Investigation of the quasi-simultaneous arrival (QSA) effect on a CAMECA IMS 7f-GEO

Rationale: IMS 7f-GEO isotope ratio applications increasingly involve analyses (e.g., S^– or O^– isotopes, coupled with primary ion currents <30 pA) for which quasi-simultaneous arrival (QSA) could compromise precision and accuracy of data. QSA and associated correction have been widely investigated for the CAMECA NanoSIMS instruments, but not for the IMS series. Methods: Sulfur and oxygen isotopic ratio experiments were performed using an electron multiplier (EM) detector, employing Cs⁺ primary ion currents of 1, 2, 5 and 11.5 pA (nominal) and a variety of secondary ion transmissions to vary QSA probability. An experiment to distinguish between QSA undercounting and purported aperture-related mass fractionation was performed using an EM for ¹⁶O^– and ¹⁸O^– plus an additional ¹⁶O^– measurement using a Faraday cup (FC) detector. An experiment to investigate the accuracy of the QSA correction was performed by comparing S isotopic ratios obtained using an EM with those obtained on the same sample using dual FCs. Results: The QSA effect was observed on the IMS-7f-GEO, and QSA coefficients (β) of ~0.66 were determined, in agreement with reported NanoSIMS measurements, but different from the value (0.5) predicted using Poisson statistics. Aperture-related fractionation was not sufficient to explain the difference but uncertainties in primary ion flux measurement could play a role. When QSA corrected, the isotope ratio data obtained using the EM agreed with the dual FC data, within statistical error. Conclusions: QSA undercounting could compromise isotope ratio analyses requiring ~1 × 10⁵ counts per second for the major isotope and primary currents <20 pA. The error could be >8‰ for a 1 pA primary current. However, correction can be accurately applied. For instrumental mass fractionation (IMF)-corrected data, the magnitude of the error resulting from not correcting for QSA is dependent on the difference in secondary ion count rate between the unknown and standard analyses.