Increase of the particle hit rate in a laser single-particle mass spectrometer by pulse delayed extraction technology
Abstract. A single-particle mass spectrometer (SPMS) can provide a
wealth of valuable information on chemical and physical parameters of
individual particles in real time. One of the main performance criteria of
the instrument is efficiency of particle detection (hit rate). Most SPMS
instruments use constant electrical field (DC) extraction, where stationary
high voltage is applied to the extraction electrodes. As the aerosol
particles initially carry a certain charge, those with a high amount to
charge can be deflected by this electric field and lost, thus decreasing the
hit rate. We realized that the delayed extraction technique can eliminate the
stochastic dispersion of the particle beam caused by their deflection in the
stationary electric field. As the result, the hit rate of the instrument can
be significantly improved. Also, as the effect of the deflection in the
electric field is mass dependent, it can cause distortion of the measured
size distribution of the particles. Hence, the delayed extraction technique
can bring the recorded distribution closer to the actual one. We found that
the delayed extraction technique provides a mass resolution improvement as well
as increases the hit rate. The gain in the hit rate depends on the type of
particles. It can be 2 orders of magnitude for model particles and up to
2–4 times for ambient particles. In the present work we report experiments
and results showing the effect of the delayed extraction on the beam
divergence caused by particle charge, the hit rate improvement, and the
effect of the delayed extraction on the measured particle size distribution.