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Notes > XRF Method
XRF
Analysis of Rocks and Minerals for Major and Trace Elements
on a Single Low Dilution Li-tetraborate Fused Bead
Johnson,
D.M., Hooper P.R., and Conrey,
R.M.,
GeoAnalytical Lab, Washington State University,
Advances in X-ray Analysis, vol 41, p. 843-867, 1999 (PDF
Format)
Abstract
The
precision and accuracy of a low (2:1) Li-tetraborate fused bead
technique by X-ray fluorescence analysis for 27 major and trace
elements is demonstrated by comparison to accepted values of standard
samples and to values acquired by other techni ques in other laboratories.
The increased efficiency of using a single bead for major and trace
elements is achieved without loss of precision or accuracy and
the beads may be stored for tens of years without degradation.
Introduction
Of the many advantages in applying
X-ray fluorescence (XRF) to the analysis of rocks and minerals,
one of the most obvious is the versatility of the instrumentation.
Methods can be developed to satisfy a wide variety of needs.
In the GeoAnalyti cal Laboratory of Washington State University,
the method developed over a period of more than 30 years (e. g.
Hooper, 1964) was originally designed to distinguish the subtle
chemical differences between flows of the Columbia River Basalt
Group. To trace these flows over the Columbia Plateau required
larger than normal numbers of analyses for the maximum number of
elements and the highest possible analytical precision, while retaining
the best available absolute accuracy. The approach finally adopted
incl udes three separate components which differ from the more
commonly employed methods based primarily on the work of Norrish
and Hutton (1969). First, a single low dilution (2:1 diLi-tetraborate
: sample) fusion is used for both major and trace elements, pr
oviding maximum efficiency without loss of accuracy. Second,
a constant voltage on a Rh target is used for all elements to achieve
maximum long term stability and precision, despite this causing
less than perfect conditions for a few trace elements. Third
, the oxidation state of iron and the volatile content of the rock
are ignored. The original major element concentrations are then
normalized to 100%, volatile-free, with all the iron expressed
as FeO.
Each of these three factors is independent of the
others. Adoption of one does not require the adoption of any other.
The advantages and disadvantages of each factor are discussed and
all three are implicated in the ultimate accuracy of the analyse
s discussed below.
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Single
bead low-dilution fusion technique
Sample Preparation
Fresh chips of the sample are
hand picked and a standard volume of chips (approximately 28 g)
is ground in a swing mill with tungsten carbide surfaces for 2
minutes. Three and a half grams (3.5 g) of the sample powder is
weighed into a plastic mixing jar with 7.0 g of spec pure dilithium
tetraborate (Li2B4O7) and, assisted by an enclosed plastic ball,
mixed for ten minutes. The mixed powders are emptied into graphite
crucibles with internal measurements of 34. 9 mm diameter by 31.8
mm deep. Twe nty four (24) filled crucibles are placed on a silica
tray and loaded into a muffle furnace only large enough to contain
the tray. Fusion takes 5 minutes from the time the preheated furnace
returns to its normal 1000oC after loading. The silica plate and
graphite crucibles are then removed from the oven and allowed to
cool. Each bead is reground in the swingmill for 35 seconds, the
glass powder then replaced in the graphite crucibles and refused
for 5 minutes.
Following the second fusion, the cooled beads are
labeled with an engraver, their lower flat surface is ground
on 600 silicon carbide grit, finished briefly on a glass plate
(600 grit with alcohol) to remove any metal from the grinding
wheel, washe d in an ultrasonic cleaner, rinsed in alcohol
and wiped dry. The glass beads are then ready to be loaded into
the XRF spectrometer. Preparation of a single bead takes, on average,
45 minutes.
A number of practical points in this process need
emphasis. Hand picking of fresh chips after the use of steel hammers,
hydraulic press, and steel jaw crusher should prevent significant
iron, chromium or nickel contamination, which resides mainly i
n the finer dust. It has long been recognized that tungsten carbide
mills cause contamination with tungsten and cobalt and these elements
are not analyzed. Niobium contamination has also been reported
from tungsten carbide mills (Joron et al., 1980; Hicks on and Juras,
1986) and tests using pure vein quartz suggest that different mills
cause variable degrees of Nb contamination, which is typically
of the same order of magnitude (2%) as the precision of the method
(one standard deviation < 1.0
ppm). Tantalum contamination is apparent (Table
1). Alumina ceramic mills can be substituted for tungsten carbide
but are brittle and therefore costly and only achieve the fine
and even-grained powder required over a much longer period. Fine
and even grinding is surprisingly important. Coarse powders result
in separation of mineral phases during fusion (even double fusion)
and can be a cause of high totals.
