABLE IV
Analysis of BCR 144 and BCR 146 Sewage Sludge
Element
Co
Cr
Cu
Hg
Pb
Zn
Sewage sludge
Measured
(µg/g)
9.2
482
716
1.62
487
3228
BCR 144
Certified
(µg/g)
9.06 ± 0.6
494 ± 61
713 ± 26
1.49 ± 0.22
495 ± 19
3143 ± 103
Sewage sludge
Measured
(µg/g)
11
790
914
10.4
1250
4056
BCR 146
Certified
(µg/g)
11.8 ± 0.4
769 ± 79
934 ± 24
9.5 ± 0.76
1270 ± 28
4059 ± 90
TABLE V
Analysis of RM Soil 1
Element
Cu
Hg
Ni
Zn
Full Digestion of Soil Sample
Sample (Table V):
0.1 g soil, RM Soil 1
Measured (µg/g)
30
0.124
40
226
Certified (µg/g)
30 ± 5
(0.13)
45 ± 8
223 ± 10
191
Reagents: 2 mL HNO3 +
1 mL HF
Temperature: 280°C for
2 hours
This is an example of the use
of glassy carbon vessels which are
suitable for all applications using
hydrofluoric acid.
For complete recovery of the
elements after HF digestion, it is
often necessary to perform additional treatment of the samples
after digestion. For complexing
the hydrofluoric acid, about 6 mL
cold-saturated boric acid per mL
HF is added to the digestion solution which is then briefly heated
again. This will dissolve sparingly
soluble fluorides and the elements
are made accessible for subsequent
determination. Evaporation of the
hydrofluoric acid, with its inherent
risk of errors, is unnecessary.
Digestion of Platinum Metals
Sample: 0.1 g Rh, Ru, Ir
(powder or structured
material)
Vessels: 50-mL quartz
Reagents: 12 mL HCl +
0.7 g KClO3
REFERENCES
Temperature: 280°C for
3 hours
This is an application which,
in the field of noble metalS analysis,
can replace the powerful but inconvenient Carius tube digestions.
Using a gas phase reaction, the
potassium chlorate located in an
insert vessel will release chloric
acid, which converts the noble
metals Rh, Ru, and Ir into soluble
chlorides (Figure 5). This reaction
requires very precise temperature
control and distribution within
the vessels. In the HPA-S, this is
performed in the open heating
blocks. In the same way, ferrochrome and other refractive
metals or alloys, which will decompose very slowly or not at all in
aqua regia, can be dissolved with
this method.
CONCLUSION
This HPA-S system provides
robust, reproducible decompositions for a wide variety of sample
types. It is safe and easy to use.
These features have made it the
preferred decomposition tool in
many laboratories.
Fig. 5. Gas phase digestion.
192
1.
G. Knapp and A. Grillo, Am. Lab.
4, 76 (1986).
2.
G. Knapp, Microchim. Acta 2, 445
(1991).
3.
T. White, J. Assoc. Off. Anal.
Chem., 72 (1989).
4.
G. Knapp, Trends in Anal. Chem.
3, 182 (1984).
5.
G. Knapp, Intern. J. Environ.
Anal. Chem. 22, 71 (1985).
6.
P. Tschöpel, Labor 4, 9 (1988).
7.
P. Schramel, S. Hasse,
and G. Knapp, Fresenius’ Z. Anal.
Chem. 326, 459 (1987).
8.
M. Würfels, E. Jackwerth,
and M. Stoeppler, Fresenius’
Z. Anal. Chem. 329, 459 (1987).
9.
HPA-S List of Applications
(B25ia01b), Anton Paar GmbH,
Graz, Austria,
Tel: +44 316 257 360
Fax: +43 316 257 257
e-mail: kettisch@anton-paar.com
http://www.anton-paar.com/ap
10. I. Ciurea, Y. Lipka, and
B. Humbert, Mitt. Geb. Lebensm.
Hyg., 77 (1986).
A Simple Closed-Vessel Nitric Acid Digestion Method
for a Polyethylene/Polypropylene Polymer Blend
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. Therefore, wet
digestion methods for trace metal
determination performed on such
samples must include a rigorous
sample preparation step. In most
cases, this includes a microwave
digestion procedure (1–3). For
example, one of these procedures
specifies that a 0.5-g sample of polyethylene is transferred to a closedvessel acid digestion bomb. Three
milliliters of concentrated nitric
acid is added to the vessel, followed
by an additional 8.0 mL of concentrated sulfuric acid. Microwave
power is applied for a 15-minute
period. The sample is allowed to
cool, and then an additional 5.0 mL
of nitric acid is added. A second heating stage is applied, the sample is
allowed to cool, diluted with water,
and analyzed by ICP or AAS techniques.
A similar method for the digestion of poly(vinyl chloride) has also
been reported (4,5). Samples were
digested using H2O2. Polymeric circuit boards were analyzed for trace
metals on the surface by immersing
the board in a mixture of hot HNO3
and HCl, followed by plasma OES
analysis (6).
Inductively coupled plasma
optical emission spectrometry
(ICP-OES) methods for trace metal
determination are advantageous
due to their low detection limits
and fast analysis times (7–10). The
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
A simple, high pressure
asher digestion method was
developed for a polyethylene/
polypropylene polymer blend.
A 0.15-g sample has been
digested in 2.5 mL nitric acid in
a quartz closed vessel under high
pressure and high temperature.
The digestate was then filtered
and analyzed by inductively
coupled plasma optical emission
spectrometry (ICP-OES). Twentythree elements were determined
in the sample. 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 11 test elements.
Recoveries were in the 90–100%
range for all elements. The digestion technique eliminated the
need for the harsh acid mixtures
(including H2SO4 + HNO3 and
H2O2) that are routinely used for
polymer samples.
effectiveness of ICP-OES methods is
further increased when analyzing
samples with minimal dissolved
solids and residual carbon. Closed
vessel high pressure asher digestion
technology maximizes sample
decomposition by heating at
elevated pressures (11–15). The
decreased time for sample digestion, coupled with the ability to
control reaction parameters, makes
HPA digestion an excellent means
of sample preparation for ICP-OES
determinations.
193
Digestion systems capable of
operating at elevated pressures allow
the decomposition of samples without time-consuming predigestion
steps. The process of developing
HPA digestion procedures is streamlined by system control of reaction
parameters such as temperature
and pressure. Customized programs
can be developed for specific sample matrices utilizing multi-stage
programs and temperature ramping
capabilities.
A further advantage of highpressure, closed-vessel systems is
the ability to decompose the sample
matrix with a minimal amount of
acid. The digestion may also be
accomplished with a single acid as
opposed to a mixture of acids. Limiting the amount and types of acids
used in sample preparation reduces
the dilution of the analytes in the
final solution, 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 the development
of a method for the complete
dissolution of the polymer sample.
Such a technique would ensure that
trace amounts of metals were not
somehow permanently “bound” in
an inorganic or organic matrix and
therefore not detected 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
a polymer sample by ICP-OES. The
procedure requires only nitric acid
and a closed vessel HPA digestion
system. The technique is much
easier 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
observed for spiked real samples.
EXPERIMENTAL
Instrumentation
Inductively Coupled Plasma
Optical Emission Spectrometer
Perkin-Elmer Optima 3000™ DV
ICP-OES (16) using the axial configuration. Table I shows the operating parameters. The samples were
atomized with a GemTip™ crossflow nebulizer assembly. A Rainin®
Dynamax peristaltic pump, Model
RP-1, was used with a pump speed
of 31.19 RPM. A Perkin-Elmer AS-90
autosampler was used. Table II
shows the wavelengths and background correction points.
High Pressure Asher
HPA-S High Pressure Asher™
(17) (Perkin-Elmer/Paar) digestion
system, equipped with an autoclave
and microprocessor control unit.
The autoclave can hold seven 50-mL
quartz, five 90-mL quartz, fourteen
15-mL, or twenty-one 20-mL glassy
carbon vessels for acid digestion
of samples at temperatures up to
320°C and pressures up to 130 bar.
The microprocessor control unit
allows for programming, storage,
and selection of up to four digestion
programs and controls the autoclave
during the sample digestion. Quartz
vessels with a 50-mL volume were
used for the digestions.
Reagents
Nitric Acid
Optima grade (Fisher Scientific,
(Pittsburgh, PA USA).
Calibration Standards
SPEX Certiprep (Metuchen, NJ
USA), Custom Multielement ICPgrade Standards, diluted to produce
working solutions with 10% HNO3
by volume.
Preparation of Sample
The polyethylene/polypropylene
blend samples (0.15 g) were accurately weighed into clean, dry HPA-S
quartz vessels. HNO3 (2.5 mL) was
added using an Optifix Basic dis-
penser (EM Science, Gibbstown, NJ
USA). Ultrapure Teflon® tape was
placed over the opening of the
vessel and smoothed. A quartz lid
was placed on top of the tape.
Teflon tape was then wound three
more times around the lid and the
top of the vessel. This wrapping
technique provides for optimum
tightness and minimal risk of contamination. The optimized digestion
program was employed for the
polymer sample being analyzed.
The final program for the polymer
sample is shown in Figure 1. Upon
completion of the digestion step,
the heating block with the samples
was removed from the autoclave,
then the vessels were removed.
The Teflon and the quartz lids were
slowly removed from the fume
hood, allowing the nitrogen oxide
to escape slowly. 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
TABLE I
ICP-OES Operating Parameters
Parameters
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
1 – Step to 80°C
2 – Ramp to 160°C over 30 minutes
3 – Step to 280°C and hold for 90 minutes
4 – Step to 0°C and end
Fig. 1. HPA-S digestion program for polymer sample.
194
Vol. 19(6), Nov./Dec. 1998
with dd H2O. The solutions were
filtered with Nalgene 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 HPA-S program used for
digestion of the polymer samples
involved optimization of several
parameters: time, temperature,
sample size, and acid volume. These
factors contributed to the total
digestion of the sample and the
pressure inside the vessel. The system controls the external pressure
at 130 bar by filling the autoclave
with nitrogen. Thus, the inside
pressure of the vessels can reach
nearly 130 bar without risking any
loss of sample.
TABLE II
Emission Wavelengths, Background Correction Wavelengths Relative to Emission Wavelengths,
Points per Peak, and Processing Mode for Each Element Determined
Element Emission Background Process
Points Element Emission
Background Process
Points
Wavelength Correction
Mode
per
Wavelength Correction
Mode
per
(nm)
Relative to
Peak
(nm)
Relative to
Peak
(nm)
(nm)
Ag
Al
328.068
396.152
–0.036
+0.050
Area
Area
1
2
Mo
Na
202.030
589.592
As
188.979
Area
2
Ni
231.604
Au
242.795
+0.018
–0.012
–0.020
Area
1
P
177.428
B
Ba
249.773
233.527
Area
Area
2
1
Pb
Pd
Be
Bi
Ca
313.042
223.061
317.933
Cd
–0.022
–0.060
+0.062
–0.025
Area
Area
2
2
Area
1
Area
1
220.353
340.458
–0.020
+0.020
–0.025
–0.040
Area
Area
2
1
Area
Area
Area
1
1
1
Pt
265.945
–0.025
Area
Sb
217.581
2
Area
2
Sc
361.384
Area
1
Co
228.616
+0.025
Area
2
Se
196.026
Area
1
Cr
205.552
–0.023
Area
2
Si
288.158
Area
1
Cu
Eu
Fe
K
324.754
381.967
259.940
766.491
Area
Area
Area
Area
2
1
1
2
Sn
Sr
Te
189.933
407.771
214.281
Area
Area
Area
1
1
1
La
379.478
Area
1
Tl
276.787
–0.017
Area
1
Li
670.781
Area
2
V
292.402
–0.030
Area
1
Mg
Mn
279.553
257.610
+0.033
–0.047
–0.035
–0.140
+0.129
–0.041
+0.047
–0.110
+0.102
–0.040
+0.026
–0.018
+0.025
–0.035
+0.035
–0.015
+0.023
–0.027
+0.028
+0.020
–0.038
+0.060
Area
226.502
–0.030
–0.030
+0.030
–0.050
–0.020
–0.030
+0.030
+0.030
Area
Area
1
2
Zn
Zr
213.856
343.823
–0.021
–0.030
Area
Area
2
1
195
In developing the programs for
digestion of the polymer, a conservative approach was used to prevent
excessive pressure increases. 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 for the dilution
step after digestion. Small dilutions
were required to measure the 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 polymer sample
used 2.5 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 by adding 10 mL
of HNO3 and placing the vessels in a
warm ultrasonic bath for 20 minutes.
This was 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 between
each sample run. All samples
and materials were handled with
powder-free, acid-resistant gloves.
Results
The detection limits, concentration levels, and relative standard
deviations (RSD) for the cosmetic
samples are reported in Table III.
The concentration levels were
below the detection limit for those
elements where no value is
reported. The data in Table III
were calculated using the average
of three samples where each sample was analyzed three times by
ICP-OES. Blank subtraction was
performed automatically by the
computer software. Rubidium
TABLE III
Detection Limits, Concentration Levels (ppm), and
Relative Standard Deviations for the Polymer Sample
Polymer Blend
Detection
Concn.
