Quantitative Determination of Platinum by XRF Techniques

Transcription

Platinum Manufacturing Process
Platinum Day Symposium Volume IX N5
Quantitative Determination of Platinum
by XRF Techniques
Greg Normandeau & David Ueno Imperial Smelting & Refining Co. of Canada Ltd.
•
•
•
The quantitative determination of platinum by x-ray fluorescence (XRF) methods involves
analysis of a solid sample to determine content in commercial
alloys at a repeatable absolute accuracy level of 0.10%. This level
is considered quantitative while
1-2% relative accuracy is considered qualitative. The goal is to
supplement classical wet chemical methods (A.A & I.C.P) with
XRF techniques as an accepted
method for demonstrating compliance to international precious
metals marking requirements.
Coverage of the topic includes
the following sections:
• A general definition of XRF
concepts.
• Explanation of our analytical
methods related to x-rays.
(secondary target, filters,
high total counts, long count
times, triplicate analysis).
• Explanation of the principles
of our analytical methods re-
•
•
lated to regression models,
correlation coefficients and
the number of standards required.
Discussion of current alloy
applications and future potential.
An outline of our general
techniques for standard
preparation, assay via A.A &
I.C.P. and polished surface
preparation.
Explanation, with graphics,
of the routine for running a
sample, printing results, storing data, examining and confirming quality of results
Explain results of commercial applications in 1999,
2000, 2001. We’ll emphasize the number of tests,
show range, standard deviation and accuracy with statistical terms.
Conclusions and Further Research.
DEFINITION OF
TERMINOLOGY:
Consideration of this topic
must be based on understanding
some basic terms and technical
concepts of the trade. The following are considered significant:
X-Ray fluorescence spectroscopy: (XRF) The detection and
analysis of x-rays emitted from a
sample after impingement by a
controlled x-ray energy source.
X-Ray: unique and characteristic
energy emitted from a solid sample when it is irradiated with a
high energy electron or x-ray
beam.
Accuracy: the uncertainty limits
on an analytical result for a
specific platinum content.
e.g.: 95.20% Platinum ±0.10%
Precision: the repeatability of an
analytical result through the application of a technique.
These terms will be referred to
throughout the document. Figure
1 outlines the concept of x-ray
generation. It is beyond the scope
of the article to present the atomic
physics that underlie the photoelectric effect phenomenon. Entire
texts have been written on the
subject. Energy from an x-ray
source hits the electrons in an
atom’s orbital and displaces a particular electron. A nearby electron
in a neighboring orbital shell
Figure 1
© Platinum Guild International USA 2002
All Rights Reserved
1
will replace the missing electron
to preserve energy stability. The
change to a lower energy level
when filling the void created by
the missing electron causes emission of a specific characteristic xray for that element. Electrons
coming from different shells create the Kα, Kβ, Lα and Lβ, type
x-rays that all have distinctive energy levels unique for each element. Figure 2 shows a typical xray spectrum with labeled energy
peaks at various points along the
spectrum. Note that the signals are
very tall, distinct energy peaks
while the general background
noise is minimal. This high signal
to noise ratio is desirable for accurate analysis.
Figure 2
Figure 3 illustrates the general schematic of a typical x-ray
fluorescence unit. Figure 4 shows
the general size and shape of a
commercial XRF unit. This particular type deploys a secondary
target and
filter system. The primary x-rays
from a Rh tube source impinge on
a tin secondary target. That target
emits x-rays that are filtered before hitting the sample. This
technique produces clean monochromatic x-rays that enhance
accuracy. The x-rays emitted by
the sample are captured by a liquid nitrogen cooled detector that
translates them into electrical
pulses.
The specimen emits an x-ray
spectrum characteristic of its
atomic constituents from an area
of approximately 8mm round.
The depth of about x-ray penetration is dependent on
the matrix and power used. This
point is critical when considering
the validity of results. Surface
treatments and some history of
the sample must be known.
Figure 3
2
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Solid-state pulse processing
equipment and a PC manipulate
the emitted counts through statistical means to compute the quantity of metal present in the sample. Specific computational
routines must be developed to establish a high degree of accuracy
for a specific set of x-ray generation and acquisition conditions.