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Analytical
Procedure
The concentrations of 27 elements in the unknown samples are
measured by comparing the X-ray intensity for each element with
the intensity for two beads each of nine USGS standard samples
(PCC-1, BCR-1, BIR-1, DNC-1, W-2, AGV-1, GSP-1, G-2, and STM -1,
using the values recommended by Govindaraju, 1994) and two beads
of pure vein quartz used as blanks for all elements except Si.
The 20 standard beads are run and used for recalibration approximately
once every three weeks or after the analysis of abou t 300 unknowns.
The intensities for all elements are corrected automatically for
line interference and absorption effects due to all the other elements
using the fundamental parameter method. Operating conditions (Table
2) are unremarkable and the values used for the standards are listed
in Table 5.
Precision
Two
standard beads (BCR-P and GSP-1) are used as internal standards.
Kept in the same position in the automatic loader, they are run
between every 28 unknown samples and so provide a continuing check
on instrumental performance. They also provide a measure of instrumental
precision within a single run (Table 3) and between runs over much
longer periods (Table 4).
The other critical aspect of analytical
precision is the ability to reproduce the same concentration values
in many separate beads prepared from the same rock or mineral sample.
The important factors here are the homogeneity of the original
sample and the ability to make a homogeneous bead. Clearly, coarse
grained rock samples need to be homogenized adequately before mixing
with the tetraborate flux. Assuming that the sample powder is perfectly
homogenous, then the analysis of multiple beads prepar ed from
that powder should provide a realistic measure of the overall precision
of the technique (Table 4). A quick visual illustration of the
variation in elemental concentrations between two beads made from
the same powd er is provided in the vertical discrepancies between
each of the two beads made from the ten standard samples used to
create the calibration curves (Fig. 1).
In a laboratory dedicated
to the analyses of up to one hundred samples a week, every week
of the year, one of the most acute concerns is the possibility
of mixing samples. This can occur at any stage, but in the preparation
procedure used he re the most obvious step in which samples may
get mixed is the placing of the 20 samples in unmarkable carbon
crucibles in the furnace for fusion. As a precaution against mixing
at this stage the plate on which the samples are loaded is notched
and sample numbers recorded on a paper template. In addition, a
second bead is made from one, randomly chosen, sample from each
tray and reported as a "repeat" analysis.
Such repeat beads also provide the user with an immediate measure
of the precision of the analy ses and whether small variations
in the composition of two samples are analytically significant
or not.
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Accuracy
Unlike precision, a definitive measure of the accuracy of
geologic samples is not possible. We can best estimate accuracy by
comparing our results to the "given" values of standard
(reference) samples, compiled from numerous analyses by different
w orkers in different laboratories using a variety of techniques.
Reliance on oxide totals approximating to 100% as a measure of accuracy
is of limited value. While the use of totals as a test of accuracy
was the only such check available to the classical w et chemical
analyst, it should only be used in instrumental analysis as a rough
guide to locate gross errors, as in the weighing of sample and
flux. This is particularly true if, as in the methods outlined
here, the volatile content and oxidation state of iron are ignored.
The modern instrumental analyst has better methods of estimating
accuracy.
In the WSU GeoAnalytical Laboratory we estimate
the accuracy of our analyses in two ways: First, by the scatter
of the standard samples around the calibration curve for each element
(Fig. 1); and second, by comp aring our values to those of the
same samples analyzed by other workers in different laboratories
and using different techniques.
(1) Accuracy estimated by use of
standard samples
By treating the ten calibration standards as
unknowns and comparing the values so obtained to the "given" values
(that is, other peoples' best estimates) we gain an immediate visual
impression of accuracy (Fig. 1). In essence this is the amount
of scatter of any one sample from the calculated calibration curve
drawn through all 20 analyzed standard beads (Fig. 1). These results
are quantified in Table 5, where the observed WSU XRF values are
compared to the best or "given" values compiled by Govindaraju
(1994), normalized to 100% on a volatile-free basis.
For most major
elements (Fig. 1) the variation between the two standard sample
beads is of the same order as their variation from the given value,
with the inference that imprecision resulting from the preparati
on of the beads (as recorded in the overall precision, Table 4)
is equal to or greater than inaccuracies caused by inadequate matrix
and interference corrections. With the exception of Na, the total
discrepancies from the "given" values
are less than might reasonably be expected between two random samples
collected in the field from the same rock unit - lava flow, igneous
intrusion, etc. Hence, this degree of inaccuracy may be regarded
as insignificant for most purposes of geologic correlations or
petrogenetic modeling.
