Abstract Trace element impurities in high-purity antimony were determined employing three different methods
for the removal of matrix; on Dowex 50WX 8 by adsorption from 0.1 mol/L HF and elution with 4 mol/L HNO3
;
on Chelex-100 resin (in NH4
+
form) Bi, Cd, Co, Cu, and
Pb were separated in the presence of tartaric acid at a pH
of 9.0 ± 0.1 with subsequent elution with 2 mol/L HCl;
these determinations were carried out by GFAAS. The
separation of trace impurities from Sb by volatilization of
the matrix from H2
SO4
and HBr medium was also investigated. ICP-MS was used for the determination in these
cases.
All the three procedures showed that the removal of
the antimony matrix was nearly quantitative (> 99.99%).
The recoveries of trace elements were found to be > 95%.
The relative standard deviations were in the range 2–7%.
Standard addition calibrations were used. The levels of
process blanks indicate that with careful optimization, the
volatilization procedure coupled with ICP-QMS can be
used for trace impurity characterization of 6N+ Sb.
Introduction
Trace analysis of high-purity metals is important in the
microelectronic industry as elemental impurities even at
ultra-trace levels can have a detrimental effect on the performance characteristics of the end products. With increasing requirement of high-purity metals for semiconductor
applications, the assessment of purity at improved detection limits is a must.
The analytical requirements of such high-purity metals
are met to a large extent by glow discharge mass spectrometry (GD-MS), but the high cost of a high resolution
GD-MS system has restricted its availability. Further the
lack of suitable solid standards for a variety of matrices,
requirement of specific shapes, need of accurate RSF values and contamination from binding materials are certain
limitations in GD-MS. An alternate approach for analysis
of solid samples is laser or spark ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), but this
suffers from the large variations in analytical results, which
depend on the homogeneity, quality and surface states of
the solid samples. However freedom from chemical blanks
and minimal sample preparation are the favorable features
of GD-MS and LA-ICP-MS.
ICP-MS with quadrupole facility (ICP-QMS) offers
simultaneous multielement capabilities, a large dynamic
range, excellent sensitivities and detection limits for solution based analyses. Despite these favorable features, the
reported applications of trace characterization of high-purity materials by ICP-MS are up to now only a few
(Al2O3
,GaAs, TiO2
etc.) [1–3]. This is primarily due to the
restriction imposed by the need to dissolve the samples
using high-purity reagents, process (chemical) blanks,
molecular ion interferences, matrix related suppressions
on ion intensities and memory effects. Double focussing
ICP-MS is one way to remove molecular ion interferences
for specific elements improving the figure of merit. However, in addition to the high cost, the resolution requirements of some elements may not be met fully even by the
enhanced resolution.
Sample handling under clean room conditions, use of
high-purity reagents, and element specific chemistry are
some means by which these restrictions may be possibly
overcome, affording the use of ICP-QMS for ultra-trace
level impurity characterization in high-purity materials.
Cross-validation using alternate techniques of similar sensitivity is also a must.
This paper reports on the analysis of antimony metal
samples (nominally 5 N) for their purity assessment using
GFAAS and ICP-QMS to upgrade an existing facility to
produce 6N+ antimony. Different chemical separation
procedures viz., ion-exchange separation and matrix
volatilization have been used to assess their suitability for
the determination of trace impurities in high-purity antimony.
M. V. Balarama Krishna · D. Karunasagar ·
J. Arunachalam
Studies on the determination of trace elements
in high-purity Sb using GFAAS and ICP-QMS
Fresenius J Anal Chem (1999) 363 :353–358 © Springer-Verlag 1999
Received: 25 May 1998 / Revised: 23 September 1998 / Accepted: 26 September 1998
O R I G I N A L PA P E R
M. V. Balarama Krishna · D. Karunasagar · J. Arunachalam (Y)
National Centre for Compositional Characterisation of Materials,
E.C.I.L. Post, Hyderabad-500 062, IndiaExperimental
All the sample handling was done at the ultra-trace analytical laboratory (UTAL) of our Centre which is equipped with class 100
rooms and clean laminar flow work benches better than class 10.
Instrumentation
ICP-QMS measurements were made with a VG-Plasma Quad 3
(VG Elemental, Winsford, Cheshire, U.K.) system with a Meinhard concentric nebulizer and a Scott double pass cooled spray
chamber. The optimized parameters are given in Table 1. All the
measurements were performed using the peak jump mode. Multielement standards were applied for quantification.
