DAPPI

Several other ionization methods have been developed based on DESI, including desorption atmospheric pressure chemical ionization (DAPCI), desorption atmospheric pressure photo-ionization (DAPPI), laser ablation electrospray ionization (LAESI), and extractive electrospray ionization (EESI). Each technique uses variations of the solvent, how the charged beam is formed, and how the beam is nsed to facilitate the prodnction of analyte ions. Because these are surface methods (except EESI), they are incompatible with LC. 

Methods Ambient ionization methods, of which there are now over 20, e.g., desorption electrospray ionization (DESI), desorption atmospheric pressure chemical ionization (DAPC), desorption atmospheric pressme photo-ionization (DAPPI), and direct analysis in real time (DART), are now joined by paper spray, a method where ESI is initiated at the pointed tip of a piece of filter paper. A drop of blood ( 15 pi) is dried on the paper, and then the paper is moistened with 25 pi of a solvent suited to both the extraction of the analytes from the blood and the ESI process (e.g., 90% methanol 10% water with either 100 ppm acetic acid or 200 ppm sodium acetate). When the paper is exposed to high voltage (3-5 kV) while held close ( 5 mm) to the entrance of the mass analyzer, a spray (similar to electrospray) is induced at the tip of the paper as capillary action carries extracted compounds through the paper (Figure 4.5). The spray is maintained for 30-90 s at a flow rate comparable to that used in nano-electrospray. 

Most of the mass spectrometry analyses are conducted under vacuum environment. However, ambient mass spectrometry is a rapidly growing field that provides fast and direct analysis of solid sample surfaces or liquid samples introduced on a suitable surface (Alberici et al. 2010 Weston 2010 Huang et al. 2010 Chen et al. 2010). For that, different ambient ionization MS methods, such as atmospheric pressure desorption/ionization on porous silicon (AP-DIOS) (Huikko et al. 2003), desorption electrospray ionization (DESI) (Takats et al. 2004), direct analysis in real time (DART) (Cody et al. 2005), desorption atmospheric pressure chemical ionization (DAPCI) (Takats et al. 2005), and desorption atmospheric pressure photoionization (DAPPI) (Haapala et al. 2007), have been successfully used in the direct analysis of compounds fi"om various samples, such as body fluids (Cody et al. 2005 Chen et al. 2006), finiits, plant leaves (Luosujarvi et al. 2010), milk (Yang et al. 2009), banknotes (Cody et al. 2005), textiles (Cody et al. 2005 Chen et al. 2007), and pharmaceutical formulations (Ifa et al. 2009 Gheen et al. 2010), just to mention a few, without any sample pretreatment. 

Ambient MS is another advance in the field. It allows the analysis of samples with little or no sample preparation. Following the introduction of desorption electrospray ionization (DESI) [108,109], direct analysis in real time (DART) [110], and desorption atmospheric pressure chemical ionization (DAPCI) [111, 112], a number of ambient ionization methods have been introduced. They include electrospray-assisted laser desorption/ionization (ELDI) [113], matrix-assisted laser desorption electrospray ionization (MALDESI) [114], atmospheric solids analysis probe (ASAP) [115], jet desorption ionization (JeDI) [116], desorption sonic spray ionization (DeSSI) [117], field-induced droplet ionization (FIDI) [118], desorption atmospheric pressure photoionization (DAPPI) [119], plasma-assisted desorption ionization (PADI) [120], dielectric barrier discharge ionization (DBDI) [121], and the liquid microjunction surface sampling probe method (LMJ-SSP) [122], etc. All these techniques have shown that ambient MS can be used as a rapid tool to provide efficient desorption and ionization and hence to allow mass spectrometric characterization of target compounds. 

Desorption electrospray ionization (DESI) [1] was introduced at the end of 2004, and direct analysis in real time (DART) [2] soon after in 2005. The apparent potential of both DESI and DART in high-throughput applications soon led to the development of some derivatives with the intention to broaden the field of applications or to adapt the underlying methodology to specific analytical needs. Now, the repertoire of methods includes variations of the DESI theme such as desorption sonic spray ionization (DeSSI) [3], later renamed easy sonic spray ionization (EASI) [4] or extractive electrospray ionization (EESI) [5,6]. Then, there are the DESI analogs of APCI and APPI, i.e., desorption atmospheric-pressure chemical ionization (DAPCI) [7,8] and desorption atmospheric pressure photoionization (DAPPI) [9]. 

The mechanism of ion generation in DAPPI has been proposed to be a combination of thermal and chemical processes. After thermal desorption of the analytes from the surface they can be photoionized in the gas phase. However, analytes with no UV chromophore may only be ionized by ion-molecule reactions with dopant ions. Dopant molecular ions, as formed from toluene for example, may promote charge exchange, while protonated dopant ions will yield [M+H]" ions. 

Fig. 13.11. Schematic of (a) the DAPPI setup and (b) photograph of the microchip nebulizer. The small solvent flow is mixed with nitrogen gas and vaporized in the microchip nebuhzer by resistive heating of the platinum wire. The loypton UV lamp irradiates the sample surface that is in contact with the hot reagent gas. Reproduced from Ref. [9] with permission. The American Chemical Society, 2007.

Acidic analytes can undergo deprotonation, while electronegative molecules are prone to anion addition or electron capture [27]. (The rules governing these processes have already been dealt with in Chap. 7.) The factors influencing the desorption and ionization in DAPPI such as the microfluidic jet impinging geometry, the thermal characteristics of the DAPPI surfaces, as well as chemical aspects like spray solvent have been examined for both positive- and negative-ion mode [27]. 

Example DAPPI is capable of analyzing dried sample spots of compounds of different polarities from various surfaces, may serve for the direct analysis of pharmaceuticals and illicit drugs from tablets and other preparations (Fig. 13.12), and of many other applications [9,27-29]. 

Fig. 13.12. Analysis of a hashish slab with DAPPI using toluene (a)and acetone (b) at flow rates of 2 ml min" as spray solvents. The insets show the tandem mass spectra of (a) the ion of cannabinol and (b) the [M+H] ion of cannabinol the ions at m/z 314 and 315 are attributed to the respective ionic species of tetrahydrocannabinol (THC). Reproduced from Ref. [29] with permission. John Wiley Sons, Ltd., 2008.

This section on direct analysis in real time (DART) [2] has not been placed at the end of this chapter because it would be inferior to the preceding techniques, but because DART relies on much different phenomena. In fact, DART delivers analytical results very similar to those of DESI, even more so to those of DAPCI or DAPPI, and thus presents an alternative concept of ambient MS. 

Finally, the analysis of nonpolar confounds such as steroids and hydrocarbons makes DART a very strong competitor for DAPPI [69]. 

DAPPI Desorption atmospheric pressure photoionization Sample surface exposed to APH [9]... 

Several modifications of the ESI principle were described, such as the desorption electrospray ionization (DESI), the desorption atmospheric pressure photoionization (DAPPI), the electrospray-assisted pyrolysis ionization (ESPI), the ambient sonic spray ionization (SPI), " the electrosonic spray ionization (ESSI), but also combined MALDI/ESI techniques, such as the matrix-assisted laser desorption electrospray ionization (MALDESI). ... 

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