Mechanisms of Ion Formation in DESI

Three processes of ion formation have been proposed for DESI [17] depending on the conditions and applied solvent-analyte pair one of these will predominate i) droplet pickup, ii) condensed phase charge transfer, and Hi) gas phase charge transfer. 

Hi) Gas phase charge transfer means that ion formation occurs after volatilization or desorption of neutral species from the surface into the gas phase through ionization via proton/electron transfer or other ion-molecule reactions at atmospheric pressure. Indeed, the assumption of ion-molecule reactions, eventually purely in the gas phase above the sample, has led to the development of an DAPCI source (Chap. 13.2), in the first place to prove this mechanism of ion formation [7]. The solvent pH can be used to positively affect the vapor pressure of the analyte, e.g., the vapor pressure of volatile plant alkaloids is increased by addition of abase. 

DESI can serve for the identification of natural products in plant material [1,12,13], of lipids in animal tissue [16,19], for high-throughput analyses of pharmaceutical preparations [20], and for drug metabolite identification or even quan- 

The limit of detection for small peptides is in the order of 1 pg absolute or 0.1 pg mm , for proteins in the order of 1 ng absolute or 0.1 ng mmT small molecules such as pharmaceuticals and explosives are detectable in the range of 10-100 pg absolute or 1-10 pg mm [10,12]. These numbers indicate a footprint of about 10 mm roughly corresponding to a circular spot of 3 mm in diameter. Smaller footprints can be realized - sanple and sensitivity of the instrument permitting. The number of DESI papers is steadily growing. A few representative examples of DESI applications are conpiled below. 

Example The negative-ion DESI mass spectrum of the over-the-counter drug acetylsalicylic acid (aspirin) was measured with a home-built DESI source attached to a seasoned triple quadrupole instrument (Fig. 13.5). The sarrple was presented on paper to a methanol spray. Whereas this sample even allowed tandem mass spectra of the [M-H] ion, m/z 179, to be readily obtained, other samples were beyond the instrument s sensitivity limits [15]. 

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