RSD
Element
limit (µg/g)
(µg/g)
(%)
Ag
As
Au
B
Ba
Be
Bi
Ca
Cd
Co
Cr
Cu
Eu
Fe
K
La
Li
Mg
Mo
Na
Ni
P
Pb
Pd
Pt
Sb
Sc
Si
Sn
Te
Tl
V
Zn
Zr
0.004
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
0.011
0.006
0.005
0.076
0.022
0.008
0.016
0.032
0.0002
0.019
0.007
0.048
0.063
0.002
0.001
0.002
(30 µg/g) and ytterbium (5 µg/g)
were used as the internal standards.
Table IV shows spike recovery values
for 11 elements in the sample.
196
120
2.4%
3.4
0.4%
146
132
122
365
109
16%
2.6%
3.0%
1.2%
3.0%
94.0
280
6.6%
0.9%
139
56.8
151
136
103
1804
7.1%
2.8%
9.1%
2.0%
20%
1.5%
332
9.5%
162
1.9%
Vol. 19(6), Nov./Dec. 1998
TABLE IV
Spike Recovery Table of Polymer
Al
Cd
Co
Cr
Cu
Fe
Mn
Ni
Pb
Se
Zn
96.1%
95.0%
93.3%
94.5%
90.9%
90.8%
92.9%
91.9%
94.9%
96.4%
95.0%
CONCLUSION
Sample preparation is often the
most time-consuming part of an
analysis. The speed of the analysis
has been shortened with the
advances in simultaneous ICP-OES.
Reduction in sample preparation
time requirements can result in an
overall improvement in sample
throughput. In these methods for
specific polymers, the complete
sample preparation process takes
approximately three hours. Another
advantage of this method is that
harsh acid mixtures are not
required. This also helps to avoid
matrix-induced interferences that are
possible with axial view ICP-OES.
Generally, HNO3 is the acid of
choice in ICP-OES analysis.
ACKNOWLEDGMENTS
The authors gratefully acknowledge financial support from The
Perkin-Elmer Corporation and a
grant from the NSF-GOALI program
(CHE-9710218).
REFERENCES
1.
2.
3.
Application Note OP-6, Revision
10-88, CEM Corporation,
Matthews, NC USA.
R.J. Fordham, J.W. Gramshaw,
L. Castle, H.M. Crews, D. Thompson,
S.J. Parry, and E. McCurdy, J. Anal.
At. Spectrom. 10, 303 (1995).
J. Anal. Am. Spectrom. 7 (5), 247R
(1992).
4.
Analyst 117, 27, (1992).
5.
J. Anal. At. Spectrom. 7 (7),
329R (1992).
6.
Adv. Lab. Autom. Rob. 5,
185 (1989).
7.
Modern Methods for Trace
Element Determination,
C. Vandecasteele and C.B. Block,
John Wiley and Sons (1993).
Using the HPA-S system to
digest samples for elemental analysis has definite advantages when
compared to traditional sample
preparation procedures. The use
of quartz closed 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.
197
8.
J.W. Milburn, At. Spectrosc. 17 (1),
9 (1996).
9.
J.R. Jordan, Referee, 7 (June 1995).
10. J.C. Ivaldi and J.F. Tyson,
Spectrochim. Acta Part B,
50, 1207 (1995).
11. Introduction to Microwave Sample
Preparation: Theory and Practice,
H.M. Kingston and L.B. Jassie, eds.,
American Chemical Society (1988).
12. Methods of Decomposition in
Inorganic Analysis, Z. Sulcek and
P. Povondra, CRC Press, Inc. (1989).
13. R.T. White, Jr., and G.E. Douthit,
J. Assoc. Off. Anal. Chem. 68,
766 (1985).
14. H.M. Kingston and L.B. Jassie,
Anal. Chem. 58, 2534 (1986).
15. D. Chakraborti, M. Burguera, and
J.L. Burguera, Fresenius’ J. Anal.
Chem. 347, 233 (1993).
16. Perkin-Elmer ICP-Emission Spectrometry Optima 3000 Hardware
Guide (1993).
17. Perkin-Elmer HPA-S High Pressure
Asher Instruction Handbook (1996).
The Effect of Digestion Temperature on
Matrix Decomposition Using a High Pressure Asher
Meredith M. Daniel, James D. Batchelor, Charles B. Rhoades, Jr., and Bradley T. Jones*
Department of Chemisty, Wake Forest University
Winston-Salem, NC 27109 USA
INTRODUCTION
For many years, acid digestion
has been the sample preparation
method of choice for most trace
metal determinations. More
recently, pressure-assisted acid
decomposition has become the
norm. This technique can be performed in the high pressure asher
with sealed, Teflon®-lined stainless
steel vessels heated in an aluminum
block (1), but usually closed-vessel
microwave digestion systems are
employed (2). For most trace metal
techniques, the highest possible
temperature and pressure provides
the highest accuracy. As expected,
more complete digestion is
achieved under these conditions.
Interestingly, however, complete
decomposition of the organic
matrix is almost never accomplished
with conventional microwave
digestion systems. For biological
and botanical matrices, a few very
stable decomposition products of
nitric acid digestion are usually
observed. The most common of
these are the nitrobenzoic acids
(NBAs) which result from the
breakdown of proteins and
aromatic amino acids (3,4). The
decomposition of carbohydrates,
proteins, and lipids to produce
such products typically occurs at
temperatures in the 140–160oC
range (3). Destruction of the NBAs
occurs at temperatures well above
200oC, so most conventional sample preparation equipment cannot
decompose them.
Remarkably, very limited information exists regarding the qualitative and quantitative determination
of the organic products of nitric
acid digestion. Wurfels, Jackwerth,
and Stoeppler (4) used a combination of IR, NMR, and GC results to
show that the organic matrix com*Corresponding author.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
A high pressure asher system
was employed for the nitric acid
digestion of two standard reference materials (NIST SRM 1547
Peach Leaves and NIST SRM
1632b Coal). The effect of the
final ashing temperature
(180–300°C) upon the extent of
digestion was investigated. The
organic matrix decomposition
products are monitored by high
performance liquid chromatography. For the peach leave samples,
the isomers of nitrobenzoic acid
appear as the major products at
low temperature. They degrade
at elevated temperatures to a variety of by-products, until only one
product is left after digestion at
300oC. The coal sample has a much
more complicated post-digestion
matrix that is also reduced to a
single component at the maximum ashing temperature. Trace
metal recoveries for each sample,
at each ashing temperature, are
determined by inductively coupled plasma optical emission
spectrometry. In most cases, the
recovery for a variety of test metals varies between 90 and 110%,
regardless of ashing temperature.
ponents of biological materials
are nearly completely mineralized
at 180oC with nitric acid digestion.
The only organic digestion products remaining were the NBAs
and a few carboxylic acids. Pratt,
Kingston, et al. (5) demonstrated
that the major products from the
microwave nitric acid digestion of
bovine liver were o-, m-, and p-NBAs.
Methyl isobutyl ketone (MIBK)
extracts of the nitric acid digests
were evaporated under nitrogen
and re-dissolved in methanol for
high performance liquid chromatography (HPLC) determinations. The
reported chromatograms were
remarkably clean, showing peaks
198
for only the three NBA isomers and
a peak for residual MIBK. More
recently, Reid, Greenfield, and
Edmonds (3) used IR and thin-layer
chromatography to determine that
a mixture of carboxylic acids and
NBAs resulted from the nitric acid
digestion of food samples. The carboxylic acids could be eliminated
by post-digestion treatment with
hydrogen peroxide, but the NBAs
required treatment with perchloric
acid (a procedure that most laboratories omit due to safety requirements). Without the perchloric
acid step, the NBAs were found to
greatly interfere with trace metal
determination performed by
voltammetric methods and occasionally by inductively coupled
plasma optical emission spectrometry (ICP-OES) (3,5). Some of the
ultra-trace techniques (such as
GFAAS, anodic stripping voltammetry, and ICP mass spectrometry)
can be very matrix-dependent as
well, and thus could show dependence on NBA concentrations.
The matrix interference effects
associated with the common
microwave digestion decomposition products could be greatly
reduced or eliminated if a high
pressure, high temperature device
were employed. The high pressure
asher device is capable of performing acid digestions at temperatures
up to 320oC and pressures up to
130 bar (6–10). Under these extreme
conditions, effective sample decomposition can be obtained using a
minimal amount (5 mL) of a single
acid (nitric acid). The effect of the
high pressure asher temperature on
the extent of nitric acid digestion of
two standard reference materials
(NIST SRM 1547 Peach Leaves and
NIST SRM 1632b Coal) was investigated. The organic matrix decomposition products were monitored
by HPLC, and trace metal recoveries
were determined by ICP-OES.
Vol. 19(6), Nov./Dec. 1998
EXPERIMENTAL
Instrumentation
A HPA-S High Pressure Asher ™
system (Perkin-Elmer/Paar) was
employed. The HPA-S includes an
autoclave and a microprocessor
control unit. The autoclave holds
five 90-mL quartz vessels. Five mL
of Optima grade nitric acid (Fisher
Scientific) was added to 0.3 g of
sample. The samples were directly
weighed into quartz vessels. The
vessels were placed into the autoclave under 100 bar of nitrogen.
The high temperatures obtainable
using the autoclave allow for digestion temperatures up to 320oC. For
each digestion, the temperature
was slowly ramped over 2.5 hours
and then held at the desired temperature for one hour. In this manner, a different aliquot of each
sample was digested at each final
temperature between 180 and
300oC at 10oC intervals. The temperature reported is the measured
temperature of the stainless steel
block heated by the autoclave. A
thermopile measures the temperature of the block. A pressure gauge
monitors nitrogen pressure within
the high pressure asher. After each
digestion, the nitric acid was evaporated to less than 2 mL. The samples were diluted to 100 mL and
then passed through a 0.45-micron
filter. The samples were stored in
de-ionized water-rinsed polyethylene tubes.
HPLC analysis was accomplished
using a Hewlett Packard®1090D
liquid chromatograph with a builtin photodiode array for detection.
The HPLC trace was recorded at
260 nm and the absorption spectrum was saved from 200–400 nm.
Data acquisition was performed
with an HP486 PC using HP ChemStation software. A 25 cm x 4.6 mm
Whatman C18 HPLC column was
employed for all separations. The
mobile phase was a mixture of
methanol, 10% acetic acid (v/v),
and water with a flow rate of
TABLE I
Mobile Phase Gradient
1 mL/min. The mobile phase
gradient is shown in Table I.
The chromatogram was collected during the first 30 minutes. The last two steps are
used to equilibrate the column
before the next sample.
When the coal samples were
analyzed, a 15-minute rinse
with methanol, followed
by a 15-minute reequilibration
time, was added to the
program after the first
30 minutes.
Trace metal recoveries
were determined with
a Perkin-Elmer Optima 3000™
DV ICP-OES, equipped with
an AS-90 autosampler. The
samples were aspirated into
the axially viewed torch using
a GemTip™ cross-flow nebulizer. A Rainin® Dynamax
peristaltic pump was used
at a pump rate of 31.19 RPM.
Three replicates were analyzed
for each sample. The remaining ICP instrumental parameters are listed in Tables II
and III.
Time
(%)
(%)
Methanol Acetic
acid
(10%)
(%)
Water
Ramp
ort
Constant
0–5
0
5–25
50
25–27
50
27–27.5
100
27.5–31.5 100
31.5–32
0
5
5
5
0
0
5
95
45
45
0
0
95
Constant
Ramp
Constant
Ramp
Constant
Ramp
32–35
5
95
Constant
0
TABLE II
ICP-OES Parameters
Parameter
Instrument Setting
Auxiliary gas flow
0.5 L/min
Nebulizer flow
0.7 L/min
Plasma flow
15 L/min
RF power
Sample rate
1360 W
1.6 mL/min
Wash time
30 sec
Sample read delay
50 sec
Processing mode
Peak area
TABLE III
ICP-OES Parameters for the Elements Determined
Element
Emission
Upper
Lower
Points
Wavelength
BGC
BGC
per Peak
(nm)
Point
Point
Al
396.152
+0.050
2
Ba
233.527
-0.030
+0.030
1
Ca
317.933
-0.043
2
Fe
259.940
-0.035
1
K
766.491
-0.140
+0.129
2
Mg
279.079
-0.033
+0.030
1
Mg
279.553
-0.040
1
Mn
257.610
+0.026
2
Na
589.592
-0.060
+0.062
2
Ni
231.604
-0.025
1
P
177.020
-0.020
+0.020
1
S
180.669
-0.020
+0.015
2
Sr
407.771
-0.038
1
BGC = Background Correction.
199
RESULTS
Peach Leaves
Chromatograms were collected
for each ashing temperature as
described. Figure 1 shows the chromatograms for the samples digested
at three different temperatures
(180, 250, and 300oC). Figure 2 is
a three-dimensional chromatogram
showing retention time (21–27 minutes) versus HPA-S temperature.
