Desorption atmospheric pressure photoionization

DESI works best with polar analytes that are easy to protonate or deprotonate, although analytes of low polarity are accessible to a certain extent. This was the rationale for developing DAPCI. In order to improve the efficiency of ambient MS in the regime of low-polarity compounds even further, desorption atmospheric 

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. 

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]. 

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Kauppila, T.J., Arvola, V., Haapala, M., Pol, J., Aalberg, L., and Saarela, V. 2008. Direct analysis of illicit drugs by desorption atmospheric pressure photoionization. Rapid Commun. Mass Spectrom., 22 979-985. 

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. 

Haapala M, Pol J, Saarela V, Arvola V, Kotiaho T, Ketola RA, Franssila S, Kauppila TJ, Kostiainen R (2007) Desorption atmospheric pressure photoionization. Anal Chem 79 7867-7872 Hsieh Y, Li F-B, Korfmacher WA (2010) Mapping pharmaceuticals in rat brain sections using MALDI imaging mass spectrometry. Methods Mol Biol 656 147-158 Hu LG, Xu SY, Pan CS, Zou HF, Jiang GB (2007) Preparation of a biochip on porous sihcon and application for label-free detection of small molecule-protein interactions. Rapid Commun Mass Spectrom 21 1277-1281... 

Luosujarvi L, Kanerva S, Saarela V, Franssila S, Kostiainen R, Kotiaho T, Kauppila TJ (2010) Environmental and food analysis by desorption atmospheric pressure photoionization-mass spectrometry. Rapid Commun Mass Spectrom 24 1343-1350 Manicke NE, Kistler T, Ifa DR, Cooks RG, Ouyang Z (2009) High-throughput quantitative analysis by desorption electrospray ionization mass spectrometry. J Am Soc Mass Spectrom 20 321-325 Martynov IL, Karavanskii VA, Kotkovskii GE, Kuzishchin YA, Tsybin AS, Chistyakov AA (2011) Ion mobility spectrometer with ion source based on laser-irradiated porous silicon. Tech Phys Lett 37 15-18... 

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]. 

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

Luosujarvi, L. Arvola, V. Haapala, M. Pol, J. Saarela, V. Pranssila, S. Kotia-ho, T. Kostiainen, R. Kauppila, T.J. Desorption and Ionization Mechanisms in Desorption Atmospheric Pressure Photoionization. Anal. Chem. 2008, 80, 7460-7466. 

Luosujarvi, L. Laakkonen, U.M. Kostiainen, R. Kotidio, T. Kauppila, T.J. Analysis of Street Market Confiscated Drugs by Desorption Atmospheric Pressure Photoionization and Desorption Electrospray Ionization Coupled with Mass Spectrometry. Rapid Commun. Mass Spectrom. 2009,23,1401-1404. 

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). ... 

DGE a AC AMS APCI API AP-MALDI APPI ASAP BIRD c CAD CE CF CF-FAB Cl CID cw CZE Da DAPCI DART DC DE DESI DIOS DTIMS EC ECD El ELDI EM ESI ETD eV f FAB FAIMS FD FI FT FTICR two-dimensional gel electrophoresis atto, 10 18 alternating current accelerator mass spectrometry atmospheric pressure chemical ionization atmospheric pressure ionization atmospheric pressure matrix-assisted laser desorption/ionization atmospheric pressure photoionization atmospheric-pressure solids analysis probe blackbody infrared radiative dissociation centi, 10-2 collision-activated dissociation capillary electrophoresis continuous flow continuous flow fast atom bombardment chemical ionization collision-induced dissociation continuous wave capillary zone electrophoresis dalton desorption atmospheric pressure chemical ionization direct analysis in real time direct current delayed extraction desorption electrospray ionization desorption/ionization on silicon drift tube ion mobility spectrometry electrochromatography electron capture dissociation electron ionization electrospray-assisted laser desorption/ionization electron multiplier electrospray ionization electron transfer dissociation electron volt femto, 1CT15 fast atom bombardment field asymmetric waveform ion mobility spectrometry field desorption field ionization Fourier transform Fourier transform ion cyclotron resonance... 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Earlier methods of ionization applied to carotenoids, including electron impact (El), chemical ionization (Cl), a particle beam interface with El or Cl, and continuous-flow fast atom bombardment (CF-FAB), have been comprehensively reviewed elsewhere (van Breemen, 1996, 1997 Pajkovic and van Breemen, 2005). These techniques have generally been replaced by softer ionization techniques like electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), and more recently atmospheric pressure photoionization (APPI). It should be noted that ESI, APCI, and APPI can be used as ionization methods with a direct infusion of an analyte in solution (i.e. not interfaced with an HPLC system), or as the interface between the HPEC and the MS. In contrast, matrix-assisted laser desorption ionization (MALDI) cannot be used directly with HPEC. 