These routines are usually developed in consultation with the xray supplier.
SCOPE OF APPLICATION:
We used XRF technology to
quantify platinum content of
wrought prepared samples representative of the following alloys
and melt lot sizes:
95Pt-5Co 2000-6000g lots
95Pt-5Ru 12000-21000g lots
90Pt-10Ir 12000-21000g lots
XRF methods were not used
on 95Pt-5Pd, 95Pt-5Ir or 95Pt-5
heat treatable alloys.
In all cases, samples were extracted from wrought (cold
worked and annealed) sections of
a billet. The pieces were approximately 1” long, ½” wide
and 3/16” thick. They were always rolled flat, ground on a
metallographic lapping wheel
with 240, 400 and 600 grit silicon
carbide papers followed by polishing with 6 and 1 micron diamond paste. The mirror bright
and flat nature of the prepared
surface was critical to enhance
precision and accuracy levels.
The selected sample piece must
be big enough to hold in the fingertips for this standard preparation method. The actual x-ray
process was non-destructive to
the sample, allowing for recycle
back into subsequent melt lots.
The segregated and nonhomogenous nature of cast articles was not a factor in any of our
analysis.
Figure 4
Figure 5
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3
Because of the wide 8mm xray spot size, finished jewelry articles with concave or convex surfaces, that were potentially rhodium plated were not used for any
analysis.
ANALYTICAL PRINCIPLES:
From an x-ray source perspective, a 50Watt system does
not have enough excitation energy to use a secondary target.
This was found to be a critical
factor contributing to high precision and accuracy. A 400Watt
system produced enough x-ray
power to excite a secondary target
and produce high energy filtered
monochromatic x-rays. The clean
Tin x-rays produced a high signal
to noise ratio and were able to excite the heavy PGM elements.
Contrast Figure 5 with Figure 2.
The spectrums represent the same
specimen. Figure 2 clearly shows
distinct peaks with minimal noise
and overlap versus Figure 5.
From a statistical perspective,
we always tried to achieve 1 million x-ray counts for analysis.
This was achieved through 200800 seconds of x-ray acquisition.
Analysis was done in triplicate to
confirm precision on each x-ray
session. A standard which was
validated by alternative analytical
methods was used to confirm
minimal detector drift.
Statistical manipulation of the
x-ray counts is based on linear
regression analysis techniques to
maximize accuracy. Other methods such as standard less fundamental parameters (SLFP) have
10-20% relative accuracy. Even
using the NBS-GSC method
where a known standard is analyzed in tandem with the unknown specimen only yields 510% relative accuracy. To fulfill
the requirements of the linear regression model, a specific number
of standards that are validated by
4
Figure 6
a separate analytical procedure
must be used. The model dictates
that (n+1)2 standards are required, where n equals the number of distinct elements in the alloy, not just the number being
analyzed. For a typical binary
platinum alloy, 9 standards must
be fabricated, validated by some
accepted method such as I.C.P or
AA spectroscopy and their spectrums acquired by XRF methods.
For a three element alloy, 16
standards are required. The re-
All Rights Reserved
sults of these acquisitions were
numerically managed to produce
a regression curve with a high
coefficient of co-relation. This
greatly enhanced accuracy. The
goal was to establish a curve
where unknowns are analyzed by
interpolation as opposed to extrapolation. Figure 6 illustrates
the concept. All elements in the
alloy must be accounted for because the software will normalize
the results to 100% and create inaccuracies. The same basic
statistical principles are used
with wet chemical methods such
as A.A or ICP.
FABRICATION OF
STANDARDS:
Our platinum binary alloys
were made to ±0.50% of the
nominal platinum content in
0.10% increments. We validated
the standards by A.A. methods
internally and had an external lab
use I.C.P. to confirm our values.
For the 90Pt-10Ir we had standards from 89.5% Pt to 90.5% Pt
in 0.10% increments. Likewise
for the 95% alloys, we had standards from 94.5%Pt to 95.5%Pt
in 0.10% increments. This furnished high quality regression
curves where linear interpolation
was used to determine the platinum content of unknown specimens. All standards received surface preparation by grinding and
diamond polishing from wrought
microstructures produced through
a series of cold mill rolling and
furnace annealing operations to
minimize potential segregation
issues.