Cross-validation of the results for some trace elements was carried out using a Varian model AA-875 atomic absorption spectrophotometer, equipped with GTA-95 Graphite Furnace Atomizer
and a programmable autosampler. Pyrolytically coated graphite
tubes and deuterium lamp background correction were used. The
operational parameters have been optimized for individual elements.
A JY-38, ICP-AES (Jobin-Yvon, France) system equipped
with a 56 MHz R.F. generator (Durr-JY) was used for the determination of Cu, the assessment of the recovery of trace elements. The
operating parameters were: output power 1.7 kW, plasma gas flow
18 L/min, slit width 10 μm/10 μm, integration time 0.3 s.
Eppendorf micropipettes and wherever feasible PTFE/PFA
containers were used. All the containers were cleaned by soaking
in 20% (v/v) nitric acid for a week and rinsed with high-purity water before use. A digital pH meter was used for pH measurements
and a halogen hot plate (Bibby Sterling Ltd., U.K.) was used for
heating purpose.
Reagents
Sub-boiled acids prepared using quartz sub-boiling units in our
laboratory and Suprapur grade (E. Merck, Darmstadt, Germany)
HNO3
, HCl, H2
SO4
, HBr and HF were used throughout. Ultra-pure
water (> 18 MΩ resistivity) was obtained by passing water through
a combination of RO system-mixed bed ion exchanger and Millipore water purification system (Millipore, Bangalore, India).
All stock standards of 1 mg/mL were prepared by dissolving
99.99% pure metals. Standards of required strength were prepared
by sequential dilutions every day.
~2.0% APDC (ammonium 1-pyrrolidine dithiocarboate) solution was prepared by dissolving 100 mg of APDC in 5 mL of water and filtered through a Whatman 42 filter paper.
Tartrate solution was prepared by dissolving 12.5 g of tartaric
acid in high purity water and the pH was adjusted to 9.0 ± 0.1 by
dropwise addition of isopiestic ammonia. The final volume of the
solution was adjusted to 50 mL. The solution was purified by complexing the impurities with 5 mL of 2% APDC and extracting
three times with 10 mL of MIBK (methyl isobutylketone).
Dowex 50WX 8 of 50–100 mesh in H+
form (Sigma, USA) and
Chelex-100 of 50–100 mesh in Na
+
form (Bio-Rad, USA) were
used for trace element sorption studies after suitable purification
steps.
Studies on the removal of Sb matrix
Ion-exchange separations
Studies using Dowex 50WX 8
A matrix-analyte separation technique using Dowex 50WX 8 in
HF medium for the trace characterization of titanium(IV) oxide reported by Wildhagen et al. [3] indicated that antimony was not retained on the column. This method was adopted for the separation
of trace impurities from antimony.
Sample dissolution. About 500 mg of the Sb sample was transferred into a teflon beaker containing 5 mL of HNO3
. Conc. HF
was added dropwise while heating gently at 80 °C till Sb was dissolved completely. After removing the nitrates by evaporation, the
volume of the solution was made up to 20 mL using 0.1 mol/L HF.
Corresponding blanks were prepared in the same way without
samples.
Column preparation. A polyacrylic column (25 × 1.0 cm i.d) with
a leak proof stop-cock made of self-lubricating Teflon was used.
About 4 g of Dowex 50WX 8 in H+
form of mesh size of 50-100
was loaded in the column. The approximate height of the resin bed
was 5 cm. The separation column was cleaned thoroughly with
10 mL of 7 mol/L HNO3
to remove any trace impurities. Then the
resin was washed with water followed by passing 20 mL of 0.1 mol/L
HF solution to convert the resin into the fluoride form. The sample
solution was passed through the column at 0.3 mL/min and washed
with 10 mL of 0.1 mol/L HF and 20 mL of water. The adsorbed
ions were then eluted with two 10 mL portions of 4 mol/L HNO3
at a flow rate of 0.3 mL/min. Sample aliquots spiked with known
amounts of trace elements and blanks were processed for obtaining
standard addition plots. The amounts of trace elements recovered
after treatment were determined based on the standard addition
calibration method using GFAAS.
