A Reassessment of the Quasi-Simultaneous Arrival Effect in Secondary Ion Mass Spectrometry

TL;DR

This paper reevaluates the quasi-simultaneous arrival effect in secondary ion mass spectrometry, showing that it deviates from the Poisson model and proposing a standards-based correction method to improve isotope measurement accuracy.

Contribution

It introduces pulse-height distributions and time-series data to challenge the Poisson model of QSA, and develops an emission-transmission-detection model for better correction strategies.

Findings

QSA effects are over-dispersed and follow a negative binomial distribution.

Poisson model does not adequately explain experimental data.

Standards-based correction effectively minimizes QSA impact.

Abstract

Quasi-simultaneous arrival (QSA) effects in secondary ion mass spectrometry can create mass-indepedent inaccuracies in isotope measurements when using electron multiplier detectors (EMs). The simple Poisson statistical model of QSA does not explain most experimental data. We present pulse-height distributions (PHDs) and time-series measurements to better study QSA. Our data show that PHDs and the distribution of multiple arrivals on the EM are not consistent with the Poisson model. Multiple arrivals are over-dispersed compared to Poisson and are closer to a negative binomial distribution. Through an emission-transmission-detection model we show that the QSA correction depends on the non-Poisson emission of multiple secondary ions, the secondary ion energy distribution, and other factors, making an analytical correction impractical. A standards-based correction for QSA is the best approach, and we show the proper way to calculate standards-normalized $\delta$ values to minimize the effect of QSA.