Atmospheric-pressure desorption ionization

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 ) to the surface to be analyzed. Analytes are desorbed from the surface and analyzed by mass spectrometer (Eig. 1.15). 

Huikko, K., Oestman, P., Sauber, C., Mandel, F., Grigoras, K., Franssila, S., Kotiaho, T., and Kostiainen, R. (2003). Feasibility of atmospheric pressure desorption/ionization on silicon mass spectrometry in analysis of drugs. Rapid Commun. Mass Spectrom. 17 1339-1343. 

From the time of the second edition published in 2001 until now, much progress has been achieved. Several techniques have been improved, others have almost disappeared. New atmospheric pressure desorption ionization sources have been discovered and made available commercially. One completely new instrument, the orbitrap, based on a new mass analyser, has been developed and is now also available commercially. Improved accuracy in low-mass determination, even at low resolution, improvements in sensitivity, better detection limits and more efficient tandem mass spectrometry even on high-molecular-mass compounds are some of the main achievements. We have done our best to include them is this new edition. 

Traditionally, products and adsorbates had to be volatile enough so that they could be carried from the cell into the mass spectrometer, either by headspace sampling, or, more commonly for near-simultaneous analysis (referred to as differential electrochemical mass spectrometry), across a nanoporous, gas-permeable membrane (e.g., Teflon) supported at the tip of a microcapillary placed close to the electrode. Alternatively, a Pt-coated membrane electrode can be used. But the advent of the so-called soft atmospheric pressure desorption/ionization techniques associated with liquid chromatography-mass spectrometry has allowed the sampling of the solvent and involatile solutes directly. The spectra are more... 

A turning point in atmospheric pressure desorption-ionization techniques for the analysis of chemical compounds was reached very recently with the introduction of DESI (U.S. Patent Application 20050230635). In this approach, charged droplets from an electrosprayed solution are directed toward a solid sample by means of a high-velocity gas stream. The charged droplets ablate the exterior of the sample, removing and ionizing chemical compounds present on the surface. Accordingly, DESI permits the direct analysis of condensed-phase samples with minimal or no sample preparation. 

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. 

Ostman P, Pakaiinen JMH, Vainiotalo P, Franssila S, Kostiainen R, Kotiaho T (2006) Minimum proton affinity for efficient ionization with atmospheric pressure desorption/ionization on silicon mass spectrometry. Rapid Commun Mass Spectrom 20 3669-3673... 

PDI), the sample is dissolved in a volatile solvent and deposited on a nitrocellulose target material, which is subsequently bombarded with the fission fragments [33-37]. Maity mass spectra of PDl-MS show the occurrence of [M+Na]+ next to [M+H]+ [37], PDl-MS has been extensively used for the analysis of biological macromolecules, but has been superseded by MALDl. Atmospheric-pressure desorption ionization methods are briefly discussed in Sect. 7.2.7. 

In addition to DESI and AP-MALDI, a large variety of other, sometimes closely related, atmospheric-pressure desorption ionization techniques have been introduced in the past decade, connected to a huge number of acronyms. Van Beikel et al. [76] tried to classify these emerging techniques into four categories, i.e., (1) thermal desorption ionization, (2) laser desorption/ablation ionization, (3) liquid-jet and gas-jet desorption ionization, and (4) hquid extraction surface sampling probe ionization. 

El = electron ionization Cl = chemical ionization ES = electrospray APCI = atmospheric-pressure chemical ionization MALDI = matrix-assisted laser desorption ionization PT = plasma torch (isotope ratios) TI = thermal (surface) ionization (isotope ratios). 

Figure 2.1 Mass spectrometric approach. Dl, direct inlet GC, gas chromatography HPLC, high performance liquid chromatography CZE, capillary zone electrophoresis El, electron ionization Cl, chemical ionization ESI, electrospray ionization DESI, desorption electrospray ionization APCI, atmospheric pressure chemical ionization MALDI, matrix assisted laser desorption ionization B, magnetic analyzer E, electrostatic analyzer...

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... 

Ion genera lion can be achieved in a number of ways electron impact (Eh ionization, chemical ionization (CI). fas I atom bombardment (FAB), matrix assisted taser desorption ionization (MAI.DI), eleclrospray ionization (ESI) and atmospheric pressure chemical ionization (APC I are the most common methods,... 

Figure A.3A.1 Flow chart illustrating the selection of a suitable ionization technique for the mass spectrometric analysis of a sample. Abbreviations APCI, atmospheric pressure chemical ionization Cl, chemical ionization El, electron impact FAB, fast atom bombardment MALDI, matrix-assisted laser desorption/ionization.

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

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. 

An ideal interface should not cause extra-column peak broadening. Historical interfaces include the moving belt and the thermospray. Common interfaces are electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCl). Several special interfaces include the particle beam—a pioneering technique that is still used because it is the only one that can provide electron ionization mass spectra. Others are continuous fiow fast atom bombardment (CF-FAB), atmospheric pressure photon ionization (APPI), and matrix-assisted laser desorption ionization (M ALDl). The two most common interfaces, ESI and APCI, were discovered in the late 1980s and involve an atmospheric pressure ionization (API) step. Both are soft ionization techniques that cause little or no fragmentation hence a fingerprint for qualitative identification is usually not apparent. 

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. 

Identification of the forms thus obtained using complementary molecule-specific techniques (nuclear magnetic resonance infrared [IR] matrix-assisted laser desorption/ionization electrospray ionization [ESl]/atmospheric pressure chemical ionization mass spectrometry [MS])... 

The dynamic development of mass spectrometry has had a huge impact on lipid analysis. Currently, a variety of suitable mass spectrometers is available. In principal, a mass spectrometer consists of an ion source, a mass analyzer, and an ion detector. The typical features of each instrument (Fig. 2) result mostly from the types of ion source and mass analyzer. To date, the ionization techniques apphed to lipid analysis include Electrospray Ionization (ESI or nano-ESI), Atmospheric Pressure Chemical Ionization (APCI), Matrix-Assisted Laser Desorption/Ionization... 

MALDI), and, more recently, Atmospheric Pressure Photo Ionization (APPI) and Desorption Electrospray Ionization (DESI). For the majority of analytical tasks in hpidomics, ESI and nano-ESI are the most common choices. 

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