Ambient mass spectrometry

In 2004 a novel ambient ionization technology, called Desorption ElectroSpray Ionization (DESI), was proposed by the Cooks group at Purdue University [63,64]. For the first time, surfaces can be probed and analysed by MS under real native ambient conditions, without any sample preparation or matrix addition. The typical lay-out of a DESI source is shown in Figure 6. In short, a pneumatically assisted electrospray is used to produce charged microdroplets

Solvent

Solvent

moving sample stage in air

Figure 6 Schematic view of a DESI source.

moving sample stage in air

Figure 6 Schematic view of a DESI source.

and gas-phase solvent ions that are directed onto the sample at a surface. The ionization process closely resembles conventional ESI-MS; however, the sample is not present in the solvent nor ionized during the electrospray process, and is therefore less vulnerable to ionization suppression caused by the presence of salts and other interfering matrix components. Instead, ionization of the sample molecules takes place at or above the surface as a result of proton transfer, electron transfer, or ion transfer by gas-phase ions [65]. Reagents can be added to the electrospray solvent in order to tune the selectivity and the sensitivity of the DESI process [66]. That versatility is a great advantage over the direct analysis in real time (DART) ionization alternative in which helium or nitrogen is used as a reaction gas [67]. Depending on the capabilities of the MS, targeted (selected ion mode), untargeted (full-scan mode) or accurate mass analysis is possible, yielding for example, new biomarkers from differential metabolomics with the aid of principal component analysis (PCA) [68]. The ion transfer line to the atmospheric inlet of the MS can be extended and integrated with the electrospray nozzle, thus creating a probe for in situ sensing of any native or cut surface. Widely variable substrates such as paper, metal, Teflon, human skin, animal organs such as pancreas, liver and brain, plant parts such as stems, seeds, flowers and roots, glass, leather, bricks, polymers (Nylon, latex, PDMS) and silica gel have been sampled successfully. Measurements take on average 3 s but high-throughput analysis (3 samples/s) with a moving belt has also been described [69]. Automated chemical imaging of surfaces is possible. Depending on the dimensions of the electrospray tip and the distance to the surface, spot sizes will vary from 2 x 2 mm down to 50 x 50 mm. In terms of analytes, the technique is more versatile than standard ESI-MS, as also highly non-polar molecules such as carotenoids, steroids, terpenes and even volatiles can be analysed. The sensitivity of the technique lies in the low picogram or high femtogram range. DESI-MS is also believed to be quantitative but we have some reservations in that respect. So far, the applicability of DESI technology has been demonstrated in forensics (detection of explosives, of drug residues on banknotes, Ecstasy), pharmaceutical [69-71], plant sciences (in vivo analysis of plant alkaloids) [72], and clinical analysis, diagnosis of cancerous tissue [73] and metabolomics [68].

The option of direct surface analysis of plant alkaloids, pesticide residues and natural toxins will be very attractive for chemical food contaminant analysis. For complex, mixtures the combination of TLC and DESI can be considered, as recently demonstrated for alkaloid dietary supplements [74].

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