[22] Electrospray ionization mass spectrometry

This chapter discusses electrospray ionization mass spectrometry, and in the case of noncovalently bonded species, the mass spectra show a distribution of charge states which usually spans about one-half the number of the most abundant charge state.

Abstract

This chapter discusses electrospray ionization mass spectrometry. Electrospray ionization (ESI) occurs during the electrostatic nebulization of a solution of charged analyte ions by a large electrostatic field gradient (approximately 3 kV/cm). Highly charged droplets are formed in a dry bath gas, at near atmospheric pressure. These charged droplets shrink as neutral solvent evaporates until the charge repulsion overcomes the cohesive droplet forces leading to a “Coulombic explosion.” Large oligopeptides and small proteins so far successfully examined by ESI-MS show a distribution of multiply charged molecular ions arising (in positive ion mode) by either proton or alkali ion attachment and no evidence of fragment ions because of fragmentation unless dissociation is induced during transport into mass spectrometer vacuum by collisions at higher energy. The largest protein so far examined is the native covalent dimer of bovine serum albumin with a Mr of more than 130 kDa. It should be noted that in the case of noncovalently bonded species, such as protein subunits, the mass spectra show only the contributions of the individual subunits. These mass spectra show a distribution of charge states which usually spans about one-half the number of the most abundant charge state.