Use of Portable XRF: Effect of Thickness and Antecedent Soil Moisture on Measured Concentration of Trace Elements.

The use of portable X-ray fluorescence (PXRF) for rapid measurements of concentrations of trace elements in soils is increasing. The purpose of this study was to assess in a systematic way the influence of soil moisture and sample thickness on concentrations of Ni, Zn, Cd, and Pb in a mixture of glass beads as well as Windsor loamy sand and Webster loam soils. In this context, the efficacy of the Compton Normalization (CN) calibration method to correct for changes in soil moisture was determined. Despite CN calibration, an inverse correlation between PXRF measured concentrations and soil moisture was observed. The magnitude of this effect depends upon the energy of the characteristic X-ray fluorescence of each element, with those emitting lower energy X-ray fluorescence being more greatly influenced. Moisture contents ranging from 0.09 to 0.26 cm³ cm^-3 were found to generate trace element concentrations with no statistical difference, with significantly lower concentrations reported at greater moisture contents. In addition, measured concentrations of each trace element increased with increasing sample thickness until a constant measured concentration was attained, which was also correlated with the energy of the characteristic X-ray fluorescence of the corresponding trace element. Ni, with the lowest energy fluorescence, obtained constant measured concentration at 3 mm for all matrices, whereas a sample thickness of up to 10 mm was needed for Cd. In order to account for energy differences across a range of trace elements and matrices, the use of PXRF should be limited to soil samples having a thickness of at least 10 mm.