As previously reported (3–5), the
NBA isomers dominate the
chromatograms after digestion at
temperatures up to 220oC. In order
to verify the assignment of these
three peaks in the chromatograms,
a standard solution of the three isomers was analyzed under the same
HPLC conditions. The retention
time corresponded to that observed
in the peach leaves digestate at low
temperature (180oC). Additional
positive verification was obtained
by comparing the ultraviolet
absorbance spectrum collected
on-the-fly for each
suspected nitrobenzoic
acid isomer with that of
the standard solution. For
digestion temperatures
ranging from 230–250oC,
additional analyte peaks
reach their maximum
value at retention times
of approximately 7.9,
10.0, 22.0, 22.7, 23.1,
25.0, and 26.0 minutes.
Almost all analyte peaks
vanish at an HPA-S temperature of 270oC, except for
a single compound that
remains present even at
300oC (23.2 minutes).
Fig. 1. Chromatograms of peach leaves HPA-S
digestates at 180oC, 250oC, and 300oC. The isomers of nitrobenzoic acid are labeled.
The degradation and formation
of the nitric acid decomposition
products in peach leaves, as a function of final ashing temperature,
can be seen in Figures 3 and 4. The
plots show the "fraction of component" present versus the digestion
temperature. The "fraction of component" was calculated from the
peak height. The peak height for
a given analyte was determined
at each HPA-S temperature;
then it was normalized to the
maximum peak height observed
for that compound. Figure 3 shows
the degradation trend for the three
nitrobenzoic acids as the digestion
temperature increases from 180
to 240oC. Figure 4 shows the compounds that are forming as the temperature increases until about
270oC where these compounds
fully degrade. One component at
23.2 minutes remains after digestion at 300oC.
The degradation of the nitrobenzoic acids (Figure 3) complements
the product formation (Figure 4).
One might expect that the formation products are probably the
result of nitrobenzoic acid decomposition. To test this hypothesis,
Fig. 2. Three-dimensional chromatogram of peach leaves digestates showing the relationship between retention time and HPA-S temperature.
200
Vol. 19(6), Nov./Dec. 1998
Fig. 3. Degradation of 2-nitrobenzoic acid (■), 3-nitrobenzoic
acid (◆), and 4-nitrobenzoic acid (▲) in peach leaves.
Fig. 4. Formation and subsequent degradation of unknown
organic compounds in peach leaves.
the individual isomers of nitrobenzoic acid were digested at temperatures of 180, 220, and 300oC. The
degradation of the nitrobenzoic
acids results in compounds with
retention times identical to those
in Figure 1. However, the three
individual isomers yield different
decomposition products. For example, both the 2-nitrobenzoic acid
and the 4-nitrobenzoic acid yield
a peak at 7.9 minutes, while the 3nitrobenzoic acid isomer does not.
Each component observed in the
peach leave digestate was also
observed in the digestate of the
three NBA standard solutions.
Fig. 5. Chromatograms of coal digestates at HPA
temperatures of 180oC, 250oC, and 300oC.
Coal
The coal samples were prepared
in exactly the same manner as the
peach leaves. However, the coal
digestion matrices were completely
different, and much more complicated than the peach leave matrices. The chromatograms (Figure 5)
demonstrate the large amount of
organic constituents present in this
sample. Most of these compounds
degrade completely at an HPA-S
temperature of 260oC (Figures 6
and 7). However, a group of new
compounds reaches a maximum
peak height at 280oC and then
201
disappears at 290oC. These
compounds have retention times
of 8.2 and 10.5 minutes (Figure 6),
and 22.4, 23.6, 25.6, and 26.7 minutes (Figure 7). Interestingly, only
one constituent exists after digestions at 300oC (23.64 min). Based
on its absorption spectrum, this
compound is different from the
one with a retention time at 23.6
minutes. As with the peach leaves
digests, the fraction of component
for each constituent was calculated
across the range of digestion temperatures. Again, groups of compounds
followed similar decomposition
trends. However, the coal digestates
contained so many components
that plots of the "fraction of component" present at different HPA-S
temperatures would be too
crowded for clarity. It is expected
that a comprehensive identification
of all of these components would
demonstrate that the degradation
trends would indicate a different
class of organic compounds. Such
a comprehensive analysis would
require HPLC with a mass
spectrometry detector.
Trace Metal Recovery
The trace metal recoveries for
seven test elements in peach leaves
and coal are shown in Figures 8
and 9. The selected test elements
are those that can be determined
with only nitric acid used in the
digestion and they are present at
levels that allow quantitation
(Table IV). To recover other
Fig. 6. Three-dimensional chromatogram of coal digestates showing the relationship between retention time (6–16 min) and HPA-S temperature.
Table IV
Certified Values for Standard
Reference Materials
Peach
Leaves
Coal
SRM 1547
SRM 1632b
EleConcn
Concn
ment
(µg/g)
(µg/g)
Al
249±8
8550±190
Ba
124±4
67.5±2.1
Ca
15600±200
2040±60
Fe
218±14
7590±450
Mg
4320±80
383±8
Mn
98±3
12.4±1.0
Na
24±2
515±11
Ni
0.69±0.09
6.10±0.27
P
1370±70
K
24300±300
748±28
S
(2000)
18900±600
Sr
53±4
(102)
Fig. 7. Three-dimensional chromatogram of coal digestates showing the relationship between retention time (21–27 min) and HPA-S temperature.
elements, the combination of
nitric acid with hydrofluoric acid,
hydrochloric acid, or sulfuric acid
would have been necessary. The
figures demonstrate that metal
recovery is not dependent upon
HPA-S temperature for either sample type. This suggests that the
complexity of the peach leave and
coal matrixes does not adversely
affect the ICP-OES analytical procedure for the metals determined.
202
CONCLUSION
The HPA-S acid digestates of
peach leaves and coal were examined by HPLC. Increasing the acid
digestion temperature led to an
increased decomposition of the
organic compounds present in
the digestate for both samples. The
digestion temperature and related
organic compounds did not influence the trace metal recoveries
for the selected test elements for
Vol. 19(6), Nov./Dec. 1998
with conventional microwave
digestion procedures. Future
studies of interest will seek to
determine the effect of digestion
upon trace metal recoveries for the
more "matrix-interference-prone"
techniques such as GFAAS, voltammetry, and ICP-MS.
ACKNOWLEDGMENT
This work was supported by
grants from the NSF-GOALI
program (CHE-9710218); The
Perkin-Elmer Corporation; and
R. J. Reynolds Tobacco Company.
The authors would like to thank
Jannell Rowe and John Martin,
R. J. Reynolds Tobacco Company,
for their useful discussions.
REFERENCES
Fig. 8. ICP-OES percent recoveries for Al (◆), Ba (■), Ca (▲), Mg (✕), Mn (✻),
Sr (●), K (+), and P (–) in peach leaves.
1. M. Wurfels, E. Jackwerth, and M.
Stoeppler, Anal. Chim. Acta 226,
1 (1989).
2. H.M. Kingston and L.B. Jassie,
J. Res. Nat. Bur. Stds. 93, 269
(1988).
3. H.J. Reid, S.Greenfield, and T.E.
Edmonds, Analyst 120, 1543
(1995).
4. M. Wurfels, E. Jackwerth, and M.
Stoeppler, Anal. Chim. Acta 226,
17 (1989).
5. K.W. Pratt, H.M. Kingston, W.A.
MacCrehan, and W.F. Koch,
Anal. Chem. 60, 2024 (1988).
I6. ntroduction to Microwave Sample
Preparation: Theory and Practice,
H.M. Kingston and L.B. Jassie (ed.),
American Chemical Society (1988).
7. Methods of Decomposition in Inorganic Analysis, Z. Sulcek and P.
Povondra, CRC Press, Inc. (1989).
8. R.T. White, Jr. and G.E. Douthit,
J. Assoc. Off. Anal. Chem. 68, 766
(1985).
Fig. 9. ICP-OES percent recoveries for Ba (■), Ca (▲), Fe (✕), Mg (●), Mn (+),
Na (–), Ni (✻), S (◆), and Sr (O) in coal.
either standard reference material.
This suggests that the ICP-OES
determination of these metals in
these sample matrices is not dependent upon acid digestion tempera-
ture. In other words, the ICP-OES
technique is not particularly sensitive to the changes in digestate
composition observed at temperatures higher than those associated
203
9. H.M. Kingston and L.B. Jassie,
Anal. Chem. 58, 2534 (1986).
10. D. Chakraborti, M. Burguera, and
J.L. Burguera, Fresenius' J. Anal.
Chem. 347, 233 (1993).
Determination of Arsenic and Selenium in
Foodstuffs – Methods and Errors
P. Fechera and G. Ruhnkeb
aLandesuntersuchungsamt Nordbayern, D-91054 Erlangen, Germany
b
Chemisches Untersuchungsamt, D-67346 Speyer, Germany
INTRODUCTION
Because of recurring problems
in the determination of arsenic
and selenium in foodstuffs due to
matrix and element compounds,
the “Inorganic Components” group
of the Society for Foodstuff Chemistry (Lebensmittelchemische
Gesellschaft), a subchapter of the
Society of German Chemists, set
out to investigate and establish
suitable analytical methods. At first,
comparative laboratory tests were
performed with 25 laboratories
participating from Germany and
Switzerland. The materials investigated were homogenates of mussel
tissue, egg powder, and Brazil nuts.
Since each laboratory was free to
select the appropriate digestion
and determination procedures,
the method combinations varied
widely. Each laboratory was
required to provide five individual
results per material and per
element. The “expected” results
for mussel tissue and egg powder
homogenates referred to in the
text and in the figures are based
on prior analyses by a reference
laboratory.
EXPERIMENTAL
Digestion and Determination
Procedures
The combinations of methods
used in the comparative laboratory
tests for the determination of
arsenic and selenium in foodstuffs
are listed in Table I, together with
the respective number of participating laboratories. Some of the participants used several different
procedures, resulting in a total
of over 25 combinations.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
The accurate determination of
total arsenic and selenium in
foodstuffs provides basic information for further investigation with
respect to speciation analysis or
toxicity. Homogenates of mussel
tissue, egg powder, and Brazil
nuts were investigated to determine their arsenic and selenium
concentration. Multiple digestion
procedures at different temperatures were applied and the elements subsequently determined
using atomic absorption, voltammetry, and ICP-MS. The multitude
of methods employed led us to
expect a wide range of results.
A differentiation by digestion
and determination procedures
showed that accurate results can
be expected only if the procedures are attuned to each other.
For example, foodstuffs of marine
origin require a digestion temperature of 320°C to allow full
arsenic determination by hydride
AAS. Such significant dependency
was not observed for selenium;
but here too an incomplete digestion may cause considerable
problems during its subsequent
determination. Since arsenic and
selenium concentrations are very
low in vegetables, hydride AAS
should preferably be used in
place of graphite furnace AAS.
In the majority of cases, the
following procedures were used:
HPA: High temperature digestion
in the HPA-S High Pressure Asher™
system (Perkin-Elmer/Paar) with
nitric acid in quartz vessels at different temperatures, maximum possible temperature 320°C.
Microwave: Instruments from
various manufacturers, working at
different pressures and temperatures; digestions in Teflon®, PFA,
or quartz vessels with nitric acid.
Tölg bomb: Digestion under
pressure with nitric acid in
Teflon bombs and temperatures
up to 200°C maximum.
Open digestion: Boiling with
nitric acid under reflux conditions.
Temperature gradient: Dry
ashing with magnesium oxide/
magnesium nitrate as ashing agent.
None of the participants used
perchloric acid to digest the samples
due to the risk of explosion with
organic matter. Although it is mentioned in the literature, the use of
perchloric acid is avoided in modern
and safety-conscious laboratories.
Moreover, since powerful digestion
systems are available (e.g., high
pressure ashers), which permit
digestion temperatures above
300°C, there is no need to use this
acid. Perchloric acid must not be
used in pressurized systems.
The results of these comparative
laboratory tests for the determination of arsenic in homogenized
mussel tissue indicate a strong
dependence on the digestion
temperature. The samples were
analyzed by hydride AAS.
204
Vol. 19(6), Nov./Dec. 1998
TABLE I
Method Combinations Employed for the Determination of Se and As
Number of
Digestion
Determination
laboratories
method
procedure
Se
As
1
7
1
1
0
1
2
7
0
4
2
1
1
3
7
0
1
1
0
3
5
1
3
3
1
2
HPA
HPA
HPA
HPA
HPA
HPA
Microwave
Microwave
Microwave
Tölg bomb
Tölg bomb
Open digestion
Temperature gradient
Graphite Furnace AAS
Hydride AAS
Hydride ICP-MS
ICP-MS
ICP-OES
Voltammetry
Graphite Furnace AAS
Hydride AAS
ICP-MS
Graphite Furnace AAS
Hydride AAS
Hydride AAS
Hydride AAS
TABLE II
Results for Se in mg/kg of Dry Substance – Survey
Minimum
Median
Maximum
Reference Value
Mussels
Egg homogenate
Brazil nuts
0.05
0.01
0.95
2.72
0.90
4.35
In another comparative laboratory
test, the digestions were performed
in the HPA-S system, only at different temperatures. Five digestions
each were performed at 280°C and
320°C, respectively. The elements
were subsequently determined
using hydride AAS, graphite furnace
AAS, and ICP-MS.
