A technique for estimating contamination tolerance limits for stable isotope ratio determinations

A model of the interaction between the precision of an isotope ion signal measurement and the accuracy of an isotope ratio determination was developed and used to derive the equations required to calculate the maximum tolerable amount of contamination for stable isotope ratio determinations. Comparison of the calculated tolerance limits and the blank estimated amount of contaminant will establish whether or not a correction for the contribution of the contaminant to the gross signals will be required. The 1000:1 sample-to-contaminant concentration tolerance limit used in stable isotope ratio plasma mass spectrometry will, in some situations, underestimate the contamination error compared to the model calculations.Derivation of the limit equations required an empirically determined relation between the signal strength and the signal's relative standard deviation (signal noise function). A single hyperbolic signal noise function was used to describe the behaviour of isotope ion signals of Ni, Cu, Tl and Pb measured using a plasma source double focusing magnetic sector mass spectrometer. The derivation could be extended to accommodate different signal noise functions.