1. technology and computing

Accurate Determination of Titanium as Titanium
Dioxide for Limited Sample Size Digestibility Studies
of Feed and Food Matrices by
Inductively Coupled Plasma Optical Emission Spectrometry
With Real-Time Simultaneous Internal Standardization
Wim van Bussel*, Francien Kerkhof, Theo van Kessel, Henk Lamers, Dorke Nous,
Han Verdonk, and Biek Verhoeven
CCL Nutricontrol, P.O. Box 107, 5460 AC Veghel, The Netherlands
Nik Boer and Hans Toonen
PerkinElmer, Inc., P.O. Box 5205, 9700 GE Groningen, The Netherlands
INTRODUCTION
Use of inert markers for the measurement of digestibility is a less
labor-intensive method than total
faecal collection. Apart from the
elimination of errors in obtaining
exact measurements of feed intake
and total faeces output in the traditional total collection method, the
use of markers to determine nutrient digestibility of feeds in animal
species would fit into animal welfare considerations. Animal welfare
studies should be designed to use
the minimum number of animals
required to achieve the objectives
of the study. The appropriate number of animals depends on several
factors including effective size (e.g.,
difference in means between two
groups), variability of data (e.g.,
standard deviation), desired significance level (probability of finding a
significant result by chance when
there is really no effect, usually 5%),
desired power (probability of finding a significant result when it actually exists), and the minimum
required sample size for the analytical laboratory to perform the analyses. This likely depends on the
requested parameters such as main
nutrients and amino acids, dry matter, ash content, crude protein,
crude fat, starch, total carbohydrates, minerals, and inert markers
such as TiO2, Cr2O3, or Y2O3 (1).
For digestibility studies, animals are
*Corresponding author.
E-mail: wim.van.bussel@ccl.nl
Atomic Spectroscopy
Vol. 31(3), May/June 2010
ABSTRACT
Inductively coupled plasma
optical emission spectrometry
(ICP-OES) offers excellent possibilities for the quantitative high
precision analysis of feed and
food. In the present study, an
analytical method was developed
for the simultaneous determination of titanium as titanium dioxide when only a limited sample
size is available.
Sample preparation was performed by wet acid digestion.
In-house standard nutrient reference materials (RMs) were prepared to verify the accuracy and
precision of the quantitative
method. Improved analytical
results in terms of precision and
accuracy were obtained using
real-time simultaneous internal
standardization.
The final result shows a quantitative method for the selected
element with a precision of typically 1% or better and RSDs of
0.04–0.10%. Detection limits
were in the range of
0.003–0.008 mg/kg. Wide working ranges (ppb to ppm range)
and low detection limits (ppb)
were obtained for the ionic lines
investigated (Ti 334.940 nm, Ti
336.121 nm, Ti 337.279 nm, Ti
334.903, and Ti 368.519 nm).
The analytical procedure developed provides a quick, sensitive,
precise, and economic method
for the simultaneous determination of titanium dioxide, even
when only a limited sample size
is available.
81
often pooled for an adequate and
representative sample size. Ileum of
several rats are often pooled for an
adequate sample size for analysis.
TiO2 is a well-known accepted
digestion marker for all kinds of
veterinary and digestibility studies.
Chromic oxide (Cr2O3) and
titanium dioxide (TiO2) have been
widely used as dietary markers in
animal digestibility studies (1–4).
Jagger et al. (2) demonstrated that
Cr2O3 had a lower faecal recovery
(quantity collected from a total collection of faeces expressed as a proportion of that ingested, an
important indicator of the marker
reliability) than TiO2 (75% versus
98%), and that TiO2 induced lower
standard errors for apparent ileal N
digestibility than Cr2O3 (3). Overall,
TiO2 has been suggested as a more
appropriate marker for animal
digestibility studies. Acid-insoluble
ash or celite (diatomaceous earth)
has also been proposed as a reliable
marker which can be used for both
animals and humans (4–6). However, the large amount of digesta
sample (1.5−2 g) required for analytical determination is a limiting
factor (1993). Jagger et al. (2) compared Cr2O3, TiO2, and acid insoluble lignin as inert markers in
determining the nutrient digestibility in pigs. They found that the
smallest difference between faecal
CCL-Nutricontrol is a leading laboratory in
the field of feed and food analyses in The
Netherlands and RvA accredited – Dutch
Accreditation Council - ISO 17025.
digestibility of nitrogen and amino
acids can be determined by total
faecal collection and by the use of
markers for TiO2 with a recovery
rate of 97%. They concluded that
the most appropriate marker to use
in digestibility studies was TiO2 (2).