RESULTS AND CONCLUSION
Selenium
For all matrices, the minimum,
median, and maximum Se values,
including the reference values, are
listed in Table II. The reference
values are based on the results
obtained from a reference laboratory.
Table II lists the median range in
5.72
3.20
8.19
3.2
1.2
–
mg/kg of all Se results obtained for
mussel tissue, egg powder, and
Brazil nuts. The median includes all
values. The reference values were
determined by an independent laboratory. It can be seen that the minimum and maximum values differ
widely for all materials.
The selenium results for all matrices scattered widely, probably due to
incomplete digestion, inappropriate
use of modifiers, temperature programs, or incomplete prereduction.
Arsenic
Arsenic was determined in
mussel tissue only. In the other
materials, the concentrations were
below the detection limits for the
205
procedures employed. The range in
variation of the arsenic concentration for mussel tissue was markedly
wider than for selenium. The
lowest value found was <0.2, the
highest 24.3 mg/kg in the dry substance. The expected concentration
was given as 12.6 ± 0.6; the probable value was calculated to 12.5 ± 1
mg/kg in the dry substance.
Figure 1 shows the results
obtained for digestions performed
using HPA-S, microwave, Tölg
bomb, and dry ashing, with subsequent analysis by hydride AAS and,
for comparison, by graphite furnace
AAS. The individual results from the
different laboratories are presented
as individual dots arranged vertically. The gray zone represents the
range of expected values. The
graph shows that, when using the
hydride technique, markedly lower
results were observed when digestion was incomplete. The arsenic
compounds, arsenobetain and
arsenocholine, in many digestion
systems are not quantitatively
decomposed by nitric acid down to
the ionogenic species (5–7). These
species are known to be nontoxic
and it would be more important to
know the amount of the toxic part
(8). But an accurate determination
of the total amount of As is required
for speciation analysis and for quality
control procedures (9). But in some
laboratories, even graphite furnace
AAS analysis results were very low,
which is due both from incomplete
digestions and, as already discussed
for selenium, from inadequately
adapted modifiers and graphite
furnace parameters (10,11).
In order to highlight the importance of the digestion temperature
for subsequent hydride AAS determination, another comparative
laboratory test was performed. In
this case, only the HPA-S was used
for the sample digestion using two
temperatures, 280°C and 320°C.
Figure 2 shows the results obtained
for the two temperatures by the
unknown and the water content
of the sample is usually not determined, a complete digestion can
be achieved with certainty only
at a temperature of 320°C. The
results demonstrate that state-ofthe-art digestion procedures do
not need the addition of ashing
aids like sulphuric acid or the dangerous perchloric acid. The application of a high-temperature nitric
acid alone can completely destroy
all of the organic compounds present in biological materials.
REFERENCES
1.
V. Krivan and S. Arpadjan,
Fresenius J. Anal. Chem. 342,
692 (1992).
2.
D.L. Styris, L.J. Prell, D.A. Redfield,
J.A. Holcombe, D.A. Bass, and
V. Majidi, Anal. Chem. 63, 508
(1991).
3.
V. Majidi and J.D. Robertson, Spectrochim. Acta 46B, 1723 (1991).
4.
M. Sager, Analytiker-Taschenbuch,
Band 12; 257, Springer-Verlag
(1994).
5.
P. Schramel and S. Hasse, Fresenius
J. Anal Chem. 346, 794 (1993).
6.
M. Ihnat, and H.J. Miller, J. Assoc.
Off. Anal. Chem. 60, 813 (1977).
7.
M.L. Cerver and, R. Montoro,
Fresenius J. Anal. Chem. 348,
331(1994).
8.
A.G. Howar and C. Salon, Anal.
Chim. Acta 333, 89 (1996).
9.
U. Ballin, R. Kruse, and H.A.
Rüssel, Fresenius J. Anal. Chem.
350, 54 (1994).
Fig. 1. Arsenic content in mussels.
10. V. Krivan and S. Arpadjan, Fresenius
J. Anal. Chem. 335, 743 (1989).
Fig. 2. Arsenic contents in mussels – HPA-S digestions at 280 and 320°C and
determination by hydride AAS.
same laboratories, respectively. It
is quite obvious that accurate
results with hydride AAS are decisively influenced by the digestion
temperature and that a temperature
of 320°C is necessary to break up
the stable compounds arsenocholine and arsenobetain. In addition,
acid and water also influence the
digestion, particularly with regard
to the digestion of dried materials.
When sufficiently high amounts of
acid are used, even lower temperatures may be sufficient. Since the
acid amounts required for routine
digestions of fresh materials are
206
11. J. Sneddon and K.S. Farah,
Spectrosc. Lett. 27, 257 (1994).
Determination of Lead and Cadmium in Food Products
by Graphite Furnace Atomic Absorption Spectroscopy
C. Blake and B. Bourqui
Nestlè Research Centre, Quality and Safety Assurance Department
P. O. Box 44, 1000 Lausanne 26, Switzerland
INTRODUCTION
Overall exposure to lead and
cadmium is a public health
concern. The lead content in food
products has been gradually
reduced due to the phasing out
of lead-soldered cans as well as the
use of unleaded gasoline (11).
However, cadmium levels in the
environment appear to be increasing. Burgatsacaze et al. (12)
recently reviewed the role of cadmium in the food chain.
Various international organizations, e.g., Codex Alimentarius, the
European Union (E.U.) (1,2), are
debating and reviewing the maximum allowable concentration of
lead and cadmium in raw materials
and food products. Future norms
will set the limits of metals concentration, particularly for lead, which
will be rather low. This is expected
to be an important factor in international trading, i.e., grain exporters
must increasingly be able to certify
that the grain shipments are in
compliance with regulatory requirements for toxic metals content (9).
The technique most commonly
used by Nestle laboratories for the
determination of lead and cadmium
is graphite furnace atomic absorption spectroscopy (GFAAS). The
current Laboratory Instructions (LI)
were published in 1989 and have
been implemented in many regional
laboratories. These LI are similar to
methods published in the German
food analysis handbook (3) and by
ISO (4–7).
Impending EU legislation for
toxic metals determination will set
strict method performance criteria
(1,2), based on the criteria
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
Two sample wet ashing techniques for mineralization of food
products and raw materials were
evaluated using high pressure
ashing and microwave digestion
with Teflon vessels, fitted with
quartz inserts.
Similar accuracy and precision
for the determination of lead and
cadmium were obtained when
analyzing a range of certified food
reference materials by graphite
furnace atomic absorption spectroscopy (GFAAS). The high pressure asher method is preferred
due to the higher sample
throughput, selection of 14 or 21
tubes, depending on the type of
heating block used.
The limits of quantification for
lead and cadmium by GFAAS
with Zeeman correction were
improved using end-capped
graphite tubes and an electrodeless discharge lamp in place of a
hollow cathode lamp. A single
matrix modifier (magnesium
nitrate and ammonium dihydrogen phosphate) was found to be
suitable for the determination of
both lead and cadmium.
The limits of detection and
repeatability for Pb and Cd are
close to the requirements currently being proposed by the
European TC 275 working group
for heavy metals methodology.
described in ISO 3535–1993.
Thus, the current LI will need to
be updated to meet future norms.
In the past few years, a number
of instrumental developments have
contributed to providing more reliable results and higher detection
limits for trace determination of
207
lead and cadmium by GFAAS. These
include (a) improved electrodeless
discharge and hollow cathode
lamps for increased light output;
(b) transversely heated graphite
tubes with end caps for higher sensitivity; and (c) improved wet ashing sample preparation techniques,
e.g., microwave digestion and high
pressure ashing. These aspects have
been evaluated in the current study.
EXPERIMENTAL
Instrumentation
Sample preparation
HPA-S High Pressure Asher™
system (Perkin-Elmer/Paar),
equipped with stainless steel
heating blocks for 14 or 21 tubes
and Suprasil mineralization tubes
(15 mL).
Microwave digestion system,
MLS 1200 Mega (Milestone), with
a temperature control system. The
high pressure (HPV 80) vessels are
equipped with QS 50 graduated
quartz liners with caps.
The mineralization tubes were
decontaminated before use in
a nitric acid vapor decontamination
system (Trabold Ltd, Berne,
Switzerland).
Atomic absorption
instrumentation
A Perkin-Elmer Model 4100 ZL
atomic absorption spectrometer
was used, equipped with
transversely heated graphite
furnace and Zeeman background
correction, AS-70 autosampler,
closed-circuit cooling system, fume
extraction system, and System 2
electrodeless discharge lamp
power supply .
Transversely heated, pyrolyzed
graphite tubes with integrated platform, and transversely heated
pyrolyzed graphite tubes with end
caps were used.
Reagents
All solutions were prepared in
polypropylene volumetric flasks,
using ultra-pure water, prepared
with a Barnstead Nanopure system.
Nitric acid: Suprapure (Merck).
Hydrogen peroxide: Analytical
grade (30%), (Merck).
High Pressure Ashing
Samples of 300 mg each were
weighed into decontaminated 15mL Suprasil tubes. Two mL of concentrated double-distilled nitric acid
was added. The tubes were sealed
with Teflon® tape, capped, and
then wet-ashed in the HPA-S. A typical temperature program used is
shown in Figure 1. The acid solution was diluted to 10 mL with
water. Further dilutions were made
with 10% (m/v) nitric acid solution,
if required to be within the calibration range.
Lead and cadmium stock solutions, 1 g/L (Spex).
Cadmium and lead working solutions were prepared by dilution of
the cadmium and lead stock solutions with 10% (v/v) nitric acid.
Matrix modifier: Various
mixtures of ammonium dihydrogen
phosphate (NH4H2PO4) and magnesium nitrate [Mg (NO3)x.6H20] in
10 % nitric acid solution.
Reagents were of Suprapure
quality (Merck).
Reference Materials
A range of reference materials
with certified lead and cadmium
content was used for method evaluation. These products were
obtained from IRMM, NIST, IAEA,
and NRCC. Nestec reference materials (cereals with milk) of known
lead and cadmium content were
also used.
Fig. 1. HPA-S temperature program.
Sample Preparation
All sample weighings were carried out in a class 100 laminar flow
cabinet (Skan Model EVZ 180). This
cabinet was installed in a clean-air
room (class 1000 air quality) under
positive air pressure with a filtered
air inlet.
The samples were wet ashed
using the following two methods:
Fig. 2. Program for Milestone MLS.
208
Microwave Digestion
Samples of 300 mg each
were weighed into a Milestone
QS5 quartz insert with 3 mL concentrated double-distilled nitric
acid. The insert was then
introduced into a Milestone HPV 80
Teflon vessel. One mL hydrogen
perioxide (30 %) and 4 mL water
were added inside the Teflon vessel, but at the exterior of the insert.
The microwave vessel was closed
with its cap. Six vessels were
placed into the rotor and heated in
the microwave oven according to
the temperature program shown in
Figure 2.
Vol. 19(6), Nov./Dec. 1998
The rotor was removed from the
microwave oven and allowed to
cool to room temperature. The vessels were carefully opened in a
fume cupboard. The quartz inserts
were removed with the Tefloncoated tweezers provided. The
inner wall of the quartz insert was
rinsed with de-ionized water and
made up to the 10-mL mark with
water. Further dilutions were made
as required with 10% (m/v) nitric
acid solution. All subsequent dilutions were prepared in polypropylene volumetric flasks.
GFAAS Determination of Lead
and Cadmium
The GFAAS operating parameters for the determination of
lead and cadmium are listed in
Tables I and II. Transversely heated
pyrolyzed graphite tubes were used
for the lead and cadmium determinations. The GFAAS was calibrated
by the external standards method
with a zero blank and five standard
concentrations. All analyses were
performed by triplicate firings. During an analytical series, a mid-range
QC standard solution was injected
every 10 analytical sample solutions
to verify the calibration slope.
RESULTS AND DISCUSSION
Graphite Furnace Method
Development
Optimization of matrix modifier
The current LI for lead requires
the use of ammonium dihydrogen
phosphate as the matrix modifier.
The LI for cadmium requires
a palladium matrix modifier. The
main purpose of these modifiers is
to stabilize the element during the
graphite furnace cycle and to permit increases in the charring and
atomization temperatures. This
allows a better separation of the
element from interferences.
However, since two different
modifiers are used with the current
LI, a separate graphite tube is
required for each element. In order
to simplify these procedures, a
mixed modifier has been evaluated
for the determination of both elements. The mixed matrix modifier
(magnesium nitrate and ammonium
dihydrogen phosphate) has been
reported in several publications in
different ratios. In the present study,
several concentrations of the mixed
matrix modifier were evaluated:
1. Magnesium nitrate 0.06% and
ammonium dihydrogen phosphate
0.5%.
2. Magnesium nitrate 0.6% and
ammonium dihydrogen phosphate
0.5%.
3. Magnesium nitrate 0.10% and
ammonium dihydrogen phosphate
1.3% (9).