Studies using Chelex-100
As an alternative method, we have also examined the extent of matrix removal using Chelex-100, which possesses a very high affinity for transition elements as well as for other elements like Pb, Cd,
Hg and Bi. Adsorption of transition elements on Chelex-100 increases sharply between pH 2 to 4, reaching Kd
values .10
4
–10
5
above pH 4.0 [4]. In an earlier study, it was found that Chelex-100
is highly effective in retaining the critical trace impurities while
not retaining Ga, As and Sb in the presence of tartrate [5]. Hence
we have adopted this procedure for the trace element characterization in high-purity antimony.
Since Chelex-100 strongly adsorbs many critical elements such
as Bi, Cd, Co, Cu and Pb even at high pH values (ca. 9.0) and tartaric acid forms stronger complexes in the alkaline pH ranges with
Sb, the use of tartaric acid to complex and separate antimony while
retaining the trace elements on Chelex-100 was attempted. The
minimum amount of tartrate required to complex antimony to retain it in solution without getting hydrolyzed was found to be twice
the amount of the antimony. The detailed studies using Chelex-100
are presented below.
Sample dissolution. 500 mg of the sample was dissolved in 6 mL of
aqua regia by heating gently at 80–100 °C. After removing nitrates
by evaporation, the pH of the solution was adjusted to 9.0 ± 0.1 using ammonia after addition of purified tartaric acid solution to prevent hydrolysis of Sb. Corresponding blanks were also prepared.
354
Table 1 Operating conditions for the ICP-MS
ICP-MS system
Instrument VG plasmaquad PQ3
Torch type Fassel
Plasma FW power 1350 W
Reflected power < 10 W
Glas flow rates
Coolant gas 13.4 L min
–1
Aux. gas 0.66 L min
–1
Nebulizer gas 0.85 L min
–1
Sampler cone 1.0 mm Ni
Skimmer cone 0.7 mm NiColumn preparation. About 2 g of Chelex-100 in the Na
+
form
(mesh size of 50–100) is poured with water into a poly-propylene
column (25 × 1.0 cm i.d.) with an approximate length of the resin
bed of 4 cm. After the resin particles had settled, 5 mL of 2 mol/L
HCl was passed to remove any trace element impurities, followed
by washing with water until the pH was around 5.0. Then 20 mL
of 2 mol/L isopiestic ammonia was passed to convert the resin into
the NH4
+
form. Excess of ammonia was removed by water.
20 mL of loading solution (pH 9.0 ± 0.1) containing 500 mg of
sample and twice the amount of tartaric acid was passed through
the column at 1 mL/min. The column was washed with 20 mL of
2 mol/L ammonia. The elution of adsorbed elements was performed with two 10 mL portions of 2 mol/L HCl. Sample aliquots
with known amounts of trace elements and blanks were treated as
above for standard addition calibration plots.
Volatilization studies. Although metal bromides are known to have
higher boiling points than the corresponding chlorides, it is often
found easier to distill an element as its bromide from a solution
containing HCl. This is particularly true for antimony and tin [6].
Therefore the removal of the Sb matrix by volatilization was investigated for the separation of impurities.
3 mL of conc. H2
SO4 was added to 200 mg of Sb sample, and
up to 3 mL conc. HCl was added dropwise under warm conditions
till the Sb sample was completely dissolved. Sb was removed as
volatile bromide by three successive additions of HBr of 1 mL each
maitaining the temperature at 200 °C. Excess Br2 was removed by
adding 0.5 mL of conc. HNO3
and evaporating to near dryness.
These volatilizations were carried out under open conditions in a
clean fume hood. On cooling, 0.5 mL of conc. HNO3 was added to
the residue. The solution was made upto 10 mL with water in a
volumetric flask.
Blanks were also prepared. The blank and sample digests were
analyzed by ICP-QMS for trace elemental impurities since the
analysis of these by GFAAS was complicated in the presence of
sulfuric acid.
The concentrations of some of the impurity elements were determined by a semi-quantitative technique employing the response
curve of the ICP-QMS previously constructed using standards over
the entire mass range. Rh, which was not present in the sample,
was used as an internal standard.
Results and discussion
Direct determination of trace elements in antimony is not
feasible using ICP-QMS under the dilutions required for
sample introduction, as Sb gets hydrolyzed at lower acidities.
