Moisture, organic matter, and large particle correction for accurate Pb portable X-ray fluorescence assessment in urban soils

The need for accurate quantification of lead concentrations in urban centers around the world is turning attention towards rapid methods for contamination assessment. Portable X-ray Fluorescence (pXRF) has been recognized as a more accessible alternative to traditional digestion methods. This study was conducted to produce measurement corrections for in-situ pXRF measurements conducted on field soils under varying conditions of moisture, organic matter (OM) content, and particle size heterogeneity. Urban forest soils were analyzed for Pb under increasing soil moisture, OM, and coarse fraction contents with two different pXRF and the results were compared to reverse aqua regia digestions. While both instruments showed a strong correlation despite using different calibration methods, correction of pXRF data to digestion values improved accuracy but potentially underestimated Pb concentrations. We propose an empirical correction equation derived from the regression of variance between theoretical and observed concentrations to correct for moisture effects. OM and particle size heterogeneity did not produce a significant effect on Pb measurement and did not warrant correction. Our study confirms other work showing that pXRF provides an accurate and precise alternative to traditional digestion methods for Pb screening, provided that moisture is corrected.