4. Magnesium nitrate 0.20% and
ammonium dihydrogen phosphate
2.0% (8).
TABLE I. Instrumental Parameters for the Determination of Pb
Furnace Time/Temperature Program
Parameter
Dry 1
Dry 2
Char 2
Atomize
Clean
o
Temp ( C)
100
130
750
1600
2300
Ramp (sec)
2
20
10
0
5
Hold (sec)
20
60
25
5
3
250
250
250
0
250
Argon gas flow
(mL/min)
Wavelength:
Lamp:
Slit Width:
Read time:
Signal
measurement:
Graphite tube:
Peak area
Pyrolytic graphite, end-capped
Calibration
Standards
I.D.
Calibration
Blank
Std 1
Std 2
Std 3
Std 4
Std 5
Reslope
Standard
283.3.nm
Electrodelss discharge
0.7 nm
5s
STD 1
STD 2
STD 3
STD 4
STD 5
STD 3
Concn.
(µg/L)
1.0
2.5
5.0
7.5
10.0
Pipette speed: 100%
Injection temperature: 20oC
Volume
(µL)
Diluent
volume
(µL)
Modifier
volume
(µL)
20
2
5
10
15
20
18
15
10
5
0
5
5
5
5
5
5
15
5
5
Calibration type: Linear
Note: The calibration range may be increased to 20 µg/L; for higher concentrations,
a non-linear calibration curve was obtained.
209
The best results obtained in
terms of peak profile was with a
mixture of 0.6% magnesium nitrate
and 0.5% ammonium dihydrogen
phosphate; although the non-specific background was somewhat
higher than with modifier 1.
Modifier 1 also gave good
results. Modifiers 3 and 4 were
found to give very high non-specific
backgrounds and were not further
evaluated.
With respect to the GFAAS temperature programs, the final charring and atomization temperatures
adopted are listed for lead and cadmium in Tables I and II. For lead,
the atomization temperature was
fairly critical and the peak shape
changed dramatically in the range
from 1400oC to 1600oC. This temperature needs to be optimized
carefully.
TABLE II. Instrumental Parameters for the Determination of Cd
Furnace Time/Temperature Program
Parameter
Dry1
Dry2
Char
Atomize
Clean
Temp (oC)
100
130
600
1400
2300
Ramp (s)
2
20
15
0
5
Hold (s)
20
60
20
5
3
Argon gas flow
(mL/min)
250
250
250
0
250
Wavelength:
Lamp:
Slit width:
Read Time:
Signal
measurement:
Graphite tube:
Calibration
Standards
Injection temp. 20oC
Pipette speed 100%
228.8 nm
Hollow cathode
0.5 nm
5s
Peak area
Pyrolytic graphite, end-capped
I.D.
Calibration blank
Std 1
STD 1
Std 2
STD 2
Std 3
STD 3
Std 4
STD 4
Std 5
STD 5
Reslope
Standard
STD 4
Concn.
(µg/L)
Volume
(µL)
Diluent
volume
(µL)
0.5
1.0
2.0
3.0
5.0
20
2
4
8
12
20
18
16
12
8
0
3.0
12
8
Modifier
volume
(µL)
5
5
5
5
5
5
5
Influence of end-capped graphite
tubes and lamps
The main difficulty in determining lead by GFAAS is to obtain
sufficient sensitivity. The use of
end-capped graphite tubes over the
standard graphite tubes resulted in
a significant increase in signal (by
a factor of 1.5). In addition, the use
of an electrodeless discharge lamp
instead of a hollow cathode lamp
also resulted in an increase in signal
due to increased light intensity. The
combined effect of the end-capped
tube and an electrodeless discharge
lamp (EDL) over a standard graphite
tube and hollow cathode lamp
(HCL) resulted in an increase in
sensitivity by about a factor of 2.
Figure 3 illustrates the difference in
calibration slope for lead. However,
the major improvement was in the
improved repeatability of measurements at low lead concentrations
below 2.0 ng/mL.
For cadmium, end-capped tubes
were used with a hollow cathode
lamp light source. Some further
improvement in sensitivity may be
obtained with an electrodeless discharge lamp in place of the hollow
cathode light source.
Evaluation of High Pressure
Ashing and Microwave
Digestion Techniques
A range of different techniques
has been described in the literature
(10) for the sample preparation of
foods and raw materials prior to
GFAAS determination of lead and
cadmium. For this study, the
method performance of two sample
preparation techniques for different
reference materials with certified
lead and cadmium content was
evaluated:
1. High pressure asher (HPA-S)
with new 15-mL Suprasil tubes.
2. Microwave digestion (MDS)
with Teflon vessels fitted with
graduated quartz inserts.
Calibration type: Linear
210
Vol. 19(6), Nov./Dec. 1998
The limits of quantification for
each element based on the analysis
of certified reference materials
using the stated equipment were:
Cadmium = 10 µg/kg
Lead
= 15 µg/kg
CONCLUSION
Two sample wet ashing
techniques, high pressure ashing
and microwave digestion with
Teflon vessels fitted with quartz
inserts, were evaluated.
Fig. 3. Comparison of different GFAAS conditions on calibration line.
HPA-S System
For the HPA-S technique, good
recoveries of lead and cadmium
were obtained for various reference
materials (Tables III and IV) and
two infant cereals (Nestec
reference products, MET) (Tables
V and VI). The repeatability of the
results obtained was also good and
within or close to the range of the
certified values.
Microwave Digestion System
(MDS)
The main disadvantages of this
system are that a higher volume of
nitric acid is required (5–6 mL) and
that the final volumes of the analytical solutions are often high (from
25 or 50 mL). This leads to lower
sensitivity owing to the large dilution factor. Thus, the use of Milestone QS 50 graduated quartz
inserts, which fit inside the Teflon
vessels, was evaluated. Three mL
of nitric acid was used, most of
which was consumed during the
wet ashing step. The final volume
of the analytical solution, after dilution with water, was 10 mL. Thus
a significant increase in sensitivity
was obtained due to the decrease
in total volume.
The accuracy of the results
obtained by MDS in the present
study was, in general, similar to
that of the HPA-S technique, with
occasional values being slightly
below the certified reference values
for lead (Tables III–VI). The open
quartz-tube system of the MDS may
lead to losses if the digestion unit
is not allowed to cool adequately
after completion of wet ashing. The
repeatability of the results was similar for both methods (see Tables
III–VI and VII). An overview of the
relative standard deviations (%RSD)
for lead and cadmium is shown in
Table VII. The RSD was below 25%
for concentrations <100 µg/kg lead
and cadmium and less than 10% for
concentrations >100 µg/kg. This is
quite acceptable for the trace determination of lead and cadmium.
Limits of Detection and
Quantification
An important aspect of the
method performance evaluation is
the calculation of the limits of
detection and the limit of quantification. The limits of detection
based on the repeated analysis of
blank solutions were calculated to
be:
Cadmium = 3 µg/kg
Lead
= 5 µg/kg
211
Similar accuracy was obtained
with the two methods for both lead
and cadmium when analyzing
a range of certified food reference
materials. The HPA-S method is
preferred due to the higher sample
throughput (14 or 21 tubes,
depending on the heating block
used).
The limit of quantification for
lead by GFAAS resulted in an
improvement by a factor of 2 with
respect to the current LI by using
end-capped graphite tubes and an
electrodeless discharge lamp in
place of a hollow cathode lamp.
The limit of quantification of cadmium was also improved by the use
of the end-capped tubes. Further
improvements in sensitivity may be
obtained for cadmium by using an
electrodeless discharge lamp as thelight source. A single matrix modifier (magnesium nitrate and
ammonium dihydrogen phosphate)
is suitable for the determination of
both lead and cadmium.
The limits of detection and the
repeatability for lead and cadmium
are close to the values currently
being proposed by the European
TC 275 working group for heavy
metals methodology.
TABLE III
Lead: Comparison of Results for Samples
Prepared by HPA-S and Microwave Digestion
Product
Reference
HPA-S
MLS
Lead
Lead
Lead
(µg/kg)
(µg/kg)
(µg/kg)
Bovine Muscle
NIST 8414
380 ± 240
326 ± 13
444 ± 2
n=6
n=6
Brown Bread
BCR 191
187 ± 14
199 ± 39
194 ± 10
n=6
n=6
Corn Bran
NIST 8344
140 ± 34
136 ± 2
146 ± 5
n=9
n=6
Dogfish Muscle
NRCC, DOLT-2
220 ± 2
243 ± 6
167± 3
n=6
n=9
Milk Powder
IAEA, A11
54 ± 25
57 ± 14
55 ± 5
n=6
n=3
Non-fat Milk Powder
NIST 1549
19 ± 0.3
21.5 ± 0.6
15.0 ± 0.6
n=6
n=9
Skim Milk Powder
BCR 150
1000 ± 40 1022 ± 30
910 ± 13
n=3
n=6
Whole Meal Flour
BCR 189
379 ± 12
387 ± 20
388 ± 58
n=6
n=6
Whole Egg Powder
NIST 8415
61 ± 12
66 ± 9
55 ± 5
n=6
n=6
TABLE V
Lead: Comparison of Results for Infant Cereals
Prepared by HPA-S and Microwave Digestion
Product
Reference
HPA-S
MLS
Lead
Lead
Lead
(µg/kg)
(µg/kg)
(µg/kg)
Infant Cereal
Met – 5
277 ± 52
251 ± 7
256 ± 13
n=6
n = 10
Infant Ceral
Met – 6
826 ± 52
791 ± 21
n=6
858 ± 32
n=3
TABLE IV
Cadmium: Comparison of Results for Samples
Prepared by HPA-S and Microwave Digestion
Product
Reference
HPA-S
MLS
Lead
Lead
Lead
(µg/kg)
(µg/kg)
(µg/kg)
Bovine Muscle
NIST 8414
13 ± 11
14 ± 2
11 ± 1
n=6
n=6
Brown Bread
BCR 191
28.4 ± 14
28 ± 2
30 ± 8
n=6
n=6
Corn Bran
NIST 8344
12 ± 5
9±1
11 ± 1
n=9
n=6
Dogfish Muscle
NRCC, DOLT-2 20,800 ± 500 20,247 ± 938 27,700±
560
n=6
n=9
Skim Milk Powderr
BCR 150
22 ± 14
20 ± 1
21 ± 3
n=6
n=6
Total Diet
NIST 1548
28 ± 4
28 ± 3
24 ± 3
n=6
n=6
Whey Powder
IAEA, 155
16 ± 3.5
16 ± 3
18± 3
n=3
n=6
Whole Meal Flour
BCR 189
71 ± 3
74 ± 2
58 ± 11
n=6
n=3
TABLE VI
Cadmium: Comparison of Results for Infant Cereals
Prepared by HPA-S and Microwave Digestion
Product
Reference
HPA-S
MLS
Lead
Lead
Lead
(µg/kg)
(µg/kg)
(µg/kg)
Infant Cereal
Met – 5
127 ± 22
133 ± 4
n=6
TABLE VII
Range of Repeatability Values (%RSD)
from the Various Reference Materials
Range of
Pb
Pb
Cd
Cd
Pb or Cd
RSD (%) RSD (%)
RSD (%) RSD (%)
(µg/kg)
HPA-S
MLS
HPA-S
MLS
10 –99
3 – 25
4–9
3 – 19
9 - 25
100 – 1000+ 1.5 – 20
212
127 ± 2
n=6
0.5 – 15
2
5
Vol. 19(6), Nov./Dec. 1998
REFERENCES
1. European Commission,
“Draft: Commission regulation
setting maximum limits for certain
containments in foodstuffs, amending commission regulation (EC)
194/97 of 31 January 1997 setting
maximum limits for certain contaminants in foods-maximum limits for
lead and cadmium in foodstuffs.”
European commission III/5125/95
Rev. 3 (March 1997).
2. European Community, “Commission decision 90/515/EEC of 26
September, 1990.” Off. J. Eur. Commun. 33 (L286), (1990).
3. LMBG, “Bestimmung von
Spurenelementen in Lebensmitteln.
Teil 3: Bestimmung von Blei, Cadmium, Chrom und Molybdän mit
der Atomabsorptionspektrometrie
(AAS) im Graphitrohr.” Amtliche
Sammlung von Untersuchungsverfahren nach Paragraph 35 LMBG,
(August, 19/3, 1993).
4. ISO, “Fruits, vegetables and
derived products. Determination
of lead content. Flameless atomic
absorption spectrometric method,”
ISO 6633 (1984).
5. ISO, “Fruits, vegetables and
derived products. Determination of
cadmium content. Flameless atomic
absorption spectrometric method.”
ISO 6561 (1983).
6. ISO, “Starch and derived products. Heavy metal content. Part 3.
Determination of lead by atomic
absorption spectrophotometry with
electrothermal atomization.” ISO
Norm 11212–3 (1997).
7. ISO, “Starch and derived products. Heavy metal content. Part 4.
Determination of cadmium by
atomic absorption spectrophotometry with electrothermal
atomization.” ISO Norm 11212–4
(1997).
8. G. Ellen and J.W. Van Loon,
Food Addit. Contam. 7 (2), 265
(1990).