An accurate and reproducible
method for TiO2 determination was
proposed by Short et al. (7). Many
spectrophotometric methods for
the determination of titanium are
available, but most of them are not
reliable when applied to amounts
below microgram levels in biological samples. Other reported methods are more sensitive, but have
lower selectivity. Furthermore, all
spectrophotometric methods for
the analysis of biological tissue
include excessive sample preparation to cope with the complex
matrix and are, therefore, very
time-consuming (8–10). Atomic
spectroscopic techniques, such as
flame atomic absorption spectrometry (FAAS) and electrothermal
atomic absorption spectrometry
(ETAAS) have also been used, but
they are not sensitive enough, have
severe memory effects, or (as in the
case of ETAAS when transient signals are integrated) precision is
poor (11,12). Although few food
analysts currently employ inductively coupled plasma optical emission spectrometry (ICP-OES), the
multi-element capability, wide
dynamic range, and high sample
throughput are attributes that will
prove beneficial to analysts striving
to perform these determinations.
An axially viewed plasma provides increased sensitivity and
improved detection limits
compared to a traditional, radially
viewed plasma. Inherent to axial
viewing are certain disadvantages
requiring consideration. Physical
effects that are intrinsic to radially
viewed plasma are magnified when
the plasma is viewed axially. The
progressive addition of more material to the ICP can cause undesirable
enhancements or suppressions of
the analyte signal, depending on
the nature of the affecting matrix
and type of emission monitored.
More serious in nature are matrix
effects that occur in the plasma
related to the excitation potential
of the analyte wavelength in question. As the concentration of an
interfering element increases, most
analytes affected by the matrix
effect will show a sensitivity
decrease (13). Viewing a plasma
axially extends the source path
length, thereby increasing analyte
emission intensity and improving
sensitivity. This sensitivity enhancement typically results in a 5- to 10fold improvement in detection
limits over radially viewed plasmas.
With this configuration, elements
can be determined at levels previously only attainable with ETAAS.
Radial viewing of a plasma offers
the advantages of increased linearity, reduced easily ionizable
element effects, lower physical interferences, and fewer spectral interferences.
A dual-view plasma offers the
best of both worlds resulting in
high analytical sensitivity for minor
components and extended linear
dynamic range sufficient to allow
the accurate determination of major
components. In practice, an analytical methodology is often best developed for specific elemental groups,
taking into account the matrix and
method of sample preparation
(14,15).
The objective of this study was
to improve the determination of
titanium by applying real-time
simultaneous internal standardization and to minimize sample weight
from an animal welfare point of
view. Matrix-matching of elemental
and acid composition was
performed to minimize spectral and
physical interferences. An evaluation of precision improvement
using real-time internal standardization with a dual-view inductively
coupled plasma (ICP) is presented.
82
EXPERIMENTAL
Instrumentation
All measurements were
performed using a PerkinElmer
Optima™ 7300DV ICP-OES
(PerkinElmer, Inc., Shelton, CT,
USA), equipped with standard
torch assembly, low-flow
GemCone™ nebulizer, cyclonic
spray chamber, and S10 autosampler. The Optima series of
spectrometers uses two segmented
charge coupled device (CCD)
detectors (SCDs), one for the UV
section and the other for visible
light. The detector and optics are
described in detail elsewhere (16).
The Optima 7300DV has a horizontal torch with a choice of axial or
radial view.
A GemCone nebulizer was used
because lower nebulizer gas flow
rates are useful for obtaining better
results with spectral lines having
high excitation energies. The nebulizer also reduces matrix effects by
providing more robust conditions
in the center channel of the ICP.
The tip of the low-flow GemCone
nebulizer is based on the original
conespray design by Sharp (17).
Thus, this nebulizer falls into the
general class of nebulizers known
as Babington or high solids style
nebulizers. The main characteristic
of Babington nebulizers is that the
solution flows over the surface containing the gas orifice, thereby minimizing the chances of blockage by
precipitation of the dissolved solids
or from suspended matter in the
sample (18).
The Optima 7300DV operating
conditions are listed in Table I.
The wavelengths, excitation, ionization, and total energies for the
selected emission lines are listed in
Table II (19). No attempt was made
to optimize the plasma conditions
for any particular analyte or to optimize the procedure for best sample
throughput.