All the three procedures, namely, separations on
Dowex 50WX 8 resin at 0.1 mol/L HF, Chelex-100 resin
at pH 9.0 ± 0.1 and volatilization from H2
SO4
+HBr showed
that the removal of antimony matrix was nearly quantitative (> 99.99%).
In the case of ion-exchange separation studies, using
Dowex 50WX 8 and Chelex-100 resins, the eluted solutions could not be used directly for analysis by GFAAS as
they contained 4 mol/L HNO3
and 2 mol/L HCl, respectively, and these high acidities damage the graphite tube.
Hence it is necessary to bring down the acidity to the minimum extent possible. To attain this, sample and blank solutions were kept for slow evaporation at around 80 °C, in
a clean bench. This process was continued till the solutions were nearly dry and then made up to 10 mL with water. Then the solutions were analyzed by GFAAS. The
copper content had to be determined by ICP-AES as the
impurity level of copper was much higher in the sample.
Recovery of the analytes
In the volatilization procedure, the recovery of trace elements (2 to 25 μg absolute amounts) was determined by
ICP-AES after removal of the matrix (500 mg of Sb) by
volatilization. The recoveries were found to be > 95%. In
the case of ion-exchange separations, the recovery studies
of trace elements were carried out using GFAAS after
adding ~60–460 ng absolute amounts of different elements to the matrix (500 mg of Sb). The recoveries of
trace elements spiked to the antimony sample were found
to be > 95%. The percentage recovery of various elements
added to sample were obtained comparing the signal to
that of pure standards processed under the same conditions, but without Sb.
The values of the average process blanks (1 g sample
basis) and limits of detection (LOD = 3 σ, n = 3) for ion
exchange separations estimated using GFAAS are given
in Table 2. The impurity levels of trace elements after separation of the matrix determined by GFAAS are given in
Table 3. Good agreement was observed between the results obtained by Dowex 50WX 8 and Chelex-100 procedures.
Despite the favourable removal of Sb matrix, a large
quantity of tartrate is required in the Chelex-100 procedure to prevent the hydrolysis of Sb and more analytical
steps including the purification of tartaric acid. In the
volatilization procedure, sample dissolution and removal of
the matrix could be carried out without transferring the
sample solutions which led to controlled process blanks.
Hence we did not pursue the Chelex-100 procedure. Further studies on the characterization of antimony sample
were carried out by the volatilization procedure using
355
Table 2 Comparison of process blanks and limits of detection
(LOD-3 s) of Dowex 50WX 8 and Chelex-100 procedures (GFAAS)
Element Blank (ng/mL) LOD (ng/mL)
Dowex Chelex Dowex Chelex
50W × 8 100 50W × 8 100
Bi 4.20 5.68 4.50 2.22
Cd 0.22 0.15 0.23 0.23
Co 2.01 1.56 3.35 1.56
Cu 3.50 5.67 1.19 2.42
Pb 4.05 1.69 3.45 0.79
Table 3 Determination of impurity levels after separation of the
matrix (analysis by GFAAS)
Element Impurity level (ng/g)
Dowex 50W × 8 Chelex 100
Bi 94 ± 3 84 ± 5
Cd 64 ± 1 60 ± 3
Co 124 ± 2 145 ± 4
Cu 10.7 ± 0.6
a
10.6 ± 0.3
a
Pb 432 ± 4 423 ± 12
n = 3, ± Standard deviation,
a
μg/g, ICP-AESICP-QMS. The Dowex 50WX 8 procedure was retained
as an alternate procedure for matrix removal.
Interferences during analyses by ICP-QMS
In ICP-QMS, the sample digestion procedure decides to a
considerable extent the choice of isotopes useful for quantitation, due to various molecular ionic species that can be
356
Fig. 1 Mass spectrum of a: BrOH+
(m/z = 98) and BrO2
+
(m/z =
111 and 113) species (sample: 5% HBr), b: BrOH+
(m/z = 98) and
BrO2
+
/BrS
+
(m/z = 111 and 113) species (sample: 5% HBr + 5%
H2
SO4
), c: Br2
+
(m/z = 158, 160 and 162) species (sample: 5% HBr),
d: Br2
+
(m/z = 158, 160 and 162) species (sample: 5% HBr + 5%
H2
SO4
)
1-bformed. In the volatilization procedure, both bromine and
sulfur are introduced into the sample. To determine the
masses interfered by the presence of Br and S, 5%HBr,
5% H2
SO4
and a mixture of 5%HBr and 5%H2
SO4 were
scanned for the entire mass range. The spectra obtained,
showing the molecular ionic species formed, are given in
Fig. 1a–d. Even though the real sample digests contained
much less than 5% HBr and H2
SO4
, these figures indicate
the m/z values where interferences are likely to occur.