213
9. E.J. Gawalko,T.W. Nowicki,
J. Babb, and R. Tkachuk, J. AOAC
Int. 80 (2), 379 (1997).
10. C.J. Blake, “Analysis of lead
and cadmium in foods and raw
materials – a literature review.”
R&D Note No. QS-RN 970055
(1997).
11. P.M. Bolger et al., Food
Addit. Contam. 15 (1), 53 (1996).
12. V. Burgatsacaze, L. Craste,
and P. Guerre, Revue de medicine
Veterinaire 147 (10), 671 (1996).
Determination of Trace Element Contaminants in
Food Matrices Using a Robust, Routine
Analytical Method for ICP-MS
P. Zbinden and D. Andrey
Quality and Safety Assurance Department, Micronutriments & Additives Team
Nestlé Research Center
1000 Lausanne 26, Switzerland
INTRODUCTION
Inductively coupled plasma mass
spectrometry (ICP-MS) is a very
powerful technique for obtaining
very low trace element levels and
high sample throughput. This technique is applicable for the routine
analysis of samples in quality control and safety laboratories of the
food industry, as well as of food
regulatory laboratories. However,
ICP-MS is very sensitive to different
interferences which can lead to
inaccurate results. As food samples
are very complex matrices, interferences occurring in the analysis of
such samples can be very significant.
It is obvious that ICP-MS can
become an appropriate technique
for use in food industry laboratories
when a robust analytical method is
developed. The method developed
should not be too sensitive to the
type of sample matrix to be
analyzed.
A robust method would include
a good sample preparation method
together with a detailed study of
the potential interferences.
Interferences in ICP-MS consist
of general physical interferences,
spectral interferences (1,2), and
carbon-induced interferences (3).
The general and isobaric interferences are usually well known to
ICP-MS users.
In a routine laboratory environment it is necessary to work as fast
as possible. In trace metal determination by ICP-MS, the speed of the
analysis is dependent on the speed
at which the samples are prepared.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
ICP-MS is a rapid analytical
technique that shows potential
for use in routine multielemental
analysis in the food industry.
However, in order to take advantage of its high speed of analysis,
the analytical throughput should
not be slowed down by a lengthy
sample preparation step. On the
other hand, a rapid wet ashing
method may cause interferences
due to the presence of residual
carbon, particularly in the determination of As, Se and Pb.
Arsenic and selenium measured
by ICP-MS in samples where residual carbon is present may be
determined with a higher value
up to 30%. At the same time, Pb
may be determined with a value
of 10% lower.
These carbon-related interferences were quantitatively studied.
The study shows that addition of a
set concentration of isopropanol to
wet ashed samples overcomes
interferences from residual carbon.
The accuracy and reproductibility
of the determination of As, Se
and Pb by ICP-MS was improved.
A rapid and robust analytical
method for the trace determination of As, Cd, Hg, Pb, Al and Se,
well-suited to the routine environment of the food analytical
laboratory, has been developed.
Generally, ICP-MS preparation steps
require long digestion times (e.g., 3
hours) at high temperatures to
remove carbon from the sample to
minimize matrix interferences.
214
Even under these extreme
conditions, the quantity of the
residual carbon present in solution
is difficult to evaluate.
The effect of the residual carbon
on quantitative analysis is not wellknown. Potential interferences
occurring in the determination of
27Al, 75As, 114Cd, 202Hg, 208Pb, and
82Se were studied in detail.
A robust analytical method for
the trace element determination in
food useable in a routine laboratory
environment is proposed.
EXPERIMENTAL
Instrumentation
Sample preparation
HPA-S High Pressure Asher™
system (Perkin-Elmer/Paar), maximum pressure 150 bar, maximum
temperature 320°C, used with
quartz vessels.
Decontamination of the HPA-S
quartz vessels
Decontamination of the HPA-S
quartz vessels was performed with
a decontamination system
(TRABOLD, Bern, Switzerland)
using hot HNO3 vapors.
Spectrometer
An ELAN® 6000 ICP-MS (PE
SCIEX, Concord, Ontario, Canada)
was used. A thermostated cyclonic
spray chamber fitted with a concentric nebulizer (Glass Expansion,
Australia) was used instead of the
standard cross-flow nebullizer.
Vol. 19(6), Nov./Dec. 1998
Reagents
High-purity ultrafiltered water
(18.2 MΩ, MilliQ® Plus system) was
used for dilution of the standards
and samples. Nitric acid was freshly
sub-distilled.
Reference Materials
MET 2/95, MET 6/95, Infant
Cereals; DDP 7/95, DDP 8/95, Milk
Powder. These materials were prepared by Nestlé laboratories and are
regularly used as internal reference
samples. BCR 8433, Corn Bran;
NIST 1547, Peach Leaves; NIST
1575, Pine Needles; NIST 1568a,
Rice Flour; NIST 1549, Non-Fat
Milk Powder were obtained from
PROMOCHEM, France.
Sample Preparation
All samples were prepared by
wet ashing using the HPA-S High
Pressure Asher. Except when
specified, 0.4 to 0.5 g of sample
was introduced into 15-mL quartz
HPA-S vessels, and 2 mL of subboiling nitric acid was added. The
HPA-S tubes were closed with two
PTFE strips and a quartz cap. One
strip is used to seal the tube, and
the other to close the quartz cap.
Twenty-one tubes were introduced
into the HPA-S stainless steel heating block. The HPA-S was closed
and a N2 pressure of 90 bar was
applied. The samples were then
heated according to the program
described in Table I.
TABLE I
HPA-S Heating Program
Step Initial
Time
Final
temp.
temp.
(°C)
(min)
(°C)
1
20
2
90
3
150
Total time
30
20
30
80
90
150
180
The samples were diluted with
ultrafiltered water to 10 mL in the
HPA-S quartz tubes.
RESULTS AND DISCUSSION
Automatic Addition of Indium
as Internal Standard
The normal interferences due to
the physico-chemical composition
of the sample viscosity, the difference in acid concentration or in the
quantity of matter injected are
normally corrected by the addition
of an internal standard.
After testing the different
elements as internal standards
(Nb, In, Y, Yb, Be and Ta), indium
(115In ) was found to be the best
choice for correcting the analytes
over the full range of masses.
Usually, the internal standard is
added manually to each sample,
blank, and standard. This operation
is time-consuming and prone to
manipulation errors (addition of
very small volumes). To avoid these
manipulations, a simple device was
used with the ICP-MS sample introduction system. It consists of a mixing
manifold (two ways in, one way
out) and a normal three-channel
peristaltic pump. It automatically
adds indium as an internal standard
to all sample, blanks, and standards.
This very simple system provides
interesting advantages:
•
Fewer manipulation errors and
greater sample throughout. The
internal standard is regularly
pumped at the same rate as the
sample solution. No manipulation of the samples and
standards is required.
content was always by 30% higher
in comparison to the As-certified
value (normally measured by HGAAS). The Pb value was 10–15%
lower than the value obtained by
GFAAS.
Since food samples have a high
carbon content, these problems
were presumably related to the
high carbon concentration remaining in the food samples after wet
ashing. The effect on the quantitative analysis of food matrices has
not been reported previously in
the literature.
The carbon effect on the ICP-MS
intensities of Cd, Hg, As, Pb, Se, and
In is shown in Figure 1, where the
variation of carbon concentration
was simulated by varying the concentration of alcohol and citric acid in a
standard solution. A direct relation
between analyte intensities and carbon was therefore demonstrated.
Figure 1 shows that carbon produces a strong signal enhancement
on 75As and Se (both masses tested
82Se and 77Se). The signals can be
enhanced up to seven times in the
presence of carbon in solution. This
means that measuring As and Se in
organic samples (food samples)
could lead to a value up to seven
times higher, which is unacceptable.
This enhancement effect is often
used to raise the sensitivity for As
(4,5) or Se.
Auto-dilution of the sample is
achieved. The dilution factor is
determined by the ratio of the
internal diameter of the
peristaltic tubes of both sample
and internal standard.
A change in Pb intensities was
also observed, but it is less marked
and the opposite effect occurs. For
Cd, a slight change was observed,
which follows the signal observed
for In. This suggests that for cadmium the intensity change will be
well corrected with indium as the
internal standard.
Carbon Effects on Trace
Element Determination
A systematic error was observed
in the analysis of digested organic
certified food samples. The arsenic
Carbon Effect Measured in
Food Samples
To show the carbon effect in
a more realistic analytical situation,
a Nestlé internal reference material,
•
215
Infant Cereals MET 6/95, for which
the As, Cd, Hg, and Pb concentration is accurately known, was analyzed by varying the dilution factor
of the sample. As the sample volume remains constant, a larger
amount of food sample corresponds
to more carbon in the analytical
solution.
The sample weight was varied
between 100 and 650 mg and
diluted to 10 mL. This corresponds
to a dilution factor varying from
100 to 15.
Fig. 1. Methanol, ethanol, isopropanol and citric acid effect on the ICP-MS
intensities of heavy metals (In, Y, and Se). Plot of weight (%) of alcohol or citric
acid versus analytes intensities.
A strong effect was observed
in the determination of arsenic
(Figure 2). At low dilution factors,
the measured As concentration was
higher. For a dilution near 10, the
value measured for As was 40%
higher. At low dilution factors, the
measured lead concentration was
lower. The effect on Pb is less
important as it corresponds to a
lower value of only 10%. No carbon
effect was observed in the determination of Cd and Hg.
How to Obtain Reliable
Results for 75As and 208Pb
The curve representing 75As
intensities versus isopropanol concentrations (see Figure 3) can be
split into four zones. Parts 1 and 3
are zones where relatively small
changes in carbon concentration
produced an important change in
75As intensities. On the other hand,
parts 2 and 4 are zones where
---- Control limits; .......... Reference value
Fig. 2. Effect of residual carbon on the concentration of As, Cd, Hg, and Pb
measured by ICP-MS in MET 6/95.
Fig. 3. Effect of isopropanol on 75As raw
intensities.
216
Vol. 19(6), Nov./Dec. 1998
changes in carbon concentration
result in a relatively insignificant
change for 75As intensities.
Normally, the analyses are carried out in aqueous solutions. The
normal analytical situation for food
matrices corresponds to zone 1 in
Figure 3, where the change in 75As
versus carbon concentration was
dramatic. This would explain why
As measured by ICP-MS can be
higher. This also correlates well
with the results presented in
Figure 2, which shows that the
measured As content increases
when the dilution factor decreases.
To avoid these changes in 75As
intensities, one could add known
quantities of isopropanol, corresponding to zone 2 or better to zone
4 of the isopropanol curve, where
the effects on 75As are insignificant.
Adding 2% of isopropanol to the
solution should stabilize the As
results. Adding 6% or more
isopropanol should improve the
stability of As results even more.
Nevertheless, for ICP-MS it is more
convenient to work with a low
organic solvent concentration. For
this reason, 2% of isopropanol was
added to all solutions.
Compared to Figure 2, the results
in Figure 4 show that adding 2%
isopropanol to the sample improves
both the As and Pb determination.
The results for Cd determination
remain, as expected, unchanged.
However, the determination of
mercury was unstable in 2% isopropanol. This is probably due to
electrostatic effects due to the
presence of alcohol.
To confirm the observations
obtained for the MET reference
samples, we extended this study to
other sample types such as apple
leaves, pine needles, corn bran,
infant food containing milk, peach
leaves, and rice flour.
For comparison, these samples
were measured successively in a
water solution and in 2% isopropanol.
The simultaneous determination was
extended to Se and Al (see Table II).
The results in Table II show that
the quantitative determination of
As and Pb was ameliorated in the
presence of 2% isopropanol. The Se
results were also better, although
not always well-correlated with the
certified values. This is probably
due to other interferences, which
cannot be corrected by isopropanol.
An amelioration of the repeatability
for As, Pb, and Se was also
observed. This shows that the isopropanol stabilizes the results by
stabilizing the carbon concentration
in solution.
The determination of Al and Cd
was not affected by the presence
of isopropanol.
The results show clearly that Hg
cannot be measured when isopropanol is added to the solutions. The
results obtained for the determination of Hg in normal conditions (in
water acidic solution) were also not
always good. This is especially the
case when Hg concentrations are
low. This is due to the well-known
Hg memory effect and will be the
subject of a future study.
The median of all RSDs was
calculated using the results of all
certified materials (see Table II).
These values, which can be interpreted as the in-house reproductibility, were always ≤ 5%.
Fig. 4. Arsenic, cadmium, mercury, and lead concentration measured in MET –
6/95 versus dilution factor in 2% isopropanol solutions.
