Overview about API Methods

Ionization methods that operate at AP, such as the APCI and the ESI, have greatly expanded the scope of mass spectrometry [16-19]. These API techniques allow an easy coupling of chromatographic separation systems, such as liquid chromatography (LC), to a mass spectrometer. 

A fundamental difference exists between APCI and ESI ionization mechanisms. In APCI, ionization of the analyte takes place in the gas phase after evaporation of the solvent. In ESI, the ionization takes place already in the liquid phase. In ESI process, protonated or deprotonated molecular ions are usually formed from highly polar analytes. Fragmentation is rarely observed. However, for the ionization of less polar substances, APCI is preferably used. APCI is based on the reaction of analytes with primary ions, which are generated by corona discharge. But the ionization of nonpolar analytes is very low with both techniques. 

For these classes of substances, other methods have been developed, such as the coupling of ESI with an electrochemical cell [20-31], the coordination ion-spray [31-46], or the dissociative electron-capture ionization [37-41]. The APPI or the dopant-assisted (DA) APPI presented by Syage et al. [42, 43] and Robb et al. [44,45], respectively, are relatively new methods for photoionization (PI) of nonpolar substances by means of vacuum ultraviolet (VUV) radiation. Both techniques are based on photoionization, which is also used in ion mobility mass spectrometry [46-49] and in the photoionization detector (PID) [50- 52]. 

In the past, one of the main problems of mass spectrometric analysis of proteins or other macromolecules was that their mass was outside the mass range of most mass spectrometers. For the analysis of larger molecules, such as proteins, a hydrolysis and the analysis of the resulting peptide mixture had to be carried out. With ESI, it is now possible to ionize large biomolecules without prior hydrolysis and analyze them by using MS. 

Based on previous works from Zeleny [53], and Wilson and Taylor [54,55], Dole and co-workers produced high molecular weight polystyrene ions in the gas phase from a benzene/acetone mixture of the polymer by electrospray [56]. This ionization method was finally established through the work of Yamashita and Fenn [57] and rewarded in 2002 with the Nobel Prize for Chemistry. 

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