Electrospray ionization (ESI) is a method of generating ionic forms of molecules for mass spectrometry . It generates ions directly from solution (usually an aqueous or aqueous/organic solvent system) by creating a fine spray of highly charged droplets in the presence of a strong electric field (1), as shown in Fig. 1. Subsequent vaporization of these charged droplets results in the production of multiply charged gaseous ions. The number of charges retained by an analyte can depend on such factors as the composition and pH of the electrosprayed solvent, as well as the chemical nature of the sample (2-4). For large molecules, the ESI process typically gives rise to a series of multiply charged species for a given analyte. Because mass spectrometers measure the mass-to-charge (m/z) ratio, the resultant ESI mass spectrum contains multiple peaks corresponding to the different charged states (Fig. 2).
Figure 1. The electrospray ionization process.
Figure 2. Examples of data generated on an ESI mass spectrometer. Proteins (left) typically produce positive multiply cl generate negative multiply charged ions. The insets for each part show the molecular-weight spectra computer-generated.
The extent of multiple charging that occurs in ESI is a unique characteristic of the technique that enables an analyte’s mass to be determined with great precision, as masses can be independently calculated from several different charged states (5). The multiple charging in ESI also permits the analysis of high-molecular-weight analytes, using conventional mass analyzers that are normally limited to the detection of ions with relatively low m/ z ratios. For example, a 50-kDa protein will typically retain on the order of 30 to 50 charges in ESI, yielding multiply charged species with m/z ratios between 1000 and 2000 that are easily detected with quadrupole mass analyzers . Another advantage of ESI-MS is its compatibility as an interface with liquid chromatography (see Liquid Chromatography Mass Spectrometry). Electrospray ionization is compared to the other common method of ionization in Matrix-Assisted Laser Desorption/Ionization.