217
TABLE II
Analysis of Heavy Metals in Different Types of Organic Certified Samples -Comparison Between Analysis Performed in Aqueous Soluitons (“Normal Analysis”)
and in 2% Isopropanol
Analysis N of A
Al
As
Cd
Hg
Pb
Se
N of A Al
As
Cd
Hg
Apple Leaves, NIST 1515
Peach Leaves, NIST 1546
Pb
Se
Cert-min
277,000
31
11
40
446
41
241,000
42
23
24
840
111
Cert-max
295,000
45
15
48
494
59
257,000
78
29
38
900
129
12
368,672
91
11
49
393
282
6
340,569
125
24
34
730
231
9
336,434
43
14
35
463
167
3
316,351
75
25
30
843
168
Error in water 12
±17,222
±5
±2
±2
±10
±9
6
±12,785
±10
±1
±2
±23
±39
Error in 2%
isoporpanol
±13,857
±1
±2
±1
±9
±3
3
±9052
±1
±1
±1
±24
±6
Water
Iso-OH 2%
9
Corn Bran, BCR 8433
Pine Needles, NIST 1575
Cert-min
460
0
7
2
106
37
515,000
170
n.c.
100 10,300
n.c.
Cert-max
1560
4
17
4
174
53
575,000
250
n.c.
200 11,300
n.c.
Water
3
510
3
8
46
104
54
6
591,141
207
183
78 10,144
72
Iso-OH 2%
9
636
0
13
32
138
45
6
590,780
206
188
64 10,448
65
Error in water 3
±84
±0
±1
±4
±16
±3
6
±4616
±5
±13
±2
±447
±6
Error in 2%
isoporpanol
±71
±0
±0
±3
±7
±1
6
±8903
±2
±12
±6
±147
±2
9
Infant Cereals Product, MET 2/95
Rice Flour, NIST 1568a
Cert-min
n.c.
408
460
417
160
n.c.
3400
260
20
5
n.c.
340
Cert-max
n.c.
639
531
472
339
n.c.
5400
320
24
6
n.c.
420
Water
6
n.a.
648
482
450
212
n.a.
6
3705
277
21
44
–16
347
Iso-OH 2%
3
n.a.
525
494
141
228
n.a.
6
3978
296
30
30
4
358
Error in water 6
n.a.
±7
±4
±7
±17
n.a.
6
±156
±3
±1
±3
±21
±10
Error in 2%
isoporpanol
n.a.
±3
±5
±3
±17
n.a.
6
±33
±3
±0
±2
±3
±2
3
Infant Cereals Product, MET 6/95
Cert-min
n.c. 1538
147
120
898
n.c.
Cert-max
n.c. 1769
218
175
1077
n.c.
Water
6
n.a.
2001
180
149
904
n.a.
Iso-OH 2%
6
n.a. 1719
186
–14
978
n.a.
Error in water 6
n.a.
±79
±2
±2
±25
n.a.
Error in 2%
isoporpanol
n.a.
±12
±1
±0
±11
n.a.
6
All results are expressed in mcg/kg;; n.c. = not certified; n.a. = not analyzed; N of A = Number of analyses.
218
Vol. 19(6), Nov./Dec. 1998
CONCLUSION
REFERENCES
A new method is proposed for
the analysis of food samples, which
minimizes the effect of carbon on
As, Se, and Pb by adding 2% isopropanol to the analytical solution.
1.
Meng-Fen Huang and H. Jiang,
J. Anal. At. Spectrom. 10, 31
(1995).
2.
L. Ebdon, J. Anal. At. Spectrom.
9, 611 (1994).
Cadmium can be determined
either in water or in isopropanol
with similar results. Mercury is better
determined in aqueous solution,
because of the poor repeatability and
accuracy observed in 2% isopropanol.
3.
J. Campbell, C. Demesmay and
M. Ollé, J. Anal. At. Spectrom.
9, 1379, (1994).
4.
P. Thomas, J. Anal. At. Spectrom.
10, 615 º1995).
5.
E.H. Larsen and S. St¸rup, J. Anal.
At. Spectrom. 9, 1099 (1994).
The simple sample preparation
suggested provides reliable precision and accuracy in the ICP-MS
determination of toxic minerals. It
can be applied to a wide variety of
food matrices and is well-suited for
routine food analysis.
219
Interferences in ICP-OES by Organic Residue
After Microwave-Assisted Sample Digestion
G. Knapp, B. Maichin, and U. Baumgartner
Technical University Graz, Institute of Analytical Chemistry, Micro- and Radiochemistry
Technikerstr. 4, A-8010 Graz, Austria
INTRODUCTION
Inductively coupled plasma
(ICP) is a highly robust excitation
source for emission spectrometry.
The emission can be observed
in a radial or axial configuration as
shown in Figure 1. ICP spectrometers with axial configuration have
better detection limits (by about
one order of magnitude), the
degree of precision is comparable
to radial view, and the linear
dynamic range is the same order
of magnitude, but tends towards
lower concentrations. On the
negative side, the axial arrangement
has greater matrix effects. The
OH bands are about three times
as intensive as with the radial configuration. In particular, a high concentration of organic compounds
leads to a pronounced optical background, which may interfere with
certain element lines.
Due to the increasing demand
for measurement methods that
provide high detection capabilities
for trace metal determination, an
increasing number of emission
spectrometers with axial ICP
arrangements are being offered
commercially.
State of the art for trace
element determination in organic
materials is an analytical procedure
comprised of a powerful
microwave digestion system (1)
together with a corresponding
measurement technique, namely,
an axial ICP emission spectrometer.
State of the art in sample decomposition is microwave-assisted wet
digestion with pure nitric acid in
closed pressurized vessels. Since a
high concentration of organic compounds (up to 20–30%) may remain
after microwave-assisted wet diges-
ABSTRACT
Interferences of the remaining
organic compounds after microwave-assisted wet digestion, using
axial ICP-OES measurement, were
investigated. Influences of the
organic residue on the measurement could be observed, particularly in the region of the detection limit. Also, the acid concentration of the digestion solution influences the signal intensity. Finally,
possibilities for solving these problems are discussed.
tion with low pressure vessels, we
have carried out experiments to
establish the extent of interferences
caused by the remaining organic
compounds during trace element
determination in organic sample
materials. In addition, the effect
of nitric acid on ICP-OES measurements was investigated, since a certain acid dependence is known to
occur in ICP emission spectrometry
(2).
EXPERIMENTAL
ICP Emission Spectrometry
Axial ICPs have been described
since the mid-1970s. However, this
technology did not become effective
until this decade, thanks to the
coupling of an axial ICP with an
Echelle spectrometer and a solid
state detector (3). For this study,
we used a Perkin-Elmer Optima
3000™ XL ICP-OES with an axial
plasma configuration.
Microwave-Assisted Pressure
Digestion
The sample digestion was
carried out using the Multiwave
Microwave Digestion System
(Perkin-Elmer/Paar). This
equipment permits the simultaneous measurement of pressure and
temperature in each of a maximum
of six digestion vessels (4). A 0.2-g
sample was digested using 3 mL of
concentrated nitric acid in 50-mL
quartz vessels. The automatically
controlled working pressure in the
digestion vessels was 72 bar. The
maximum temperature that can be
achieved using this technique
depends on the quantity of sample
and the sample matrix, and is about
250ºC at the given composition of
the digestion mixture. However,
the Multiwave can also be set to a
specific temperature, providing it
is lower than the maximum temperature achieved at 72 bar. In this
series of experiments, nicotinic
Fig. 1. ICP-OES with radial or axial observation.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
220
Vol. 19(6), Nov./Dec. 1998
acid was digested at 184ºC, 218ºC
and at 253ºC. Nicotinic acid was
chosen because this substance is
difficult to oxidize using nitric acid
and also because a high content of
residual carbon remains. The residual organic substances remaining
from the digestion process were
determined and given as TOC (total
organic carbon). The elements measured were arsenic at 188.979 nm
and selenium at 196.026 nm.
0.2 g of nicotinic acid was digested
using 3 mL of nitric acid at 184ºC,
218ºC, and 253ºC. The digestion
program consisted of three stages:
TOC Measurements
The remaining organic carbon
content was measured by means
of ASTRO Model 1850 total organic
carbon analyzer.
Stage 3: Cool to room temperature in 15 minutes.
RESULTS AND DISCUSSION
Preliminary experiments have
shown that of the elements As,
Cd, Co, Cr, Cu, Fe, Mn, Se and Zn,
more marked matrix effects were
displayed by As and Se. For this
reason, the experiments were
restricted to these two elements.
In later experiments, a greater number of elements was tested with
regard to interferences from matrix
constituents.
Stage 1: Ramp from 500 W in
5 minutes to 1000 W; the microwave energy is regulated back
automatically after the target temperature has been reached.
Stage 2: Hold at the target temperature for 30 minutes.
Table I lists the results of the
measurements, along with the corresponding TOC values for the
digestion solutions. The TOC values
are given as a percent of the quantity of carbon in the original sample
material. This clearly shows that
nicotinic acid is a very tough mater-
ial. At a digestion temperature of
184ºC, only about 10% is converted
into CO2, about 15% at 218ºC, and
about 60% at 253ºC. Earlier experiments have shown that for
complete oxidation of the organic
substances, which are hard to oxidize using nitric acid, temperatures
of at least 300ºC are required (5, 6).
This is currently only possible using
the HPA-S High Pressure Asher™
system (Perkin-Elmer/Paar).
The results in Table I show that
the theoretical value is approached,
and can be achieved, with decreasing TOC content. However, it
should be kept in mind that the
element concentrations given are
close to the detection limit. Analyses in nicotinic acid with ten times
the analyte concentration no longer
show this relationship to the residual carbon content. In summary,
The relationship of the signal
intensity to the nitric acid concentration was investigated first. Solutions of 500 ng/mL each of As and
Se were measured at the following
nitric acid concentrations: 5, 10,
20, 50% by volume (see Figure 2).
The digestion solution prepared
for the measurement (dilution 1:5
to 1:10) was in a concentration
range in which the variations in the
concentration of the nitric acid has
no influence on the result for measuring arsenic. When measuring
selenium, however, it is essential
to ensure that the acid concentration is the same for the standard
and the sample solution.
For testing the influence of the
remaining organic compounds,
nicotinic acid spiked with arsenic
(1.5 µg/g) and selenium (2.5 µg/g)
was digested and measured. Each
Fig. 2. Measurement of As and Se solutions with various nitric acid concentrations
at 188.979 nm (As) and 196.026 nm (Se). The emission intensity is given in % of
the signal height of the aqueous standard solution.
TABLE I
Measurement for Arsenic and Selenium
After Digestion of Nicotinic Acid at Various Temperatures
Digestion Temp. (ºC)
As (µg/g)
Se (µg/g)
TOC (%)
184
0.88 ± 0.0.6
3.9 ± 0.17
90 ± 3
218
1.20 ± 0.10
4.0 ± 0.36
85 ± 5
253
1.50 ± 0.09
3.0 ± 0.45
40 ± 8
Arsenic content: 1.5 ug/g; selenium content: 2.5 µg/g. The TOC value is the remaining organic carbon content in % of the original carbon content.
221
the most complete digestion of
the organic sample matrix is recommended if the full detection capabilities of the axial ICP emission
spectrometer are to be exercised.
CONCLUSION
Microwave-assisted wet digestion in closed vessels is state of the
art for sample decomposition in
elemental analysis. For complete
oxidation of organic materials with
pure nitric acid, temperatures of
more than 300°C are necessary.
The digestion temperature in closed
microwave heated vessels depends
on the pressure. Average temperatures in low pressure vessels at
20 bar are about 160–180ºC, and in
high pressure vessels at 75 bar they
are about 220–250ºC. At a given
pressure, the temperature depends
also on the sample weight and the
vessel volume. For axial ICP-OES
determinations at low concentration ranges, it is always better to
oxidize the organic sample as completely as possible. Therefore,
a microwave digestion system with
high pressure vessels is the better
choice. For higher element concentrations, low pressure vessels are
useful as long as the sample is dissolved completely. In this case,
organic residues of about 20–30%
are normally no problem.
222
REFERENCES
1.
Microwave Enhanced Chemistry,
S. Kingston, S. Haswell, Eds., American Chemical Society Professional
Book Series, ACS: Washington,
D.C. (1997).
2.
P. Schramel, J. Ovcar-Pavlu; Fresenius Z. Anal. Chem. 298,
(1979)28.
3.
J.C. Ivaldi, J.F. Tyson; Spctrochim
Acta Part B 50, 1207 (1995).
4.
M. Zischka, P. Kettisch, A. Schalk,
G. Knapp, Fresenius Z. Anal.
Chem. 361, 90 (1998).
5.
M. Würfels, E. Jackwerth, and
M. Stoeppler; Fresenius’ Z. Anal.
Chem. 329, 459 (1987).
6.
M. Würfels; LABO 3, 7 (1989)
Microwave-Assisted Digestion of Plastic Scrap:
Basic Considerations and Chemical Approach
Michael Zischka, Institute for Analytical Chemistry, Micro- and Radiochemistry
Graz Technical University, Technikerstr. 4, A-8010 Graz, Austria
Peter Kettisch, Anton Paar GmbH, Kaerntner Str. 322, A-8054 Graz, Austria
Peter Kainrath, Bodenseewerk Perkin-Elmer GmbH, Postfach 10 17 61, D-88662 Überlingen, Germany
INTRODUCTION
Plastic scrap, predominantly
household waste such as Tetra-pack
bottles or packing foils, is used as
an auxiliary fuel in rotary kilns in
the cement industry. The material
can replace up to 20% of the primary fuel in the clinker burning
process. The plastic material, delivered in big bales, is shredded to a
particle size of 10 to 15 mm. This
scrap has to be analyzed regularly
for several metals that may cause
environmental problems or influence the cement product quality.
