APPI is mainly used as an alternative to normal corona discharge APCI. APPI
has produced equal or better detection limits compared to APCI in selected
applications (Wang et al., 2005), however, the ionization process in APPI depends
directly on the solvent, dopant, nebulizing gas, and impurities in the gas and
surrounding atmosphere, as well as the analyte itself (Yang and Henion, 2002).
11.2.2.4 Desorption Electrospray Ionization (DESI) In DESI (Fig. 11.2a), a
fine spray of charged droplets is directed to the surface of interest, from which
small organic molecules and large biomolecules are desorbed and ionized.
Following desolvation these ions are presented to the mass spectrometer for
analysis (Cooks et al., 2006; Takats et al., 2004). The mechanism of DESI is
not yet clear but several processes have been suggested to account for the
ionization of molecules, including, droplet pick up and chemical sputtering and
evaporation followed by gas-phase ionization (Takats et al., 2005b). DESI
causes minimum fragmentation, resulting in the formation of intact molecule
ions. The main advantage of DESI compared to other desorption ionization
methods is that it does not require sample pretreatment and it can be
FIGURE 11.2 Schematics of a typical DESI (a) and a DART (b) desorption ionization
source. Reprinted from Cooks et al. 2006 with permission of American Association for
the Advancement of Science.
324 APPLICATION OF LIQUID CHROMATOGRAPHY/MASS SPECTROMETRY