Note El is electron ionization, Cl is chemical ionization, FI/FD is field ionization/field desorption, APPI is atmospheric pressure photoionization, and APCI is atmospheric pressure chemical ionization. 

Ionization of condensed-phase analytes occurs by mixing a sample in a suitable matrix and bombarding the matrix-analyte mixture with an energetic beam made of either laser photons as in MALDI, high-energy fission particles as in Cf plasma desorption, or high-energy fast atoms or ions (FAB or liquid SIMS). When an analyte is present in a solution, such as an effluent from a separation device, it can be ionized via thermospray ionization, atmospheric-pressure chemical ionization, atmospheric-pressure photoionization, or electrospray ionization. Desorption electrospray ionization and direct analysis in real time are new modes of ionization that are accomplished in ambient air. 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

The successful on-line interfacing of several ion sources has made them dominant players in quantitative analyses using mass spectrometry. These include electron ionization (El) and chemical ionization (Cl) both coupled to GC, and the atmospheric pressure ionization (API) methods of atmospheric pressure chemical ionization (APCI) atmospheric pressure photoionization (APPI), and electrospray ionization (ESI) coupled to LC. In addition, matrix assisted laser desorption ionization (MALDI) is seeing increased application in off-line LC/MS applications. 

Matrix-assisted laser desorption ionization Atmospheric pressure electrospray ionization Atmospheric pressure chemical ionization Atmospheric pressure photoionization Inductively coupled plasma Direct analysis in real time Desorption electrospray ionization... 

Triple quadrupole MS instruments have been the most common ones in studies involving lipid analysis, butnovel hybrid (quadrupoletime-of-flight, etc.)instruments are rapidly gaining popularity due to their ability for multiple precursor ion scans simultaneously. Besides ESI, atmospheric pressure chemical ionization (APCI), atmospheric pressure photoionization (APPI), and matrix-assisted laser desorption ionization (MALDI) have been employed in analysis of lipids. However, these methods seem to have an advantage over ESI only in special cases. For instance, APPI and APCI allow analysis of sterols without derivatization, which is needed for ESI. 

D, two-dimensional quadmpole field 3D, three-dimensional quadrupole field APCl, atmospheric pressure chemical ionization APPI, atmospheric-pressure photoionization ESI, electrospray ionization MALDI, matrix-assisted laser desorption ionization CID, collision-induced dissociation ETD, electron transfer dissociation. 

Tonidandel L., Ragazzi E., Roghi G. Traldi P. (2008). Mass spectrometry in the characterization of ambers I. Studies of amber samples of different origin and ages by laser desorption ionization, atmospheric pressure chemical ionization and atmospheric ptressure photoionization mass spectrometry. Rapid Commun. Mass Spectrom. 22,63(L38, ISSN 0951A198... 

In terms of the hardware, TRMS methods described in this book use most common types of ion sources and analyzers. Electrospray ionization (ESI), electron ionization (El), atmospheric pressure chemical ionization (APCI), or photoionization systems, and their modified versions, are all widely used in TRMS measurements. The newly developed atmospheric pressure ionization schemes such as desorption electrospray ionization (DESI) and Venturi easy ambient sonic-spray ionization (V-EASI) have already found applications in this area. Mass analyzers constitute the biggest and the most costly part of MS hardware. Few laboratories can afford purchasing different types of mass spectrometers for use in diverse applications. Therefore, the choice of mass spectrometer for TRMS is not always dictated by the optimum specifications of the instrument but its availability. Fortunately, many real-time measurements can be conducted using different mass analyzers equipped with atmospheric pressure inlets - with better or worse results. For example, triple quadrupole mass spectrometers excel at quantitative capabilities however, in many cases, popular ion trap (IT)-MS instruments can be used instead. On the other hand, applications of TRMS in fundamental studies often require a particular type of instrument (e.g., Fourier transform ion cyclotron resonance mass spectrometer for photodissociation studies on trapped ions). 

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