Vol. 31(3), May/June 2010
TABLE I
Operating Conditions of the
Optima 7300DV Radial ICP-OES
RF Power
1450 W
Nebulizer Flow
0.62 L min-1
Auxiliary Flow
0.20 L min-1
Plasma Flow
15 L min-1
Sample Flow
1.5 mL min-1
Source Tquilibration
Time
15 sec
Viewing Height
15 nm
Background Correction
Manual
Measurement
Processing Mode
Peak Height
Integration Time
Manual
Read Delay
45 sec
Rinse Delay
45 sec
Number of Replicates
7
Several ionic lines were selected
because the behavior of the atomic
lines is more complex, even under
robust conditions. In addition, the
ion lines are, in general, the most
sensitive lines for more than half
of the elements that can be determined by ICP-OES.
In this study, the integration
and read times for each line were
set manually to ensure a fixed total
read time per replicate and thus
allow a constant total measurement
time for all experiments. This was
done by first collecting the autointegration or pre-shot data. Manual
integration was then selected by
entering the auto-integration settings for the largest signal of all
emission lines, including the internal standard reference lines.
Real-Time Internal
Standardization
The Optima series of ICPs uses
an intelligent algorithm to prevent
over-range signals. Measurement
time is adjusted simultaneously
for each wavelength in order to
achieve optimum integration parameters and best signal-to-noise ratio
(S/N), referred as the signal-to-back-
Table II
Excitation, Ionization, and Total Energies
for Selected Emission Lines (19)
Line
Ti II
Ti II
Ti II
Ti II
Ti II
Yb II
Yb II
Wavelength (nm)
334.940
336.121
337.279
334.903
368.519
328.937
369.419
Eexc/eV
3.70
3.69
3.68
3.70
3.37
3.77
3.36
ground ratio (SBR). The segmented
array CCD (charge-coupled device)
detector (SCD) allows for the simultaneous measurement of multiple
lines and their spectral backgrounds.
This is conducted with 224 photodetector arrays, each having 20
to 80 pixels, covering the ICP spectrum from 167 to 782 nm. The full
well capacity of the SCD pixel
places a limit on the maximum
charge that can be read out for a
given integration time.
Eion/eV
6.82
6.82
6.82
6.82
6.82
6.25
6.25
Esum/eV
10.52
10.51
10.50
10.52
10.19
10.02
9.61
The sample for analysis was prepared with an internal standard and
quantified using the ICP-OES. Ytterbium was selected as the internal
standard because it emits at a wavelength close to titanium and has
first ionization potential almost the
same as titanium. Three in-house
fortified reference materials were
also analyzed.
sent at reasonably high levels. Ionization interference tends to cause
a reduction in signal intensity with
increasing concentrations of EIEs;
the effect is prominent at interferent concentrations at or above
100 mg/L. The atomic lines of Na,
K, and to a lesser extent Ca, exhibit
signal enhancement with increasing
concentrations of EIEs. The effect
can be easily minimized or eliminated on a radially viewed ICP-OES
by adjusting the viewing height.
For the more sensitive axially
viewed ICP-OES, reports of interferences due to EIEs have been
described (20-23). Reducing the
nebulizer pressure and increasing
the RF power has been reported
(24) to reduce ionization interference on the axially viewed ICPOES. Scandium as an internal
standard has also been found to
compensate for part of the signal
depression. Generally, when analyzing samples that contain high levels
of EIEs, it is recommended that all
standards have similar levels of EIEs
added (matrix matching). An alternative is to saturate the plasma with
a high concentration of another EIE
such as caesium. Therefore, the
effect of adding caesium as an ionization buffer to the standards and
samples was also investigated.
A major element in food and
feed products is sodium, which is
an easily ionized element (EIE) and
has been reported (20-23) to cause
ionization interference when pre-
For this study, caesium was chosen as an ionization buffer since it
has low ionization energy, is not
very sensitive in ICP-OES analysis
and, therefore, spectral interfer-
If the full well capacity of the
SCD pixel is surpassed, the pixel
will saturate and the conversion of
charge to signal will be degraded.
Therefore, appropriate integration
times must be chosen for the
selected emission lines. In order to
obtain the lowest possible RSDs,
the integration times were set manually to ensure real-time simultaneous measurements.
83
ence is generally not a problem.
Caesium chloride is available in
very pure form and does not build
up in the torch injector tube as
readily as with other alkali salts.
