Isotope dilution with ICP-MS—simplified uncertainty estimation using a robust procedure based on a higher target value of uncertainty

Abstract

The objective of this paper is to bring forward a simple tool for uncertainty estimation in ID-MS (isotope dilution mass spectrometry) using a robust measurement procedure. This basic form does not allow the ultimate performance to be obtained regarding uncertainty, but will still produce results that have reliable uncertainties which can be more difficult to evaluate with other calibration strategies (such as external calibration, internal standard, standard addition). The paper describes an a priori simulation process of the uncertainty that will be achieved using a validated procedure for an analyte in a matrix. For this, a valid model equation is built, where the different influence quantities are allowed to vary within a wide, robust tolerance window. For these influence quantities an approximate measurement uncertainty contribution relative to the measurand (i.e., quantity to be measured) is calculated and only those that significantly affect the measurement uncertainty (expanded, k = 2, relative) at the target level are taken into account. A target value of uncertainty of 2% is used here as an example since it is deemed to be achievable by experienced laboratories that are not primarily concerned with measuring at a lower uncertainty level. The approach is demonstrated in the determination of nickel in low-alloy steel and copper in fresh water. However, this basic approach using target uncertainty and wide tolerance windows is general and can easily be adapted for other applications.