SPLAT II: An Aircraft Compatible, Ultra-Sensitive, High Precision Instrument for In-Situ Characterization of the Size and Composition of Fine and Ultrafine Particles

This article describes SPLAT II, a new aircraft compatible single particle mass spectrometer that provides significantly improved performance when compared with SPLAT, or any other existing single particle mass spectrometer. SPLAT II detects and characterizes 100% of spherical particles and ∼30% of aspherical particles with diameters between 125 nm and 600 nm. It also brings significant increase in temporal resolution, sizing over 500 particles per second, while characterizing the composition of up to 100 of them. The increase in sensitivity to small particles makes it possible, under most conditions, to use a differential mobility analyzer upfront SPLAT II in order to simultaneously measure in addition to individual particle size and composition, a number of other particle attributes, such as density or effective density, dynamic shape factor, fractal dimension, and even hygroscopicity. SPLAT II provides sizing precision on the order of a monolayer, and makes it possible to distinguish between spherical and aspherical particles. SPLAT II uses a two-step, two-laser process to generate ions. The mass spectra of the semivolatile fraction are generated by ionization in the gas phase, reducing fragmentation and yielding highly reproducible mass spectra, while the mass spectra of the refractory fraction are simultaneously generated by ablation. The instrument control board generates size-dependent delays for laser triggers to eliminate a size-dependent hit-rate and mass spectra are recorded with 14-bit resolution. Data analysis is facilitated by SpectraMiner and ClusterSculptor; two visually driven software packages specifically designed to analyze the large datasets that this instrument produces.