Current Electrospray Mass Spectrometry: An Overview. PartB. Analyte Charging

Abstract

Electrospray ionization (ESI) is an atomization and ionization method through which a solution‐phase analyte can be transferred into the gas‐phase as an ion via minute charged droplets. The second part of this two‐part review on electrospray mass spectrometry reviews the final part of the charged droplet trajectory and covers transfer into the gas‐phase and ionization of the analyte. The now customary separation into the ion evaporation model (for small analytes) and the charged residue model (for large analytes) is followed and reviewed. These models yield insight into the charging process, but observations made with newer atomization techniques and quantitation limitations imposed by ionization suppression (matrix effect) indicate that a complete understanding is not available yet. Deviations from expected charged residue or ion evaporation behavior will be indicated. Chemical and charge‐driven surfactancy is discussed in relation to small analyte ionization and quantitation, whereas hydration and kinetic trapping of protein conformers are treated as part of the charged residue model.

Conformational effects in protein electrospray‐ionization mass spectrometry

“…The present analysis is still compatible with both the current, alternative models concerning the ESI mechanism, the ion-evaporation model (IEM) (Iribarne & Thomson, 1976) and the chargedresidue model (CRM) (Dole et al, 1968). Further studies will be required to understand which model more adequately describes the production of protein ions in the gas phase (Konermann et al, 2013;Ogorzalek Loo, Lakshmanan, & Loo, 2014;Verkerk, 2014b;Yue, Vahidi, & Konermann, 2014).…”

“…However, the ionization process is in general less effective in negative-ion rather than positive-ion mode, giving rise to CSDs centered on lower charge states (Heck & van den Heuvel 2004;Pinkse et al, 2003). This phenomenon can be ascribed to the different nature of the involved solvent components and protein functional groups in the two different instrumental settings (Verkerk, 2014b). As an example, in ammonium acetate buffer, the intrinsic basicity of protein and buffer is set more apart than their intrinsic acidity .…”

“…However, it is interesting to ask whether a similar correlation holds for gas-phase structures. To test this hypothesis, structural models for desolvated protein ions produced by ESI are needed (Verkerk, 2014a;Verkerk, 2014b). As discussed in the following sections, development of such models is complicated by the extraordinary large number of different patterns of ionized residues (protomers) corresponding to a given value of net charge.…”