Table 4 gives some of the molecular ions obtained which
cause isobaric overlap on the major isotopes of some analyte ions. The nominal resolving power required to separate these is also calculated and presented. As may be
seen, in many cases the resolution required is much higher
than available in currently available double focussing
ICP-MS (~10,000).
In the presence of sulfur, the determination of Ti is rendered impossible due to severe interferences at m/z values
48, 49 and 50 due to SO+
ions. Cu could be determined
using mass 63 alone, as mass 65 was interfered by
33
S
32
S.
The formation of SO2
+
ions interferes in the determination
of Zn isotopes.
The molecular ionic species due to Br are given in
Figs.1a, b for 5% HBr and 5% HBr + 5% H2
SO4 mixture,
respectively. Figures 1c, d represent Br2
+
ions recorded in
these two solutions.
The presence of Br gives rise to molecular ions BrO+
,
BrOH+
, BrO2
+
and Br2
+
. The BrO+
species (
79
Br
16
O and
81
Br
16
O) were very large (not shown) and the peaks occurring at m/z values 95 and 97 severely interfere with the
isobar-free isotopes of Mo. The BrOH+
occurring at
m/z = 98 is shown in Fig. 1a. The BrO2
+
peaks occurring
at m/z 111 and 113 would interfere in the determination of
Cd and In. Figure 1b shows that the height of the peak at
m/z = 98 is much lower, whereas the peaks at m/z values
111 and 113 show a marked increase from 1a to 1b. This
indicates that in addition to BrO2
+
also a formation of BrS
(
79
Br
32
S
+
and
81
Br
32
S
+
species) occurs in the presence of
S, which correspondingly reduces the available Br for the
formation of BrOH+
and Br2
+
ions (1c and 1d). This kind
of spectroscopic interferences seem to vary significantly
with the presence of the various elemental species. Thus a
reasonable calculation of the interferences based on elemental equations is very difficult in this case. In order to
simplify the mass spectrum, the elimination of bromine
from the analyte solutions was incorporated into the analysis scheme, whereas the removal of sulfur due to the
H2
SO4
proved to be quite difficult.
However, the spectra obtained for the process blanks in
the volatilization method show no significant peaks at m/z
values corresponding to the polyatomic ions formed due
to the presence of Br. The isotopic ratio of Cd 111/112 obtained by the volatilization procedure was also found to
agree with the natural isotopic abundance ratio. This shows
that the removal of residual bromide after volatilization of
the matrix is very effective and interferences occurring
due to the residual bromide remaining after volatilization
of the matrix are negligible.
Process blanks and LODs in the determinations
with ICP-QMS
The values of average process blanks and LODs for the
two procedures estimated using ICP-QMS in Table 5 show
that the volatilization procedure gave lower blanks compared to the Dowex 50WX 8 procedure. The process
blank values add up to approximately 100 ng/g. The high
blank values of the Dowex 50WX 8 procedure could possibly be attributed to the resin.
Comparison of the results
In the case of the Dowex 50WX 8 procedure, a good
agreement was achieved for the values of Cd, Co, Cu and
Pb obtained by GFAAS and ICP-QMS. The values of Mn
and Fe could not reliably be determined using GFAAS because of large variations in the blank values.
