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The secret to electrospray ionization lies in three key steps.
Electrospray ionization (ESI) belongs to a group of methodologies known as atmospheric pressure ionization techniques, in which ions or molecules in solution are transferred to the gas phase before sampling into a mass analyzer as ionized species.
When electrospray ionization is used to interface high performance liquid chromatography (HPLC), which is a solution-phase technique, to mass spectrometry, which is a gas-phase technique, two major challenges arise. First, the analytes involved may be nonvolatile, and they will need to be transferred to the gas phase. Second, a large amount of solvent must be evaporated and vented before sampling the gas phase ions, to prevent a vacuum compromise in the mass spectrometer. To overcome these issues, electrospray ionization uses three important processes, explained below.
Production of Charged Droplets
The first stage in electrospray ionization is the production of charged eluent droplets at the tip of the sprayer by applying an electric field. In "positive ion" mode, the capillary is the anode and the sampling aperture plate is the cathode. Positive ions in the eluent solution are repelled from the inner walls of the "sprayer" needle and move electrophoretically into the body of the droplet formed at the capillary tip. This mode causes positive ions (cations) to predominate in the sprayed droplet and is used in cases where the analytes (such as bases) form cations in solution. The opposite is true in "negative ion" mode.
The point at which the surface charge repulsion overcomes the surface tension of the eluent droplet at the sprayer tip is the Rayleigh instability limit. At this point, the meniscus at the sprayer tip changes to a cone shape to relieve charge repulsion. This is referred as the Taylor cone. Upon formation of the cone, a stream of droplets containing a vast excess of either cations or anions will emerge from its surface (Figure 1). This process is termed electrospray. The formation of stable spray will be highly dependent on the voltage applied to the sprayer capillary, which should be optimized (in terms of the combination of analyte, eluent, and flow rate) for each experiment.
Figure 1: Desolvation and ion production processes in electrospray ionization.
Desolvation of the Charged Droplets, Leading to Droplet Fissions
As the droplet is sprayed, it will shrink as a result of solvent evaporation (desolvation), aided by increased ambient air temperature in the ionization chamber. As the droplet shrinks, its radius decreases but its charge remains constant. This leads to an increase in the Coloumbic repulsion forces between the surface charges, until, once again, the Rayleigh instability limit is reached and the droplet undergoes Coulombic (droplet jet) fission in which a series of smaller droplets are liberated from the main droplet.
Because the resulting offspring droplets hold a greater charge per mass (volume) than the original droplet, they quickly undergo further fissions to produce successively smaller droplets. The cascade of droplet fission processes leads ultimately to very small droplets, each containing a small number of theoretical charges. The whole evaporative process occurs during the residence time of the droplet in the desolvation zone — usually a few hundred microseconds to a few milliseconds.
Production of Gas-Phase Ions
There are two popular theories regarding the mechanism by which Coulombic stress is relieved and gas-phase ions are formed.
In the "charged residue theory," further droplet fissions continue until very small droplets containing a single ion each are produced; solvent evaporation from these droplets then leads to the formation of gas-phase ions (1).
Experimental evidence most strongly supports a second mechanism, known as ion evaporation, which suggests that below a droplet radius of 10 nm, an ion is able to "evaporate" from within the droplet (2,3). The main supporting evidence for this theory comes from ion mobility studies, which show the production of significant amounts of gas-phase ions at times when most of the charged droplets are expected to have relatively large radii and multiple charges.
(1) M. Dole, L.L. Mack. R.L. Hines, R.C. Mobley, L.D. Ferguson, and M.B. Alice. J. Chem. Phys. 49, 2240 (1968).
(2) J.V. Iribarne and B.A. Thompson. J. Chem. Phys. 64, 2287 (1976).
(3) B.A. Thompson and J.V. Iribarne. J. Chem. Phys. 71, 4451 (1971).