Full uncertainty calculation on quantitative determination of tracer (111Cd) cadmium and natural cadmium in soil column effluents with ICP-MS

Abstract

In laboratory-scale migration experiments a tracer (stable ¹¹¹ Cd isotope) is added on top of a contaminated soil and the column effluents are fractionally collected. This paper describes an analytical method for the simultaneous low level quantitative determination with inductively coupled plasma mass spectrometry (ICP-MS) of contaminant (natural) and tracer ( ¹¹¹ Cd) cadmium in soil column effluents by adaptation of the isotope dilution equation. For the calculation of the full uncertainty on the quantitative determination of tracer and natural cadmium, a method has been proposed emphasising the practical, realistic approach of estimating uncertainties based on statistical assumptions. The GUM Workbench© program, the computations of which follow the rules of the ‘ISO guide to the expression of uncertainty in measurement’, was used. At low concentrations of cadmium (<0.2 ng ml ^–1 Cd) the uncertainty due to counting statistics is the major source of uncertainty. At higher concentrations the signal instability of the ICP-MS instrument, partially due to clogging of the sampling cone by calcium salts present in the leachant (1 mM CaCl ₂ ), forms the largest contribution to the total uncertainty. For concentrations of 0.5 ng ml ^–1 Cd and higher, the expanded uncertainty U amounts to ±4% (coverage factor 2; 95% probability).