Abstract
The accuracy and precision of lead isotope ratio measurements by quadrupole-based inductively-coupled plasma-mass spectrometry (ICP-MS) can be limited by any of a number of instrumental factors mass bias being one of the most relevant. Mass bias can be defined as the deviation of measured isotope ratios from the “true value” due to the different transmission of ions according to their masses before the final detection. In the present research, a systematic study aimed at obtaining a more profound insight into to what extent the potential of the ion optic system and gas-filled octapole collision cell influence the mass discrimination in lead isotopic measurements (206Pb/207Pb, 208Pb/207Pb and 206Pb/208Pb) using ICP-QMS instrumentation was carried out. From the results obtained, it could be concluded that, in most cases, the effect of ion lens potential variation in mass discrimination is not really significant when working in maximum ion intensity regions. On the other hand, the application of pressurized conditions in the octapole collision/reaction cell using He and H2 as target gases does not lead to an improvement in ion sensitivity but, instead, introduces a significant mass bias effect, particularly when using high He flow rates (6–8 mL min−1). In this latter case, the use of Tl as the internal standard (203Tl/205Tl) proved to be suitable to correct the mass bias drift and the calculated mass discrimination percentage values decreased from 3.61% to 0.33%. The use of the gas-filled octapole collision cell does not lead to an improvement in lead isotope ratio precision compared to vented conditions.
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