Optimizing pXRF sample preparation for in situ soil screening: A regulatory framework perspective

This study evaluates the impact of sample preparation methods on the accuracy and precision of portable X-ray fluorescence (pXRF) measurements for trace metals in urban soils in Hartford, CT, in the context of residential regulatory thresholds. Overall, the slopes of dry versus wet and pelletized versus dry sample comparisons ranged from 1.19 to 1.40 and 0.74 to 1.08, respectively, indicating that drying soil samples generally results in higher measured concentrations due to the removal of moisture dilution, while pelletization can slightly reduce measured concentrations through sample homogenization and increased material density. These slope values reveal the consistent influence of preparation on measured concentrations and the potential need for correction factors. Wet in situ analysis, while practical for field application, can underestimate metal concentrations in areas with moist soil conditions, as presented in the current case study, missing regulatory exceedances for lead (Pb) in up to 8% of cases at the 100 mg kg^-1 threshold and 7% at the 200 mg kg^-1 threshold compared to pelletized samples. Dry in situ measurements reduced missed exceedances to just 1.7% for both thresholds, striking a balance between practicality and accuracy. Pelletization minimized variability and increased precision but offered limited added value for screening purposes. These results underscore the need to adapt pXRF preparation methods to regulatory and contextual requirements, demonstrating its potential as a cost-effective and scalable tool for urban soil contamination assessment.