Reagents and Standard
Solutions
All chemicals used were of analytical grade (Merck, Darmstadt,
Germany). Deionized water with a
specific resistivity of 18.2 MΩ cm-1
(Milli-Q™ gradient A10, Millipore
Corporation, Bedford, MA, USA)
was used to prepare the samples
and standards. Single-element standards were prepared from
PerkinElmer single-element stock
solutions.
Sample Preparation
Samples, varying in weight from
25 mg to 500 mg, were digested
with 4 mL H2SO4 and 2 mL H2O2 in
closed 50-mL quartz vessels using a
Multiwave® 3000 microwave digestion system (Anton Paar Graz, Austria). After cooling, 0.50 mL internal
standard reagent was added and
made up to 50-mL volume with DI
water. In order to avoid acid interference effects, the acid concentration of all solutions was identical to
that in the digested samples.
RESULTS AND DISCUSSION
Choice of Internal Standard
When external calibration is performed, samples and standards must
be closely matched, otherwise, there
is an enhancement of matrix effects
that can profoundly influence the
results. The principal cause of
matrix suppression or enhancement
is a critical dependence of the total
dissolved solids (TDSs), not of dissolved solids and other physical
properties of the sample. Matrix
effects that influence the physical
position of desolvation or ionization
result in matrix-induced suppressions or enhancements. Matrix
interferences of this type are “easily”
corrected by making use of internal
standardization. In this work, ytterbium with two prominent ion lines
(328.937 nm and 369.419 nm) was
used. The use of internal standards
to correct for matrix effects is well
known and can be tailored to correct for relatively large changes in
observed intensities caused by the
properties of the matrix.
The individual replicates were
exported to Microsoft® Excel®
program by using the data manager
utility of the PerkinElmer Winlab32™
software. This allows the user to
Fig. 1. Intensity signal profile Yb II at 328.937 nm vs. replicate.
84
visualize the trend over time for
both the internal standards and the
analytes. A similar trend can be
observed for one of the internal
standards (in comparison with one
of the five ion lines studied) which
will result in the best analytical precision.
Figures 1-7 indicate the nature of
noise in the analytical signals from
the radial ICP. Figures 1 and 2 show
the intensity signal versus replicate
for the internal standard ytterbium.
Figures 3-7 show the intensity profile for the five ionic lines studied.
It can be observed that a high
degree of correlation exists in the
line signals from the ICP. Using the
ytterbium ion line at 328.937 nm
(Figure 1) as the internal standard,
the analytical precision after the
application of real-time internal
standardization is maintained
between 0.03% and 0.13% relative
standard deviation (RSD) for almost
all ion lines, except for Ti at
334.940 nm. Figure 3 shows that
the best fit for this line was found
with Yb 369.419 nm (Figure 2).
Precision improvement factors of
3 to 4 were obtained by comparing
the uncorrected results. Thus, realtime internal standardization provides significant improvements in
the RSDs of the line signals. How-
Fig. 2. Intensity signal profile Yb II at 369.419 nm vs. replicate.
Vol. 31(3), May/June 2010
Fig. 3. Intensity signal profile Ti II at 334.940 nm vs. replicate.
Fig. 4. Intensity signal profile Ti II at 336.121 nm vs. replicate.
Fig. 5. Intensity signal profile Ti II at 337.279 nm vs. replicate.
Fig. 6. Intensity signal profile Ti II at 334.903 nm vs. replicate.
ever, this is not possible for all elements by using
only a single internal standard signal. The effectiveness of real-time internal standardization is
shown to be dependent on the nature of the specific spectral line. In inductively coupled plasma
optical emission spectrometry, matrix effects and
drift are usually caused by two major factors,
namely changes in the energy transfer between
the plasma and sample and changes in the efficiency of sample aerosol formation and
transport. It is also shown that in general when
using only single correction, coefficient ‘perfect’
correction is impossible with traditional internal
standardization. The model developed can be
used to quantitatively evaluate the efficiency of
internal standardization to reduce matrix effects
and drift.
Fig. 7. Intensity signal profile Ti II at 368.519 nm vs. replicate.
