Electrospray Ionization (ESI)
ESI mass spectrometry (MS) was first published by Yamashita and Fenn1 in 1984, but it was based on earlier work by Dole, Iribarne, and Thompson. The advent of nanoelectrospray in 1996, published by Wilm and Mann, increased the use of ESI MS, particularly when samples were limited in concentration or volume. Two theories about the mechanism of ESI have been proposed: the charged residue model and the ion ejection model. In the charged residue model, the total charge of the droplet remains the same as the solvent evaporates, and it is left on the non-volatile components. In the ion ejection model, the solvent evaporates from the droplets until the force from Coulombic repulsion overcomes the surface tension of the droplet and individual ions are expelled. In either case, the resulting ions are highly charged, and each has a distribution in the number of charges (called the charge state distribution). For biological molecules such as peptides and proteins, the charge is introduced by protonation or deprotonation.
Preparation of the Samples
The analyte of interest is usually dissolved in aqueous solution and mixed with a similar volume of volatile organic solvent. This solution is then loaded into the nanospray emitter for single sample analysis or it can be subjected to liquid chromatography before it is electrosprayed (LC-ESI MS).
Analysis of the Samples
Each sample is sprayed using a voltage difference between the ESI needle and the orifice plate. In certain cases, backpressure or sheath gases are used to aid in nebulization of the spray. In this instrument, the electrospray ionization is performed at atmospheric pressure. Then, the ions are introduced into the vacuum using several directing optics, while the solvent and other neutral molecules are excluded using skimmers. In this particular instrument, the ions are transported and focused through quadrupoles (denoted as the primary beam); then, mass analysis is done using an orthogonal time-of-flight (TOF) mass spectrometer. Tandem mass spectrometry (MS/MS) can also be performed in the primary beam using collision-induced dissociation (CID), so this instrument is configured primarily for peptide sequencing.
Yamashita, M. and Fenn, J.B. (1984). Electrospray ion source. Another variation on the free-jet theme. J. Phys. Chem. 88, 4451-4459.