An investigation into detector dead time variability and mass dependency in quadrupole ICP-MS and implications for isotope ratio accuracy

Abstract

Quadrupole inductively-coupled-plasma mass-spectrometers (Q-ICP-MS) are used to analyse enormous numbers of isotope ratios, most prominently ²⁰⁶Pb/²³⁸U for age determination. With the advent of reaction-cell equipped tandem Q-ICP-MS (Q-ICP-MS/MS), the scope for isotope ratio determination has grown, especially for β-decay isotope systems (e.g., ⁸⁷Rb/⁸⁷Sr). Reaction of unwanted interfering species (e.g. ²⁰⁴Hg on ²⁰⁴Pb) has also increased the feasibility of Pb-isotope ratio measurements by Q-ICP-MS. Unlike in sector-field MS isotope ratio analysis where mass bias is corrected via a known isotope ratio or with calibrated double or triple-spikes, Q-ICP-MS(/MS) analysts generally apply an ‘external correction’ with reference to interleaved calibration standard analyses. Despite this protocol, there remains inaccuracies in derived isotope ratios when compared to reference values which are inadequately explained. The aim of this study was to investigate the origin of the inaccuracy in Q-ICP-MS isotope analysis, which has so far received surprisingly little attention. We assessed whether improved detector dead time correction can achieve more accurate isotope results and explored fractionation effects arising from steering ion beams across complex Q-ICP-MS/MS paths. We investigated detector dead time as a function of Z in both single MS and mass-shifted MS/MS mode. We document how dead time varies over time as detectors and electronic components of the ICP-MS age. We show that session specific dead times yield substantially more accurate isotope ratios for mass shifted Sr isotope ratios than when applying a generic dead time. We also provide recommendations for how to incorporate session-specific dead time analyses into an isotope ratio run. Despite the much-improved quality of dead time and conventional mass bias corrected mass shifted Sr isotope ratios, small (up to 4‰) inaccuracies remain. The inaccuracy is systematic and specific for each session. Therefore, it can be corrected with a small bias correction relative to a CRM analysed throughout the session. The origin of the Sr isotope ratio inaccuracy after dead time and conventional mass bias correction remains speculative. Our exploratory analysis suggests that voltages on lenses tuning and directing the ion beams through the MS/MS may be a source of an additional isotope fractionation process (bias) that cannot be fully corrected with conventional mass bias procedures.