TRACKING HEAVY METALS IN THE FIELD: ISSUES WITH PORTABLE X-RAY FLUORESCENCE (PXRF) SPECTROMETRY OF SOILS

X-ray florescence spectroscopy (XRF) is a non-destructive analytical technique for determining elemental composition. Originally done using laboratory-based machines requiring pressed power pellets or fusion in a glass matrix, technical advances have led to the development of field portable units (PXRF) with detection limits in the part per million (ppm) range. With current technology, the elemental signature of dry samples (≥K in air & ≥Na under vacuum) can be rapidly (<5 minutes for bulk composition) measured. This greatly increases the utility of XRF analysis, but has also increased the potential for misuse. Manufacturers often sell units as ‘point and shoot’, which mischaracterizes the technology and can led to poorly structured analytical studies and generation of flawed data.

Analysis requires application of a suitable processing technique and all quantification methods have baseline assumptions about the sample analyzed. For accurate quantification, samples must be homogeneous, uniform, very well sorted, and have flat, smooth surfaces to ensure correct beam geometry, as well as large enough to interact with the entire incident beam and ‘infinitely thick.’ Finally, the sample should be dry, as water dramatically attenuates x-rays. Studies involving in situ measurements show that for every 1% increase in soil moisture there is an 1.15 to 1.75% decrease in measured analyte concentration.

Samples often deviate from ideal, thus it is vital to ensure appropriate selection of instrument parameters to maximize data quality, while recognizing the methods limitations. In literature, PXRF studies include data collected in situ against the ground surface (sometimes with the ground-cover intact), across stratified cross-sections, or while the soil is contained in a storage bag. Note that plastic and Mylar coverings attenuate x-rays, impacting quantification of light elements. If a soil sample is analyzed in a 4 mil polyethylene storage bag, no Al will be observed because its characteristic radiation will be completely attenuated. Likewise, Si and P are also highly attenuated, by 96 and 90% respectively. No amount of post-processing and statistical analysis will make up for inappropriately chosen conditions and analysis parameters. Such misunderstandings of technique can potentially lead to serious errors.