Application
Note: 40895
Analysis of Potassium and Silicon Contaminants
in High Purity Tungsten Metal using the
iCAP 6000 Series ICP
Andrew Clavering, ICP Applications Specialist, Thermo Fisher Scientific, Scientific Instruments, Cambridge, UK.
Key Words
• ICP
• iCAP Duo
• Matrix matching
• Metal
• Alloys
Introduction
Principle
The analysis of metals by ICP ordinarily would not pose
too many difficulties for a high resolution ICP and an
experienced chemist. However, some metal matrices can
be particularly problematic with respect to the occurrence
of interferences and require careful consideration from the
analyst in order to avoid erroneous results. Specialist
metal and alloy manufacturers are often required to
determine trace element levels in metals or alloys which
produce a highly complex spectrum of lines. In these
cases, the analyst will often find that all of the major lines
have interferences as well as the requirement to correct for
complex backgrounds resulting from the metal enriched
sample matrix.
There are few instances of metals and alloy solutions
which can produce the above undesirable effects even if the
analyst is required to dissolve a high concentration of
metal to enable the analysis of trace contaminants.
However, the sample types that produce these effects have
already been analysed sucessfully using Thermo Scientific
iCAP 6000 Series instruments. Successful analyses are
often achieved by matrix matching the standards with pure
versions of the major components. These majors, e.g.
Tungsten and Molybdenum, can produce large numbers of
lines whereby it becomes optically difficult to separate all
of the twenty or thirty elements required from the
interfering matrix. Using a full wavelength coverage
spectrometer such as the iCAP 6000 Series ICP, it is
possible to provide alternate wavelengths for many of the
elements. Where this is not possible, i.e. where trace level
measurements are required, the analyst may use matrix
matching of standards and samples for major components
to compensate for the interference. The matrix matching
will, of course, also compensate for many other potential
interferences e.g. signal suppression, density differences etc.
resulting from different concentrations of metals in
standards and samples.
An alternative to matrix matching for spectral
overlaps is Inter-Element Correction or IEC which will
produce a ratio of the apparent concentration of
interferent on the analyte and compensate.
The example used here is taken from a prominent
world-wide pure metals and alloys producer who use the
iCAP 6000 Series ICP to analyse up to a hundred different
metals and alloy types for trace contaminants. This
application note describes the use of a matrix matching
technique as used by prominent world-wide producers of
pure metals and alloys to enable quantitative analyses of
selected trace metal element contaminants in various metal
enriched sample matrices.
The samples of metal alloy are dissolved in suitable acids,
either on a hotplate or preferably by high pressure
microwave digestion. The resulting clear solution is
washed into a flask and made up to volume with
deionised water. The trace elements are measured in
solution against a calibration generated by standards
containing the same concentrations of major elements as
the samples.
The samples utilised in this application note are a pure
tungsten metal, which were analysed for trace impurities
using the matrix matching method.
Instrumentation
A Thermo Scientific iCAP 6500 Duo ICP equipped with
an HF acid resistant sample introduction kit (p/n 8423
120 51821) was used for this work. The iCAP 6000 Series
includes high-resolution Echelle optics and enhanced
detector technology, which includes a fourth generation
Charge Injection Device (CID). Advancements in CID
technology allow this detector to feature higher sensitivity
and lower noise than any of its predecessors. All of these
performance improvements make it ideal for precise and
accurate measurements of major, minor and trace
components in metal samples.
Parameter
Setting
Pump Tubing
Tygon Orange-White
Tygon White-White
Pump Rate
50 rpm
Nebulizer
Miramist
Nebulizer Argon Flow
0.65 L/min.
Spray Chamber
HF Resistant spraychamber
Torch Center Tube
HF Resistant 2 mm Ceramic
Torch Orientation
Duo
RF Forward Power
1200 watts
Coolant Flow
12 L/min.
Auxiliary Flow
0.5 L/min.
Integration Time
15 seconds
Repeats Per Analysis
3
Table 1. Instrument Parameters
Reagents
• Nitric acid sg 1.42, AnalaR or Tracemetal grade
• Hydrochloric acid 35 % v/v, AnalaR or Tracemetal grade
• Hydrofluoric acid 40 % v/v, AnalaR or Tracemetal grade
• 1000 ppm single element stock standards for each element
• Stock solution of a high concentration of matrix
elements matching the sample
Equipment
• Hotplate and Teflon beakers or alternatively a
microwave digestion system
• Plastic volumetric flasks
Figure 1. Example of Potassium 766.490 nm calibration curve
Sample and Standard Preparation
The metal solid was sampled as representatively as
possible using lathes and drills and 0.25 g was weighed
out into a Teflon beaker. Acid was added to the beaker –
15 ml hydrochloric, 10 ml nitric, 5ml hydrofluoric acids –
and then simmered for several hours until the volume
reaches approximately 5-10 ml. The microwave digestion
system could be used at this stage and would produce a
similar sample in a shorter time. The sample was then
washed into a 100 ml plastic volumetric flask with
deionised water. The standards were produced by using an
intermediate stock solution to make a series of standards
at the following concentrations:
0 ppm, 50 ppm, 100 ppm, 500 ppm (in solid, µg/g).
The standards were made up to volume using a matrix
containing stock solution which would approximate the
matrix in the sample i.e. approximately 99+ % of
tungsten and matching acid content.
Method Development
Wavelength selection for a specific metal type of sample is
made easy using the interference tables in the iCAP 6000
Series iTEVA software which assists the analysts at this
early stage of method development.
This method concentrates on the two most difficult
elements in this matrix, Potassium and Silicon.
Wavelength selection was made from a larger subset of
standard lines as determined using the unique Fullframe
capability of the iCAP 6000 Series. The Fullframe and the
sub-arrays of the prospective element lines were used to
check for interferences and suitability for the analysis.
Internal standards and inter-element correction were not
required as the matrix for both the standards and the
samples was the same.
Figures 1 and 2 show the linear calibration and peak
intensities for Potassium 766.490 nm. Figures 3 and 4
show the linear calibration and peak intensities for Silicon
251.611 nm.
Figure 2. Potassium 766.490 nm subarray showing blank, standards and
samples
Figure 3. Example of Silicon 251.611 nm calibration curve
Conclusions
The iCAP 6000 Series high resolution spectrometer
demonstrates excellent sensitivity and measured analyte
concentrations are in good agreement with the expected
values. The matrix matching for the majors is shown not
to compromise the linearity of the calibrations for the
analysis. The result is that a difficult analysis is simplified
and good detection limits are easily achieved.
Figure 4. Silicon 251.611 nm subarray showing blank, standards and samples
Results
Element
K 766.490 nm
Si 251.611 nm
Sample 1 (ppm in solid)
Sample 2 (ppm in solid)
Measured
Expected
Measured
Expected
90.3
202
90.5
228
27.1
ND
32
Method DL (ppm in solid)
8.7
11.4
Note: Method Detection Limits using 10 replicates of the Matrix Blank and the 3 sigma method.
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