Accurate thresholding using a compound-Poisson-lognormal lookup table and parameters recovered from standard single particle ICP-TOFMS data

The use of time-of-flight (TOF)-based instruments in single particle inductively coupled plasma-mass spectrometry (ICP-MS) is growing quickly. While these instruments have critical advantages over their quadrupole counterparts, they present new challenges when determining thresholding values in single particle analyses. One challenge is the need to analyse the single-ion area (SIA), which is essential for accurate thresholding in single particle data sets. However, the SIA is different for each element and changes across usage, time and during detector calibration. Rapid and effective algorithms are required to determine the SIA and predict thresholds automatically. Here we introduce new tools to investigate and fit the SIA with a lognormal distribution and, for determining background signal in single particle ICP-TOFMS. First, a lookup table of compound-Poisson-lognormal quantiles was computed using a simulation of 10¹⁰ random values. This improved accuracy of thresholds at large lognormal standard deviations and was significantly faster than our previous approach. To facilitate its use, we have implemented it into our data processing software, SPCal. We also present a method to recover the SIA parameters that are required for thresholding from both raw ionic and particle data, enabling on-site SIA fitting during normal data processing. This method was tested both in simulation and experimentally, across different instruments, conditions and masses. The limitations of the method are discussed and conditions required for successful recovery determined.