X-Ray Fluorescence Services at Washington University

Introduction

The Washington University X-ray Fluorescence Laboratory provides research-quality chemical analysis of rocks and minerals. As a research facility, the aim of the laboratory is to be able to analyze the widest possible range of geological materials with confidence. Considerable effort has been expended to ensure accuracy for a very wide range of compositions and sample types. We are actively working on the mathematical methods and practice of X-ray analysis, and have made several contributions in the areas of absorption-enhancement corrections, and sample preparation.

In order to accommodate the widest possible range of sample compositions, we have been careful to use modern, general methods that ensure accurate analysis for virtually any sample. A combination of NIST and other standard reference materials is supplemented by synthetic standards, to assure accuracy for virtually any composition, regardless of whether representative, well-characterized reference materials are available. We take particular care in accurate background subtraction, spectral interference correction, and absorption-enhancement correction. We continually test our methods over composition ranges that exceed the expected range of sample compositions.

The day-to-day operation of the laboratory is under the direction of Dr. Rex Couture, who is in charge of maintenance, sample preparation facilities, and development of analytical methods. Prof. Robert Dymek, whose main interest is in Pre-Cambrian petrology, is responsible for overall direction. Click here for photos.

Available Services

X-ray fluorescence (XRF) is usually used for analysis of bulk solids, at concentrations between approximately 1 mg/kg (depending on the analyte element) and 100%. It can also be used for micro samples, thin samples, aerosols, and liquids, with detection limits of 2 to 20 ng/cm² for most elements. XRF is still the only method that can be used to analyze routinely for almost any element from Na to U, including non-metals. XRF analysis has the additional advantage that it does not require dissolution of a sample, thereby eliminating concern about insoluble residues.

For major-element rock analysis we analyze diluted, fused glass discs for Na, Mg, Al, Si, P, K, Ca, Ti, Mn, Fe. Loss on ignition (LOI) also serves as an approximate measure of volatile H₂O or CO₂, and may be used to infer the presence or absence of major concentrations of unanalyzed elements. For trace elements, we routinely analyze pressed powder pellets for V, Cr, Co, Ni, Zn, Ga, Rb, Sr, Y, Zr, Nb, Sn, Ba, and Pb, which are commonly found in many samples. On request, we can also analyze for Mo, As, La, W, Sc, and other elements; semi-quantitative analysis for sulfate is possible.

At present we offer complete major- and trace-element analysis of silicate, carbonate and oxide rocks and sediments on a routine basis, and can analyze a wide variety of other materials. Materials we have analyzed include air particulate matter, boiler condensate, chemical reagents, potsherds, soils, ocean sediments, sulfates, clay minerals, hematite and ilmenite ore materials, silicate and sulfide minerals, carbonates, and phosphates.

We are usually willing to undertake special problems involving unusual sample types or unusual compositions. Sometimes we do pass up opportunities, though: Among the most interesting samples we were ever asked to analyzed were unstable mixtures of highly explosive compounds. Oddly, no one else could find an appropriate method.

Methods

Our methods are mentioned only briefly here. The methods and accuracy of our methods for major-element analysis has been reported elsewhere (Couture, 1989; Couture et al., 1993). We have adopted a number of refinements on the usual dilution-fusion methods to assure homogeneity and gravimetric accuracy of the prepared samples. For example, the melt is continuously mixed during fusion, to assure homogeneity; the sample and flux are mixed in the platinum crucible used for fusion, thereby eliminating large, selective loss of sample material to extraneous containers; the samples are oxidized before and during fusion, to eliminate exchange of iron with platinum crucibles, which is commonly a very large source of error in XRF analysis. We also use a large, expensive platinum mold similar to that used by the U.S. Geological Survey (Taggart and Wahlberg, 1980), which assures that the cast disks are completely flat and polished without further handling. As an added benefit, the failure rate is essentially zero with virtually all silicate and oxide rock types, thereby minimizing the potential for loss of small samples.

hot platinum mold

We use a combination of mathematical methods for X-ray-absorption and secondary-fluorescence corrections, which have a solid theoretical basis (Couture et al., 1993; Couture and Dymek, 1996). This is especially important for materials which differ greatly in composition from available standards, and for samples that have unexpectedly high concentrations (>0.1%) of certain trace elements. Couture and Dymek (1996) have discussed this in detail. A method that they introduced was verified to within 1% relative for analysis of V in Fe ore. Couture (1993) also introduced a method for analysis of translucent samples, which is necessary for analysis of heavy elements such as Sn.

One of the harder aspects of trace-element analysis is in determining the background, so that analyses are reliable near the detection limit. For XRF the background region is usually curved, increasing the difficulty. By using pure compounds as references, we have construct background functions that interpolate the background region accurately for any sample (Couture, 1993).

Accuracy

Table 1 shows the results of a blind test, organized by the French Geological Survey, in which 104 labs worldwide analyzed two new reference rocks. The Washington University analyses agreed with the preferred mean to equal or better than the standard deviation for 22 of 24 elements. The results were particularly good at concentrations near detection limits.

We also achieve good agreement with other standard reference materials and with highly accurate neutron activation analyses. For a comparison of analyses with literature values and other reference values, contact Rex Couture. We also analyze a single reference sample in every run, as a check on reproducibility. These data are routinely reported with analytical results.

Submission of Samples

For a price list, description of services, and suggestions for sample preparation and submission, contact Rex Couture or Robert Dymek.

Contact Information

References

Couture, R.A. (1989) An improved fusion technique for major-element analysis by XRF. Advances in X-Ray Analysis 32, 233-238.

Couture, Rex A. (1993) Sample transparency effects in tin determination by x-ray fluorescence. X-Ray Spectrometry 22, 92-96.

Couture, Rex A. and Dymek, Robert F. (1996) A reexamination of absorption and enhancement effects in X-ray fluorescence trace element analysis. American Mineralogist 81, 639-650.

Couture, Rex A., Smith, Michael S., and Dymek, Robert F. (1993) X-ray fluorescence analysis of silicate rocks using fused glass discs and a side-window Rh source tube: accuracy, precision and reproducibility. Chemical Geology 110, 315-328.

Taggart, J.E., Jr., and Wahlberg, J.S. (1980) New mold design for casting fused samples. Advances in X-Ray Analysis 23, 257.