Figure 7
EXAMPLE OF A SAMPLE
ANALYSIS:
Samples were loaded into an
automated rotating carrousel.
Analysis can be completed overnight without supervision. Figure
7 shows the equipment. The
software is Windows based for a
standard PC. Figure 8 illustrates
a typical screen for the operator
where batches are defined and
routines selected by cursor and
mouse movement methods. The
unique sample ID can be entered
and either standard or special acquisition parameters defined for
each specimen. The results of a
600 second acquisition of 95Pt5Ru are summarized in Figure 9.
Final calculations for platinum
and ruthenium content are sum-
Figure 8
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5
marized on the screen shown in
Figure 10. These results can be
printed in report form or transferred with a computer network
for storage. The entire process of
standard acquisition and tripli-
cate analysis of 6-8 unknown
specimens can be completed in an
unsupervised overnight routine.
Each specimen may receive 30-40
total minutes of x-ray irradiation
in three separate analysis that can
Figure 9
be compared to confirm a high
level of precision. Results are
printed in the morning to quantify platinum content.
Results of Analysis 1997 &
1998 via A.A spectroscopy versus
1999 & 2000 with XRF.
Results summarized for each
platinum alloy allow comparison
between A.A spectroscopy in
1997-98 versus XRF in 19992000. Statistical indicators such
as sample size (N), average, and
standard deviation describe the
attributes of the assay results
(population). Tests such as the “F
Test” and “T Test” were run to
provide a measure of similarity
between XRF and AA assay results. Results of assay populations
can be considered in terms of the
normal distribution shown in
Figure 11. A range of
± 2σ, or 2 times the standard deviation around the average encompasses a range involving 95%
of the results. If 100 XRF analysis sessions were conducted on
the same sample, results form 95
of these would fall into the average ± 2σ range. The narrower
this range the better.
95% PLATINUM
5% COBALT
Results are summarized in
Table 1. XRF platinum assays
produced a superior average
compared to the 95.20% Pt content target, a smaller standard deviation and a smaller
± 2σ range. Figure 12 shows a
distinct trend. The results may be
indicative of actual melting skill
improvement, not specifically a
difference between analysis
methods. Sample sizes
Figure 10
6
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95Pt- 5Co
Year/method
2000/XRF
1999/XRF
1998/AA
1997/AA
T bl 1
N
30
34
37
27
# kg
66
68
115
79
were very similar. There was minimal interference between platinum
AVERAGE andSTD
DEV
+/-2SIGMA
cobalt
peaks. The
narrow 95%
0.9515 range
0.00072
and quality0.9498-0.9529
average indicate
0.9522 that0.00086
XRF handles0.9504-0.9539
Pt determination
accuracy and0.9506-0.9546
precision for this
0.9526 with0.0010
alloy.
0.9540
0.0017
0.9574-0.9506
This was symptomatic of peak
overlap and interference issues
that required attention. In 2000 a
smaller population of results had
a much lower average assay and
standard deviation comparable to
1997 values with AA methods.
After a period of adjustment the
XRF method produced results of
comparable accuracy and precision to standard AA practise.
95% PLATINUM
5% RUTHENIUM
A similar trend occurred with
this alloy. Results are summarized in Table 3 and
Figure 14. The first year of XRF
techniques (1999) showed a distinct jump in average assay and
standard deviation of the population. By the end of the second
year (2000) both the average and
standard deviation values closely
match those from AA in 1997-98
for a much large sample population. This level
Figure 11
90Pt- 10Ir
Year/method
2000/XRF
1999/XRF
1998/AA
1997/AA
N
29
50
165
62
95Pt- 5Ru
Year/method N
2000/XRF
111
1999/XRF
162
1998/AA
36
1997/AA
65
90% PLATINUM
# Kg
160
309
1380
390
AVERAGE 10%
STD
DEV
+/-2SIGMA
IRIDIUM
0.9031
0.0014 are summarized
0.9059-0.9003
Results
in Ta0.9048 ble 20.0018
0.9084-0.9012
and Figure 13.
