title: Investigation of the quasi-simultaneous arrival (QSA) effect on a CAMECA IMS 7f-GEO.
abstract: BACKGROUND
  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 for16O-and18O-plus an additional16O-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 × 105counts 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.
authors.affiliation: Department of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA.
authors.forename: Clive
authors.identifier: http://orcid.org/0000-0002-9800-344X
authors.initials: C
authors.lastname: Jones
authors.affiliation: Department of Earth & Planetary Sciences, Washington University in St. Louis, St. Louis, MO, 63130, USA.
authors.forename: David A
authors.initials: DA
authors.lastname: Fike
authors.affiliation: CAMECA, 29 quai des Gresillons, 92622, Gennevilliers Cedex, France.
authors.forename: Paula
authors.initials: P
authors.lastname: Peres