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

Clive Jones; David A Fike; Paula Peres

doi:10.1002/rcm.7828

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

Rapid Commun Mass Spectrom. 2017 Apr 15;31(7):623-630. doi: 10.1002/rcm.7828.

Authors

Clive Jones¹, David A Fike¹, Paula Peres²

Affiliations

¹ Department of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA.
² CAMECA, 29 quai des Gresillons, 92622, Gennevilliers Cedex, France.

PMID: 28093819
DOI: 10.1002/rcm.7828

Abstract

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 ¹⁶ O^- and ¹⁸ O^- plus an additional ¹⁶ O^- 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. Copyright © 2017 John Wiley & Sons, Ltd.