85
TABLE III
Results Obtained for the Analysis of In-house Reference Materials
Analyte
RM Ti
Found Ti
Recovery
RSD
(nm)
(mg/kg)
(mg/kg)
(%)
(%, n=7)
Ti 334.940
1676
1664
99.28
0.13
Ti 336.121
1676
1673
99.82
0.05
Ti 337.279
1676
1675
99.94
0.05
Ti 334.903
1676
1672
99.76
0.06
Ti 368.519
1676
1675
99.94
0.13
Ti 334.940
1810
1791
98.95
0.12
Ti 336.121
1810
1806
99.78
0.07
Ti 337.279
1810
1807
99.83
0.04
Ti 334.903
1810
1808
99.89
0.06
Ti 368.519
1810
1814
100.22
0.11
Ti 336.121
3352
3347
99.85
0.12
Ti 337.279
3352
3350
99.94
0.03
Ti 334.903
3352
3354
100.06
0.04
Ti 368.519
3352
3348
99.88
0.09
Quantitative Analysis
Correlation coefficients of
r=0.9999 or better were obtained
for all five calibration curves (see
Figures 8-12).
Since there is no certified reference material available for
nutrients, the accuracy of the proposed method was evaluated by
recovery tests. The accuracy was
measured by the recovery of inhouse made standard reference
nutrient material (RM). Three test
feeds were supplemented with
2796 mg TiO2, 3020 mg TiO2, and
5593 mg TiO2 per kg. The mean
results (n=7) obtained from the
individual and fortified nutrients are
summarized in Table III. A typical
example is presented in Table IV
and shows the obtained RSDs after
implementation of the correct internal standard correction for the
appropriate titanium ion line (as
mentioned above).
The results obtained for the
three in-house reference materials
were precise and accurate. Day-today repeatability was better than
0.0112% m/m. Within-day repeatability was 0.0074% m/m. The measured recoveries were in excellent
agreement with the RM values for
all five ion lines studied. The limit
of detection for the analytes was
determined using the abovedescribed conditions. The standard
deviation intensities were measured
in a non-spiked and spiked 0.3 w/w%
TiO2 nutrient, resulting in detection
limits ranging from 0.01–0.03 µg/kg.
The ICP-OES technique developed
in this study has been shown to
give accurate and precise results in
the determination of titanium when
a proper sampling pretreatment
procedure is applied.
TABLE IV
Typical Example of Obtained RSDs
Analyte
Mean
Calibration
Std.
Sample
(nm)
Corrected
Concen.
Dev.
Concen.
Intensity
Yb 326.937 74084.7
96.1%
0.37
Std.
Dev.
RSD
0.39%
Yb 369.419
62611.8
96.3%
334.94RADa
241343.3
9.175 mg/L 0.0121
1664 mg/kg
2.2
0.13%
336.12RADa
207673.4
9.225 mg/L 0.0050
1673mg/kg
0.9
0.05%
337.27RADa 197294.7
9.236 mg/L 0.0049
1675 mg/kg
0.9
0.05%
334.90RADa 134103.5
9.221 mg/L 0.00053 1672 mg/kg
1.0
0.06%
9.234 mg/L 0.0123
2.2
0.13%
a
368.51RAD
a
132157.4
0.40
0.42%
1675 mg/kg
RAD = radial view.
TABLE V
Results Obtained for the Variation in Sample Weighta
Sample Weight
Expected Ti
Found Ti
Recovery
RSD
(g)
(mg/L)
(mg/L)
(%)
(%, n=7)
0.5176
8.6750
8.6392
99.59
0.06
0.2148
3.6000
3.5741
99.28
0.05
0.1076
1.8034
1.7951
99.54
0.24
0.0513
0.8598
0.8630
100.37
0.34
0.0245
0.4106
0.4135
100.70
0.83
a
= Results from Ti II at 336.121 nm.
86
Vol. 31(3), May/June 2010
Fig. 8. Calibration curve for Ti at 334.940 nm using external
calibration.
Fig. 9. Calibration curve for Ti at 336.121 nm using external
calibration.
Fig. 10. Calibration curve for Ti at 337.279 nm using external
calibration.
Fig. 11. Calibration curve for Ti at 334.903 nm using external
calibration.
Fig. 12. Calibration curve for Ti at 368.519 nm using external
calibration.
Fig. 13. Sample weight vs. RSDs (Ti II at 336.121 nm).
87
To determine the lowest possible sample weight (in which still
reasonable results can be obtained),
a test was executed by varying the
sample weight from 0.5 g to 25 mg.
The results are presented in Table
V and illustrated in Figure 13. It can
be seen that even with sample
weights up to 25 mg, accurate and
precise results were obtained thus
illustrating the benefits of this
method. Both cost restrictions and
animal welfare considerations may
be factored into sample size decisions, which can dramatically
decrease excessive and unnecessary animal studies and testing.
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Inductively coupled plasma
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