Electrospray Ionization: Theory and Application

Abstract

A great deal of chemistry and most of biochemistry concerns reactions taking place in solution. The development and application of an introduction and ionization technique which allows direct sampling from solution has therefore been a prominent objective of analytical mass spectrometry for many years. Furthermore, the removal of the requirement for the volatilization of neutral analyte prior to ionization has long been recognized as the sine qua non for the analysis of many molecules of biological importance, including biopolymers such as peptides and proteins, oligosaccharides and oligonucleotides. The development by Barber and his colleagues [1,2] of fast atom bombardment (FAB) significantly extended the range of mass spectrometric analyses to less volatile compounds than were amenable to traditional techniques such as electron and chemical ionization. The elaboration of the FAB method to allow ionization from a surface continuously replenished by a flowing stream of analyte solution [3,4] introduced new capabilities (albeit rudimentary by current standards) for direct mass spectrometric analyses of solutions. Thermospray [5], expressly developed for the coupling of mass spectrometry and condensed-phase separation methods such as high-performance liquid chromatography (HPLC), addressed the need for sampling from solution but was limited with respect to the volatility and polarity of the analytes amenable to the technique.