AA methods
good results
in 1997&98.
0.9020 produced
0.0085
0.9037-0.9003
large volumes0.9054-0.8990
handled in 1998
0.9022 The 0.0016
# kg
1082
1345
198
463
exhibited a good average and comparatively narrow standard deviation. Initial application of XRF
AVERAGE techniques
STD DEV
in 1999 +/-2SIGMA
experienced a
period where
sample
0.9528 learning
0.00148
0.9557-0.9498
preparation
and
standards
were ad0.9536
0.0020
0.9576-0.9496
justed
to
compensate
for
a
high av0.9527
0.0012
0.9551-0.9503
erage
assay
and
standard
deviation.
0.9527
0.0013
0.9553-0.9501
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7
Figure 12
Fugure 13
Method
N
AVERAGE STD DEV
XRF
22
0.9518
0.0010
A.A
22
0.9516
0.0012
t-test
F-test
Pass@
95% CONF.
0.916<2.08
Pass @
95% CONF.
0.694<2.46
t-test
F-test
Fail @
95% CONF
2.93>2.09
Pass
@ 95% CONF
0.429<2.30
T bl 4
Method
N
AVERAGE STD DEV
XRF-AA
22
0.0011
0.0008
A.A
22
±0.0003
0.0012
T bl 5
8
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indicated that XRF results have a
high degree of precision and of
accuracy comparable to conventional techniques.
Another method of comparing
the two analytical methods involved applying the “F-test” and
“t-test” to a population of 22 assays of 95% Pt-5% Ru that were
executed with both methods. Results are summarized in Table 4.
The F-test evaluates the equivalence of two populations variances (1 set of results from the
XRF assay of Pt, the other from
the AA assay of the same Pt samples). Results pass the test, which
implies that the two populations
have a similar variance. In other
words the standard deviation of
Pt assays from XRF methods
closely match that of AA methods. Likewise, the student t-test
evaluates the equivalence of two
population averages. Again, the
XRF results closely match the
AA results for this population of
22 assays completed by both
methods.
One last comparison was
made between the difference
in the XRF results versus the AA
results. Results are summarized
in Table 5. The difference in the
averages for the populations was
0.0011 which closely approximated the target of 0.10% accuracy. The value of ±0.0003% was
considered the absolute level of
accuracy possible with correct
application of the standard
method. When comparing this
0.0006 range attainable to the
0.0011 range obtained those results failed the t-test for population similarity by a small margin.
The deviations in the two populations passed the F-test, indicating
a close match in values. Overall,
results from the application of the
XRF method in direct comparison with the standard AA method
for this population of 22 platinum
95Pt5Ru ASSAY RESULTS
95.4
0.0025
0.002
95.35
0.0015
95.3
0.001
95.25
0.0005
0
95.2
1997
1998
1999
2000
Figure 14
assays, compare favorably for
accuracy.
CONCLUSIONS AND
FURTHER RESEARCH:
From a large number of assay
results over a 2 year period, we
concluded that with proper
care and attention to specific details, an XRF method could quantitatively assay platinum content
to 0.10% accuracy with repeatable precision. A 4 year period of
results allowed for comparison
between population averages and
standard deviations between AA
spectroscopy and XRF. Again,
results com
All Rights Reserved
pared favorably. We believe that
a high energy x-ray source capable of generating secondary
mono-chromatic x-rays presented
to a carefully prepared, wrought
flat sample is required to attain
the highest level of accuracy and
precision. The method can be applied to binary platinum alloys
with the use of 9 standards carefully matched to the specific alloy
matrix validated by some standard wet chemical method such
as AA or ICP. The method has
been proven to work for the alloys
95% Pt-5%Co, 95%Pt-5%Ru &
90%Pt-10%Ir. The method may
also apply to such alloys as 95Pt5%Pd, 95%Pt-5%Ir and 95%Pt5%HTA. We plan to continue
testing the application of XRF
methods to additional alloys and
expand the population of direct
comparison results that confirm
accuracy and precision between
novel and conventional analytical
methods. Our ultimate goal is to
have XRF methods accepted as
standard practice in the trade.
9

Quantitative Determination of Platinum by XRF Techniques

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