A comparison of different calibration strategies for the trace analysis of soil samples by inductively coupled plasma mass spectrometry (ICP-MS)

Abstract

The National Metrology Institute of South Africa (NMISA) is expanding its capabilities in the field of the analysis of trace and ultra-trace levels of toxic and minor elements in environmental samples, with a special focus on soils to support projects of the University of Pretoria (UP) in the field of veterinary geology for the One Health Platform. The most accurate calibration strategy developed by the laboratory involves double isotope dilution analysis, where a stable isotope standard of the element of interest is used as the ideal internal standard. External calibration is the most commonly used calibration strategy for elemental analysis. To reduce the impact of instrumental drift on the accuracy of the measurement results, nominally the same amount of an internal standard is added to the samples and the standards. Standard addition is a calibration strategy that could eliminate the issue of matrix effects for trace and ultra-trace elemental analysis, because the calibration standards are added as spikes to aliquots of the sample to prepare the calibration curve. The purpose of the study was to compare the results obtained for the analysis of trace elements in soil samples using the calibration strategies of isotope dilution analysis; gravimetric external calibration with gravimetric internal standardisation; as well as gravimetric standard addition with gravimetric internal standardisation. The aim was to show that the application of gravimetric preparation to the more routine calibration approaches of external calibration and standard addition could achieve very comparable results and associated measurement uncertainties to isotope dilution analysis. The application of gravimetric sample preparation could assist routine laboratories, that have significant resource constraints, to improve the accuracy of their measurement results. These approaches could also be used in the experimental homogeneity and stability studies required for the production of reference materials, where high precision methods are required.