These metals originate from coloring agents, fillers, and stabilizers
that are added during the plastic
production.
The difficulties of the determination arise mainly from the sample
preparation step. Mixed plastic
waste is a cumbersome sample: due
to different origins, it shows great
inhomogeneity, a strongly changing
composition, and an unpredictable
reaction behavior depending on the
kind and quantity of additives. With
inadequate decomposition methods, often an insoluble residue
remains or the organic matrix is
not destroyed completely. Both
cases can cause incorrect analytical
results. Some additives form very
reactive intermediate products during wet ashing procedures, leading
to violent reactions. Pressure peaks
may occur during microwave digestion if samples with a low melting
point lump together. Under further
microwave irradiation, these lumps
will burst violently. Thus, safety is
of paramount importance.
AS
Atomic Spectroscopy
Vol. 19(6), November/December 1998
ABSTRACT
The accurate determination
of heavy metals in plastic scrap is
strongly influenced by the selection of the right sample digestion
method. The difficult and inhomogeneous nature of this kind
of sample material and its unpredictable reaction behavior are the
major obstacles in getting correct
analytical results. A sophisticated
microwave digestion instrument
that guarantees a controlled reaction in high-performance vessels
and the choice of the right
reagent mixture are key factors
for the complete mineralization
of this complex matrix. For the
determination of Cd, Cr, and Pb,
different chemical approaches
have been investigated in order
to find a universally applicable
method that is safe, reliable, and
can be used for daily routine
analysis.
Basic Requirements and
Considerations for the Sample
Preparation Step
Right choice of reagents
Plastic waste material is a complex mixture of organic and inorganic compounds for which an
appropriate reagent mixture must
be found. A simple approach with
only one acid does not work. The
organic matter must be destroyed
with oxidizing reagents, commonly
nitric acid, HNO3 (2). For the complexation of elements such as Fe,
Al, Cr, and Sb, often a small amount
of hydrochloric acid has to be
added. Hydrofluoric acid is used for
the complete dissolution of the
inorganic constituents. It also acts
as a complexing agent for several
223
metals. Before analysis, HF-containing sample solutions require
a special post-treatment which is
described later.
The use of sulfuric acid is well
known with open vessel procedures for oxidizing the organic matter and increasing the boiling point
of an acid mixture, which is a benefit in pressure-controlled closed vessel applications. A disadvantage of
H2SO4 is that some elements form
insoluble sulfates, resulting in losses
of analytes. Chemical interference
can be a problem with GFAAS.
Highest possible temperature
Basically, the reaction speed and
the efficiency of sample decomposition can be improved by a temperature increase. As a rule of thumb,
a temperature increase of 10ºC
doubles the reaction speed.
In pressure-controlled operation,
the temperature in the vessel is
dependent on the reagent mixture
and on the amount of gaseous products formed during the decomposition process. For mixed plastic
materials, the maximum oxidation
potential and, therefore, the highest
possible temperature must be
achieved. For this reason, it is beneficial to run the digestions at the
highest temperature allowed by the
vessel design.
Safe and reliable vessels
For the total digestion of plastic
scrap, an HF-resistant vessel system
has to be used. Usually, fluoroplastics (PTFE or PFA) are used, which
have temperature limitations. For
difficult-to-digest samples, it makes
a considerable difference whether
the practical temperature limit of a
vessel is 200ºC or 260ºC. Attention
must be paid to the fact that
frequent use of high temperatures
may reduce the lifetime of the vessels. The goal is to find a method
that is suitable for routine analysis
and does not result in excessive
vessel wear.
EXPERIMENTAL
Sample Digestion
A Multiwave microwave digestion system (Perkin-Elmer/Paar)
was used for the sample preparation. The system is equipped
with a six-position rotor, highperformance pressure vessels,
and simultaneous pressure and temperature control in all vessels. The
unique sensor design, which has
been described in previous works,
provides safe handling of even
“critical” samples (1,4). The vessels
were of PTFE/TFM, with operating
conditions up to 260ºC and 80 bar.
With this type of reaction vessel
(Figure 1), it was possible to
develop a method for plastic scrap
decomposition, which is suitable
for daily routine analysis.
ICP-OES Instrumentation
For the determination of the
elements, a Perkin-Elmer Optima
3000™ inductively coupled plasma
optical emission spectrometer
(ICP-OES) was used with an axial
configuration and a cross-flow
nebulizer.
Decomposition Method
Development
Samples
Two types of sample material
with a different inorganic composition were investigated:
K1: Mixed plastic with low content of inorganic components
(<10%).
K2: Mixed plastic with high content of inorganic components
(<30%).
Fig. 1. High-performance compound vessel.
The samples were chopped to
flaky, voluminous consistency. In
mixed waste applications, it is
always difficult to obtain a homogeneous and representative sample,
particularly in this case where the
applicable amount was limited to
200 mg. An optimization for higher
sample weights may be possible.
The samples were weighed into the
TFM liners and the liners placed
into the ceramic supporting vessels.
Reagents
Since the goal of this work was
to find the right chemical approach,
different reagents and reagent mixtures were tested (Table II). Suprapure subboiling acids (Merck) were
used exclusively.
The acids were added subsequently, starting with HNO3 and
ending with H2SO4. Special care
was taken to rinse the sample particles from the reaction vessel walls.
The vessels were closed with the
lip-type seals and the rotor was
prepared in accordance with the
operating instructions (6).
224
Digestion Program
All tests were performed with
one microwave power profile:
Power
Time
Power
Fan
(W)
(min)
200
1000
0
10
35
15
(W)
1000
1000
0
0
0
3
The resulting solutions were
clear and green in color. A fine
white precipitate of undissolved
inorganic compounds was
observed, but no residues from
unmineralized organic material
were present. The samples were
quantitatively transferred and
diluted to volume.
Complexation
For samples decomposed with
hydrofluoric acid, the solutions
were treated with saturated boric
acid. As a rule of thumb, 5 mL
saturated boric acid should be
added for each 1-mL HF in the
decomposition solution (7). After
the addition of boric acid, the vessels were closed and a short run
was performed on the Multiwave
Vol. 19(6), Nov./Dec. 1998
TABLE II
Reagents and Reagent Mixtures Tested
Mixture HNO3
(65%)
D
Remarks
microwave system. The samples
were then quantitatively transferred
and diluted to volume.
HCl
(30%)
HF
(40%}
H2SO4 Operating
(90%) pressure
1 mL
—
30 bar
Sulfuric acid omitted for
chemical reasons, complexation step following
Note: When this residue does
not consist of inorganic compounds,
it will not be dissolved by adding
boric acid.
Power
(W)
Time
(min)
Power
(W)
Fan
500
1000
0
5
10
15
1000
1000
0
0
0
3
A
6 mL
0.5 mL
B
6 mL
0.5 mL —
—
70 bar
Destruction of organic
material in quartz vessels
C
6 mL
—
1 mL
2 mL
30 bar
Destruction of organic
material at elevated temperature with additional attack on
organics, without the
complexation step
6 mL
—
1 mL
2 mL
30 bar
Destruction of organic
material at elevated temperature with additional attack on
inorganics, followed by the
complexation step
The procedure produces a clear,
colorless solution, free from precipitates. The samples were transferred into 25-mL measuring flasks,
as described before, and diluted
as needed.
Measurement process /
calibration
The operating conditions using
the Optima 3000™ XL ICP-OES are
listed in Table I. The measurements
were carried out at the following
wavelengths: 214.438 nm and
228.802 nm for Cd; 205.560 nm
and 257.716 for Cr; and 216.999
nm and 220.353 nm for Pb.
TABLE I
ICP-OES Operating Parameters
Fig. 2. Comparison of results obtained for Cd concentration in plastic scrap materials using different acid mixtures (reference values of Cd obtained by three independent laboratories by aqua regia leaching in open vessels and by dry ashing varied
from 5 to 36 µg/g for sample K1 and from 5 to 25µg/g for sample K2).
RF Power
1400 W
Plasma Ar gas flow
15 L/min
Auxiliary flow
0.5 L/min
Nebulizer flow
0.8 L/min
Sample flow rate
1.4 mL/min
Replicate measurements
3
Signal processing
Peak Area 3 pixels
The calibration was carried out
with a blank solution and four standard solutions within the linear
range of the calibration function.
The blank solution and the standard
solutions were prepared from a
matrix-matched solution of the
225
Fig. 3. Comparison of results obtained for Cr concentration in plastic scrap materials using different acid mixtures. (Reference values of Cr obtained by three independent laboratories by aqua regia leaching in open vessels and by dry ashing
varied from 50 to 120 µg/g for sample K1 and from 25 to 180 µg/g for sample K2.)
Fig. 4. Comparison of results obtained for the determination of Pb in plastic scrap
materials using different acid mixtures. (Reference values of Pb obtained by three
independent laboratories by aqua regia leaching in open vessels and by dry ashing
varied from 50 to 120 µg/g for sample K1 and from 20 to 85 µg/g for sample K2.)
same ionic strength, acid concentrations, and boric acid concentration
as the samples. Scandium was
added as the internal standard to
each sample solution.
DISCUSSION AND RESULTS
Results for Cd
For the recovery of Cd, the type
of reagent mixture seems less
important, only the nitric acid/
hydrochloric acid decomposition
even under the highest possible
pressure of 70 bar shows lower
Cd results (Figure 2). The same
behavior can be expected for elements with similar properties like
Hg or Tl. These elements are easily
soluble in nitric acid and form no
insoluble sulfates. Therefore, the
only prerequisite for a complete
recovery is the total digestion of the
sample material.
Results for Cr
In inorganic compounds, Cr is
often found in oxide form. When
226
nitric and hydrochloric acid is used
without hydrofluoric acid, a complete recovery of Cr can only be
obtained in samples with a low
inorganic content (sample material
K1) (Figure 3). In materials with
a greater amount of inorganic compounds (sample material K2), this
is not the case. Therefore, low
recoveries of Cr occur if hydrofluoric acid is omitted. Another reason
for low recoveries is the fact that
Cr is not soluble in nitric acid and
often reacts to form insoluble
oxides that can be adsorbed on
quartz vessel walls.
Note: Such adsorption effects
can appear at vessel walls independent of the quartz vessel material
used.
Results for Pb
For complete recovery of Pb,
acid mixture A is best suited
(Figure 4). The mixture of nitric
and hydrochloric acid (reagent B)
might be acceptable in some cases.
Lead is known to form insoluble
sulfates in digestion applications,
including H2SO4, as can be seen in
this work. Reagent mixtures containing sulfuric acid (mixtures C
and D) show significantly lower Pb
concentrations. The use of boric
acid for the complexation of insoluble fluorides shows that a great
amount of lead cannot be dissolved
because of the conversion into
insoluble PbSO4 during the digestion step (mixture C). The mixture
of nitric and hydrochloric acid
(reagent B) might be acceptable in
some cases, but acid mixture A is
best suited. Sulfuric acid, which is
used traditionally in open vessel
digestions to increase the boiling
point in order to achieve a
complete decomposition of organic
compounds, has to be avoided in
closed vessel digestion if Pb has to
be determined.
Vol. 19(6), Nov./Dec. 1998
CONCLUSION
For complete recovery of the
elements Cd, Cr, and Pb in mixed
organic-inorganic sample materials,
it is essential to select a balanced
reagent mix, a high-performance
closed vessel decomposition system, and a special sample posttreatment. Upon comparison of the
different wet chemical procedures,
only procedure A is capable of the
simultaneous determination of the
elements studied after the digestion
of the mixed samples. Simplification of the chemical approach may
be allowed if only single elements
have to be determined. In order to
obtain a complete digestion by the
use of mixture A, a digestion system
is required that allows operation at
the necessary high temperatures
over an extended period of time.
In comparison with dry ashing and
open digestion methods, it can be
seen that the combination of mixture A with a high-performance
digestion system leads to much
more reliable and precise results.
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1.
G. Knapp, F. Panholzer, A. Schalk,
and P. Kettsich, in MicrowaveEnhanced Chemistry, H.M.
Kingston and S. Haswell, Eds.,
American Chemical Society Professional Book Series; ACS: Washington, DC USA (1997).
2.
K.D. Besecker, C.B. Rhoades Jr.,
B.T. Jones, and K.W. Barnes, At.
Spectrosc. 19 (2), March/April
(1998).
3.
E. Sucman, M. Sucmanova, O. Celechovska, and S. Zima, CAS (1991).
4.
M. Zischka, P. Kettsich, A. Schalk,
and G. Knapp, Fres. J. Anal. Chem.
361, 90 (1998).
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H.M. Ortner, H.H. Xu, J. Dahmen,
K. Englert, H. Opfermann, and W.
Goertz, Fres. J. Anal. Chem. 355,
657 (1996).
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Anton Paar GmbH, Multiwave
Instruction Handbook.
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G. Knapp, ASA: Private communcation.
227