DAPPI photoionization

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. 

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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