FIELD LIMIT FOR ION EVAPORATION FROM CHARGED THERMOSPRAY DROPLETS

Thermospray ionization produces charged droplets from electrolyte solutions. As these droplets evaporate, the self induced field on the droplet surface increases and ion production from these droplets can take place either by ion evaporation or by Rayleigh subdivision, depending on the critical field strength required to initiate either process at that instant. An attempt was made to determine the field strength on the surface of the charged droplets in their final stage of evaporation, by measuring their reduced mobilities in a drift tube equipped with a mechanical chopper for gating the droplets. The relation between the reduced mobility and the surface field strength is discussed. The performance of the drift tube is such that in some cases the width of the mobility distributions is not significantly greater than the dispersion estimated from the apparatus. For the volatile and nonvolatile electrolytes studied, only one major peak was observed in the mobility spectra. The measured reduced mobilities are 0.44-0.53 cm2 V-1 s-1 and the corresponding surface field strengths are 7.6-9.1 x 10(8) V m-1. No peaks were observed with field strengths higher than 10(9) V m-1. The observed mean value, 0.83 x 10(9) V m-1, is interpreted as the critical field strength for ion evaporation and its implications are discussed.