Effects of moisture content on in situ analysis of plant samples using portable XRF

Analysis of the element composition of plant materials has many applications, including monitoring nutrient status, detecting biogeochemical indications of mineral deposits and assessing the effectiveness of phytoremediation in contaminated soils. Portable X-ray fluorescence spectroscopy (pXRF) delivers the advantage of real time in situ multi-elemental analysis at low cost, but calibration is affected by factors that include the water content of plant organs. The effect of variation in moisture content on pXRF-determined concentrations of heavy metals Zn, Fe, Cu, Sr and Th, and nutrient elements Si, S, K and Ca, have been evaluated using the foliage of A. imperialis, D. excelsa and A. macrorhiza through a controlled continuous drying experiments. A segmented linear relationship between pXRF measurements and moisture content was observed for most elements with a rapid decrease followed by a moderate decrease as moisture content increased. The position of the inflection point is dependent on leaf thickness and energy of the main X-ray peak measured. Calibration issues related to variation in moisture content comprises a combination of dilution and spectral interference effects. Dilution accounts for most of the underestimation of pXRF-determined concentrations for fresh plant samples compared with laboratory methods on dried samples. As moisture contents increase, the relative influence of spectral interferences decreases. The single-layer thickness of plant sample affects the position of inflection point of linearity and the relative contribution of spectral interference effect. This study provides new insights into the effect of moisture on pXRF-determined elemental concentrations and offers practical suggestions and recommendations for in situ analysis of plant samples using pXRF.