On the Mechanisms Involved in Spray Ionization

Several successful methods for analyzing large, nonvolatile molecules employ direct coupling of liquid chromatography to mass spectrometry by means of some kind of spray technique. These methods include thermospray [1], electrospray [2], and aerospray or ionspray [3], which involve dispersing the liquid effluent into an aerosol using heat, electrical fields, and pneumatics, respectively. Simplified schematic diagrams of the various spray techniques are given in Figure 1. In all of these instruments, molecular ions are transferred directly into the gas phase from the condensed phase as a consequence of evaporating the charged droplets produced in the aerosol. This paper attempts to summarize the present understanding of some of the processes involved in producing charged droplets in the various spray techniques as well as those involved in the subsequent release of molecular ions into the gas phase, and transmission of the gas phase ions into the vacuum of the mass spectrometer.