Achieving Size Independent Hit-Rate in Single Particle Mass Spectrometry

Recent improvements in single particle mass spectrometers make it possible to optically detect, size, and characterize the compositions of individual particles with diameters larger than a micron and smaller than 100 nm. In these instruments, two stages of optical detection are used to generate a precisely timed trigger pulse that is used to fire the ion generation laser or lasers. However, experience shows that the wide particle size range results in significant differences in laser trigger timing between small and large particles. If not treated these differences produce an instrument with size dependent hit-rate. In this case the operator is forced to optimize the instrument for the desired size range, while contending with a significantly lower hit-rate for other particle sizes. This article presents an analysis of the phenomenon and demonstrates that the dependence of laser trigger timing on particle size stems from the differences in the particle position within the detection laser beam at the instant of detection. We demonstrate that it is possible to compensate for these differences by generating, for each particle, a laser trigger delay coefficient that is a function of particle's time of flight, i.e., its vacuum aerodynamic size. The study also shows that a single function can be used to eliminate the size bias for particles with a wide range of densities.