Portable X-ray fluorescence analysis of mineral and organic soils and the influence of organic matter

Abstract

Portable X-ray fluorescence (PXRF) spectrometers with tube X-ray source are being used to determine the elemental composition of soils in the laboratory and the field. Most studies with PXRF have been with mineral soils and there is a need to assess PXRF applications to organic soils and peats. Using a commercial PXRF instrument and the manufacturer's soils programme we assessed performance by the analysis of various soils with a range of organic matter concentrations up to those of low ash peats. We first analysed seven certified reference mineral soils and a certified, low ash ombrotrophic peat. The data obtained for the certified reference mineral soils was definitive for Ti, Cr, Mn, Fe, Ni, Cu, and As, quantitative for K, Ca, Zn and Sr, and qualitative for Pb. Portable X-ray fluorescence analysis of the ombrotrophic peat gave satisfactory results for Cu (4.00±1.00mgkg−1, certified 5.28±1.04mgkg−1) and Pb (184±3mgkg−1, certified 174±8mgkg−1) but overestimated the concentrations of Ca, Ti, Cr, Ni and Zn by 2–3 times, and Fe by 5 times. To extend comparison beyond the only available low ash certified reference soil and further exemplify the differences between mineral soils and peat soils we analysed 183 Scottish topsoils that had a wide range of organic carbon (OC) concentrations (1.23 to 48.8% by weight) and compared the concentrations of K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Sr and Pb determined by PXRF with pseudo total concentrations determined by aqua regia extraction and inductively coupled plasma spectrometry. The results demonstrate that to maximise the potential of the PXRF instrument for use beyond mineral soils, to organo-mineral and peats, modifications to the manufacturer's calibrations should be made. We recommend validation using soils on a continuum of OC concentrations from those of mineral soils to peats covering the range required.