Portable X-ray fluorescence spectrometry accurately measures metal concentrations in aqueous Mehlich III soil extraction solutions

Accurate measurement of metal concentrations in soil and water is vital for healthy crop production and decision making for environmental surveys. While there are a multitude of laboratory-based soil analysis methods, such as inductively coupled plasma-optical emission spectroscopy (ICP-OES), flame emission spectrometry, and atomic absorption spectroscopy, most are time and resource intensive. Additionally, there is a lack of information for rapid analysis of elements for aqueous soil extractions. The goal of this research is to establish elemental correlations between portable X-ray fluorescence (pXRF) measurements of Mehlich III soil extractions and traditional elemental measurements via ICP-OES. We hypothesize that certain metals can be accurately measured in aqueous soil extraction solutions by pXRF to the same degree as they are measured by ICP-OES. To test this hypothesis, Mehlich III and 2% nitric acid solutions with known elemental concentrations were analyzed via ICP-OES and pXRF. Soil samples extracted using Mehlich III were compared between ICP-OES and pXRF to verify correlations. High correlations were found for As, Ca, Cd, Cr, Cu, Fe, Mn, Mo, Ni, Pb, Se, V, and Zn in both the Mehlich III and 2% nitric acid solutions at concentrations between 5 and 85 mg L-1. P, S, and Si did not show high correlations at concentrations <100 mg L-1. These results indicate that between 5 and 85 mg L-1, pXRF analysis of aqueous solutions and soil extractions is a reliable technique; however, at low concentrations (i.e., <5 mg L-1 for metals and <100 mg L-1 for P and S), pXRF is not well suited.