Among the trace elements the precision, and
therefore the accuracy, of Ni, Cr, Sc, V, and Ba is significantly
less than for Rb, Sr, Zr, Nb, Y, Ga, Cu, and Zn. This correlates
in part with the lower count rates (cts/sec/ppm) for Sc, V, and
Ba using a Rh target (Table 6). Ni, Cr, Sc, V, and Ba are regarded
as only semiquantitative below the 30 ppm level. Rb, Sr, Zr,
Nb, Y, Pb, and Th have satisfactory precision and accuracy down
to 1 to 3 ppm. La and Ce concentrations are qualitative only.
Precision
and accuracy of Sc, V, Ba, and Nb in particular, could be improved
by changing the X-ray tube target and operating conditions, but
only at a loss of some long term reproducibility for all elements.
The WSU GeoAnalytical Laboratory has an ICP/MS facility which
measures Sc, Ba, Pb, Nb and La and Ce with the other rare earth
elements much more accurately than XRF, so attempts to perfect
the XRF system for these elements have not been pursued.
(2) Accuracy
estimated from inter-laboratory comparisons
Major and Minor Elements
For each element a comparison of analyses
of the same powders by another laboratory has been attempted
using, where possible, the most appropriate technique.
For major
and minor elements other XRF laboratories have been used. Comparisons
are available from Los Alamos, the USGS (Denver), Rhodes University
(South Africa), and from XRAL (Canada). In addition, comparisons
of Fe and Na data by INAA are avail able from Washington State
University, Oregon State University, and the University of Oregon.
Na data have also been compared to ICP data from London.
Of these
various comparative data sets, that of 158 samples from the Cascade
Range supplied by Dr. Dave Sherrod, USGS, (Sherrod, 1986) and
run in Los Alamos have the widest concentration range and are illustrated
in Fig. 2. In general, the correlations are tight and within
the limits set by the precision measurements. Slight biases between
the WSU values and other XRF laboratories have been noted previously
(Hooper et al., 1993) and are not yet fully understood. The WSU
data sets appear to have consistently lower Fe (0.3% FeO) and
higher Si (0.45% SiO2) than other XRF laboratories. We suspect
this reflects differences in the Fe measurements which are then
reflected in SiO2 by the normalization procedure used. Ho wever,
no such bias is apparent between the WSU XRF data and WSU INAA
data for Fe from WSU (185 Cascade Range samples, Fig. 3a (Conrey,
1991)), nor between WSU XRF and INAA data from Oregon State University
(Hill, 1992), Fig. 3b. In all cases the biases are well within
the natural variation between two samples from the most homogeneous
flow from the Columbia Plateau and are therefore unlikely to prove
significant in petrologic studies. The Na data, while less precise
than that for other major elements, nevertheless compares well
with the Los Alamos XRF data (Fig. 2) and with INAA (WSU) and ICP
(London) data (Fig. 4).
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Trace Elements
Ni has been compared to XRF data
from Rhodes University, South Africa (J. S. Marsh, 1993, unpublished
data), XRAL, and six samples from the USGS-Menlo Park (Gardner,
1994). There is a fair scatter and the WSU data is consistently
10 to 15 ppm lower than the Rhodes values but similar or slightly
higher than the USGS (Fig. 5) and XRAL values. The Rhodes data
may not have been corrected for enhancement by Fe.
Cr XRF values
from WSU have been compared to XRF values from the USGS, XRAL,
and Rhodes University, and to INAA data from WSU and from Oregon
State University (OSU) (Fig. 6). The correlation is fairly tight
but th e WSU values appear lower, the discrepancy increasing at
higher concentrations (>100 ppm).
Sc values have been compared
to INAA values (WSU and OSU), ICP (London), and ICP/MS (WSU).
INAA should provide excellent Sc values and these comparisons are
shown in Fig. 7. ICP and ICP/MS comparisons are less tig ht, indicating
these techniques are somewhat less suitable for Sc analysis.
The main problem with the WSU XRF data for Sc is the poor precision,
a result of the low count rate caused by the combination of the
Rh target and 50 kV/50 mA settings used.
Duplicate analyses for
V are available by XRF from Rhodes University, by ICP (London,
Texas Tech. U.) (Fig. 8). Precision is again relatively poor
because of the set operating conditions, but no obvious bias is
ap parent.
Ba values are compared with ICP/MS (WSU) (Fig.
