Fig. 1.14 LC-MS analysis of a mixture of daidzein and acetophenone with a ESCi source: (A) ESI, (B) APCI, (C) photo diode array detection. Adapted with permission from reference .
An other approach has been described by Syage et al. , who investigated the potential of various ionization sources (ESI, APCI, APPI) either in simultaneous or in switching mode. They suggest that ESI/APPI is the best combination because APPI covers a broad range of analytes while ESI covers the larger molecules.
22.214.171.124 Desorption Electrospray and Direct Analysis in Real Time
Direct analysis of solid samples or analytes present on solid surfaces without any sample preparation has always been a topic of interest. Desorption electrospray ionization (DESI) is an atmospheric pressure desorption ionization method introduced by Cooks et al., producing ions directly from the surface to be analyzed, which are then sampled with the mass spectrometer [22, 37]. DESI is based on charged liquid droplets that are directed by a high velocity gas jet (in the order of 300 m s-1) to the surface to be analyzed. Analytes are desorbed from the surface and analyzed by mass spectrometer (Fig. 1.15).
Compared to atmospheric pressure MALDI (see Section 1.3.3.), no matrix is needed to perform the experiment. Direct analysis in real time (DART), a method related to DESI, has been reported by Cody et al. . This technique is based on the reactions of metastable helium atoms generated by corona discharge with oxygen/water (negative mode) or water clusters (positive mode). The formed reac-tant ions ionize the analytes either by cluster assisted desorption or proton exchange. Both methods generate mostly protonated or deprotonated molecular ions. Various applications of both techniques for the analysis of the mass spectro-metric profiling of intact biological tissue nicely demonstrated the characterization of the active ingredients in pharmaceutical samples formulated as tablets, ointments, or the sampling of plant material .
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