Sources of uncertainty in isotope ratio measurements by inductively coupled plasma mass spectrometry

A model is presented describing the effects of dead time and mass bias corr ection factor uncertainties, flicker noise, and counting statistics on isot ope ratio measurement precision using inductively coupled plasma mass spect rometry (ICPMS) with a single collector. Noise spectral analysis is exploit ed to enable estimation of the flicker noise parameters. For the instrument used, the flicker noise component exhibited a fairly weak frequency (f) de pendence (proportional to f (-0.33 +/-0.12)), but was directly proportional to the total number of counts, Q. As white noise, determined by counting s tatistics, is given by Q(0.5), the isotope ratio measurement uncertainties will actually cease to improve when Q exceeds a certain threshold. This wou ld suggest that flicker noise could become the limiting factor for the prec ision with which isotope ratios can be determined by ICPMS, However, under most experimental conditions, uncertainties associated with mass discrimina tion and dead time correction factors are decisive. For ratios up to simila r to 22 (In-115/In-113), optimum major isotope count rates are generally be low 0.3 MHz, for which precision in the mass discrimination factor is limit ing. The model derived could be used as a starting point for determining op timum conditions and understanding the limitations of single-collector ICPM S for precise isotope ratio measurements.