9a) and ICP (King’s
College and Texas Tech. U.) values (Fig. 9b). There is no discernible
bias over a large range in concentration, but again with a fair
scatter due to relatively poor precision.
Rb and Sr values indicate
both high precision and accuracy for these two elements. This
is well illustrated by a large data set for samples from Greenland
for which Dr. John Duke (University of Alberta, Edmonton) obtained
duplicate analyses by isot ope dilution (Fig. 10a and b) (Duke,
1993). Correlation with ICP/MS values is almost as tight. The
exceptionally close correlations demonstrated in these plots is
particularly significant because it implies that the rep roducibility
of the sample preparation technique must be at least that good.
And this, of course, is applicable to all other elements, so long
as the original powder was homogeneous.
Duplicate Zr values are
available by XRF (Rhodes University and U. S. G. S., Menlo Park;
Fig. 11a) and by ICP (London; Fig. 11b). No bias is apparent
but while very adequate, t he scatter on these plots is slightly
greater than expected, given a precision which is theoretically
as high as that for Rb and Sr. The answer may lie in the dispersed
nature of the principal Zr bearing phase, zircon; the powders may
not be entirely homo geneous with respect to this phase and element.
Duplicate
analyses for Y are available by XRF (U. S. G. S., Menlo Park,
Rhodes University, and XRAL), by ICP (London) and by ICP/MS (WSU).
The ICP/MS data correlates well with the WSU XRF data (Fig. 12)
although the two separate runs differ in that in one case the XRF
is slightly higher and in the other the XRF data is slightly lower
than the ICP/MS data. It is virtually impossible for this type
of variation to be due to the XRF in which the conditions are rigidl
y constant, so these differences are believed to reflect small
differences between the two ICP/MS runs.
Nb values have been compared
with XRF data (U. S. G. S., Menlo Park, XRAL, Rhodes University),
ICP data (London), and ICP/MS data (WSU). The results are scattered,
suggesting many laboratories have problems in obtaining good
Nb values. The tightest correlations of the XRF data are with the
ICP/MS values (Fig. 13), but there is a slight bias which increases
with concentration suggesting the XRF values are high. As for
Y, this bias differs significantly between the two runs suggesting
that at least a part of this problem lies with the ICP/MS values.
No
comparative data is available for Ga and duplicate Cu analyses
are only available from one XRF run (Rhodes University) which
demonstrates adequate correlation (Fig. 14a). Duplicate values
on Zn by XRF (U. S. G . S., Menlo Park and Rhodes University) and
by ICP (London) are again somewhat scattered but the relatively
good correlation with the ICP data (Fig. 14b), while implying a
small bias between the two data sets, suggests the WSU XRF data
are adequate. Clearly, more duplicate analyses are required for
Ga, Cu, and Zn to provide a better estimate of the accuracy of
the WSU XRF values.
XRF values for La and Ce are only quoted by
the WSU GeoAnalytical Laboratory because of special requests.
They demonstrate poor precision and are regarded as qualitative
only (Fig. 15a and b).
Recent XRF runs have been expanded to include
Pb and Th. Adequate comparisons are only available from the ICP/MS
(WSU). The Pb and Th values (Fig. 16a and b) show adequate correlation
and suggest the limiting fac tor in the accuracy of the values
for both elements is the precision of the XRF data.
Summarizing,
it is apparent that for the 17 trace elements analyzed on the
WSU XRF system, the accuracy imposed is that of the precision,
the limits of which are noted earlier (Tables 3 and 4). Small biases
may be present in some cases (e. g. Cr, N b) but few are significant
and none appear critical. The precision limits are, however,
important. These comparative plots serve to remind us of the high
reproducibility of XRF analyses in general but also that the XRF
technique loses precision at low con centrations (below 10 ppm
and, for some elements, below 30 ppm). At these lower concentrations
other techniques, isotope dilution and ICP/MS in particular, are
preferable.
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Stable Operating Conditions
The GeoAnalytical Laboratory
uses only a Rhodium target which is run at 50 kV and 50 mA with
full vacuum and a 25 mm mask for all elements and all samples.
The advantages for retaining the same conditions for all elements,
in addition to efficiency , is the greater stability and consequent
ability to reproduce the same intensities for the same sample
over long periods of time. This can be demonstrated for this laboratory
over a 10 year period (Table 4). In addition, the 2:1 tetraborate
beads can be stored for a demonstrable 30 years without significant
deterioration and can be re-run if and when the basic equipment,
standards, or running conditions are modified. This level of
precision has been critical to the tracin g of the subtle differences
between the many flows sampled from the Columbia Plateau over that
period.