The results obtained by the volatilization and Dowex
50WX 8 procedures using ICP-QMS are in good agreement with each other and are given in Table 6. Cu was determined by ICP-AES as well as by ICP-QMS. In the case
of the volatilization procedure, the values of Co and Bi
357
Table 4 Interferences due to formation of molecular ions in the
presence of S and Br
Isotope of Interfering mole- Resolution required
interest cular ion for separation
48
Ti
+ 32
S
16
O+
2500
65
Cu
+ 32
S
33
S
+
4100
64
Zn
+ 32
S2
+
4200
66
Zn
+ 32
S
34
S
+
,
33
S2
+
4700
95
Mo
+ 79
Br
16
O+
12800
97
Mo
+ 81
Br
16
O+
18600
111
Cd
+ 79
Br
16
O2
+
28500
113
Cd
+ 81
Br
16
O2
+
66500
113
In
+ 81
Br
16
O2
+
60000
Table 5 Comparison of average process blanks and limits of detection (LOD-3 s) of volatilization and Dowex 50WX 8 procedures
by ICP-QMS
Element
a
Blank (ng/g) LOD (ng/g)
Volatiliza- Dowex Volatiliza- Dowex
tion 50WX 8 tion 50WX 8
52
Cr 52 56 2.1 4.0
55
Mn 4 10 1.9 2.1
58
Ni 16 24 3.8 1.8
59
Co 0.5 2 0.8 0.6
63
Cu 14 35 3.8 9.7
92
Mo 3 4 2.1 0.4
112
Cd 0.4 5 0.3 1.2
138
Ba 3 3 1.8 0.9
208
Pb 5.6 8.9 3.9 2.2
209
Bi 0.5 1.5 1.1 0.7
a
= isotopes used for quantificationwere found to be much lower than those obtained using
the Dowex 50WX 8 and Chelex-100 procedures. Though
at present the reasons are not known, the values are further being checked. Semi-quantitative values for some of
the trace elements in high-purity antimony were obtained
by ICP-QMS and are given in Table 7.
Despite the use of clean benches for chemical operations, Al and Zn gave varied results and could not be determined.
Conclusion
The methods reported have been found to be suitable for
the trace element characterization of a high-purity (5N)
antimony sample. In case of the volatilization procedure,
sample dissolution and removal of the matrix (99.99%)
could be carried out without the need for changing the
sample container. This reduces the number of analytical
steps, compared to ion-exchange separation procedures,
thereby leading to controlled process blanks at lower levels. The volatilization procedure coupled with ICP-QMS
for measurements can be adopted for trace element characterization of 6N+ Sb sample with respect to many elements. An additional set of elements like rare earths could
be added to the list. This analytical procedure was validated through alternate matrix separation procedures like
Dowex 50WX 8 in HF medium and Chelex-100 at pH 9.0.
An all-quartz closed system apparatus is being fabricated
for use in volatilization studies, which is essential for lowering the process blanks further. Currently we are carrying
out studies on the trace element characterization of highpurity arsenic and tin using this procedure. These results
will be communicated shortly.
Acknowledgement The authors thank Dr. S. Gangadharan, Chief
Executive, BRIT-CCCM for his encouragement and Shri. A.C. Sahayam, who has carried out the ICP-AES measurements.
References
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(1996) J Anal At Spectrom 11: 797
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Spectrom 11: 371
4. Leydon D, Underwood AL (1964) J Phys Chem 68:2093
5. Arunachalam J (1991) Nuclear and other instrumental methods
of analysis at trace and ultratrace levels of concentrations. A thesis submitted to the University of Bombay, for the degree of
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traces of metals. General aspects; Fourth edition of Part I of Colorimetric determination of traces of metals, p 1054
358
Table 6 Impurity levels after separation of the matrix (analysis by
ICP-QMS)
Element
a
Volatilization Dowex 50WX 8
(ng/g) (ng/g)
52
Cr 184 ± 4 173 ± 5
55
Mn 114 ± 4 131 ± 6
58
Ni 527 ± 6 459 ± 8
59
Co 17 ± 5 137 ± 4
63
Cu 10.5
b
10.2
b
92
Mo 152 ± 4 141 ± 6
112
Cd 55 ± 2 73 ± 5
138
Ba 124 ± 4 130 ± 5
208
Pb 504 ± 7 503 ± 6
209
Bi 14 ± 4 82 ± 3
a
= isotopes used for computing analyte concentrations and
b
= μg/g
Table 7 Semi-quantitative determination of impurity levels after
separation of the matrix by the volatilization procedure (ICPQMS)
Element Process blank Impurity level
(ng/g) (ng/g)
82
Se 5 21
106
Pd 0.03 6
107
Ag 0.8 29
115
In 1 85
181
Ta 0.3 273
182W 0.6 20
196
Pt 0.03 2
197
Au 0.02 2
202
Hg 0.2 3
203
Tl 0.2 5
