spraying a sample solution from the tip of an electrically charged capillary in
the presence of a flow of warm nitrogen to assist desolvation and then
measured the ions by ion mobility techniques. The most recent resurgence of
interest in ESI–MS resulted from the work of Fenn and coworkers who
demonstrated, for the first time, that intact molecule ions of large peptides and
proteins can be generated by ESI and their molecular weight determined by
deconvolution of the mass-to-charge (m/z) envelope generated by multiple
charging (Whitehouse et al., 1985).
In this technique, a solution of the analyte is passed through a capillary tip
that is held at high potential (typically in the range of 2.5–5 kV) to generate a mist
of highly charged droplets. Two mechanistic models, the charge residue model
(CRM) and the ion evaporation model (IEM), have been proposed for the gas-
phase ion formation from liquids (Kebarle, 2000; Kebarle and Ho, 1997). In the
CRM, evaporation of solvents from the initially formed droplets leads to an
increasing charge density on the droplet surface. Once the droplets have reached
the Rayleigh limit (the point in which charge repulsion exceeds the surface
tension of the droplet), Coulombic explosion occurs and the droplets are ripped
apart. This process continues to the point where no further evaporation of the
solvent can occur and finally individual ions or ion clusters are formed. In the
IEM, when solvent evaporation and Coulomb droplet fissions reduce the size of
charged droplets to a certain radius (10–20 nm), direct emission of ions into the
gas-phase begins to occur. Ion evaporation replaces Coulomb fission and
evaporation of ions continues until a solid residue of ion clusters is formed. It has
been postulated that small ions are more likely formed by IEM, while large
protein ions are proposed to be formed by CRM.
Due to the inherent nature of electrohydrodynamic atomization and ion
formation, electrospray is more controlled and yields higher sensitivity at lower
flow rates. Wilm and Mann developed the ‘‘nanospray’’ technique in which a
small volume (mL) of sample was loaded into a metal-coated capillary tip and
was sprayed at the flow rate of 30–200 nL/min (Wilm and Mann, 1996). These
nanospray tips can be located very close to the nozzle that samples ions into the
mass spectrometer, thus increasing sensitivity. At these extremely low flow
rates, small amounts of sample can undergo ESI for a prolonged period of time
allowing many experiments to be performed and yielding maximum informa-
tion at very high sensitivity.
ESI is often referred to as a ‘‘soft’’ ionization technique in that little
fragmentation of intact molecular ions is produced. Despite the numerous
benefits of ESI–MS, it suffers from a shortcoming in that it is susceptible to ion
suppression effects from high concentrations of buffer, salt, and other
endogenous material in matrix solutions.
11.2.2.2 Atmospheric Pressure Chemical Ionization (APCI) In an APCI
source, liquid effluent is nebulized into fine droplets by a high gas flow and
carried into a heated chamber where the solvent droplets are evaporated.
322 APPLICATION OF LIQUID CHROMATOGRAPHY/MASS SPECTROMETRY