The disadvantages of using such constant operating
conditions are loss of precision and accuracy for some elements,
notably Sc, V, Nb and Ba, for which these conditions are not ideal.
Oxidation
State and Volatile Content - LOI
The WSU GeoAnalytical Laboratory
normally ignores the oxidation state of iron in whole rock samples,
quoting all the iron as FeO and normalizing to 100% without measuring
the volatile content. LOI and oxidation state are measured only
for parti cular purposes or on special request.
In general, we regard
the volatile content and oxidation state of igneous rocks as
a distraction for most petrogenetic work. Both are products of
post eruptive processes (alteration) in large part and serve to
distort the composition immediately pr ior to eruption which is
our principal concern. When data on the volatile content and oxidation
state are lacking, it follows that original totals can be used
only as a rough check for major errors in weighing, smaller variations
in the totals will reflec t variable oxidation states and volatile
contents. The use of normalized values has caused some concern
amongst our colleagues, especially those introduced to geochemistry
through wet chemical analysis in which the total, including volatile
content, was t he obvious check on the accuracy of the analysis.
As discussed above, there are now much better ways of measuring
precision and estimating accuracy. Incorporation of oxidation
states and volatile content so distort analyses of Columbia River
basalt, to us e but one example, that their use on the Columbia
Plateau significantly reduces our ability to correlate flows. Had
this approach been adopted our present knowledge of Columbia River
basalt flow stratigraphy would be much less precise.
Two other factors
are involved. The analysis of volatiles and the oxidation state
of iron tends to be labor-intensive, creating an unjustified
cost except in particular circumstances. Both, of course, are independent
of the X-ray analysis and can b e added or discarded any time,
so long as the totals are not relied upon as a measure of accuracy
for the whole analysis. Finally, this laboratory would argue
that in analytical comparisons inclusion of the oxidation state
of iron and the volatile content distorts the results and makes
the comparisons of little value (Govindaraju, 1994). To determine
genuine bias and analytical differences between laboratories it
is essential to calculate the iron in a single oxidation state,
eliminate the volatile (LOI) content, and normalize to 100. This
is because the methods of measuring the LOI are so variable that
differences in these values between laboratories distort the abundances
of all other elements (again, see Govindaraju, 1994).
Conclusions
We argue that the single low-dilution fusion technique is
superior to the more traditional high dilution fusion and pressed
powder technique in its much greater efficiency which is achieved
without measurable loss of either precision or accuracy.
There
are advantages and disadvantages in using stable operating conditions,
in which neither the target nor the voltage are changed between
elements. The adoption of such a procedure is likely to depend
on the specific aims of any one research pro gram. Finally, the
measurement of the oxidation state of iron and the volatile content
should not be used to distort otherwise excellent XRF analyses.
Acknowledgments
We are grateful to Drs. Wright,
Nye, Marsh, and C. and M. Barnes for use of their analytical
data. Purchase of the XRF facility at Washington State University
was supported by the Murdoch Foundation and the National Science
Foundation.
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References
Conrey, R. M., 1991, Ph. D. dissertation, Washington State
University, Pullman.
Duke, M. J. M., 1993, Ph. D. dissertation, University
of Alberta, Edmonton.
Gardner, C. A., 1994, U. S. Geological Survey
Open-file Report 94-261, 100 p.
Govindaraju, K., 1994, Geostandards
Newsletter, vol. 18, Special Issue, p. 1-158.
Hickson, C. J. and
Juras, S. J., 1986, Canadian Mineralogist, vol. 24, p. 585-589.
Hill,
B. E., 1992, Ph. D. dissertation, Oregon State University, Corvallis.
Hooper,
P. R., 1964, Analytical Chemistry, vol. 36, p 127.
Hooper, P.
R., Johnson, D. M. and Conrey, R. M., 1993, Washington State
University, Department of Geology, open-file report.
Joron, J. L.,
Briqueu, L., Bougalt, H., and Treuil, M., 1980, Initial Reports
of the Deep Sea Drilling Project, vol. LIV, U. S. Gov't., Washington,
D. C., p. 725-727.
Madin, I., 1994, State of Oregon Dept. of Geology
and Mineral Industries Geological Map Series GMS-60 with accompanying
text and table of chemical data.
Norrish, K. and Hutton, J. T.,
1969: Geochim Cosmochim Acta 33, 431.
Sherrod, D. R., 1986, Ph.
D. dissertation, Santa Barbara, 320 pp.
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