Matrix-assisted laser desorption, atmospheric ionization

Atmospheric pressure matrix-assisted laser desorpt-ion/ionization (ap-MALDI) A variant of the conventional MALDI technique is ap-MALDI in which the sample is ionized outside the vacuum system and ions are captured through a small orifice, usually into an ion trap mass spectrometer. Its advantage appears to be a considerable amount of rapid collisional cooling of the ions by the high atmospheric pressure, leading to stabilization of sensitive compounds. On the other hand, some compounds, such as carbohydrates, appear to suffer increased fragmentation under these conditions. 

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. 

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). ... 

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). 

Moyor, S. C. Marzilli, L. A. Woods, A. S. Laiko, V. V. Doroshenko, V. M. Cotter, R. J. Atmospheric pressure matrix-assisted laser desorption/ionization (AP MALDI) on a quadrupole ion trap mass spectrometer. Int. I. Mass Spectrom. 2003, 226,133-150. 

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...

In contrast to the other ion sources, the MALDI source may operate under high vacuum or under atmospheric pressure. In the latter case the acronym AP-MALDI (atmospheric pressure matrix assisted laser desorption ionization) is used. 

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... 

Atmospheric pressure matrix-assisted laser desorption/ ionization AP- MALDI Photon induced desorption/ ionization Nonvolatile molecular ions Soft method Large molecules... 

Atmospheric Pressure Matrix-Assisted Laser Desorption/Ionization... 

Analytes must be liberated from their associated solvent molecules as well as be ionized to allow mass separation. Several ionization methods enable ion production from the condensed phase and have been used for the coupling of CE to MS. Among them, atmospheric pressure ionization (API) methods, matrix-assisted laser desorption/ionization (MALDI), and inductively coupled plasma (ICP) ionization are mainly used. API techniques are undoubtedly the most widespread ionization sources and cover different analyte polarity ranges. 

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.

In direct introduction the sample can be introduced via a sample probe or plate through a vacuum lock, and can subsequently be ionized via El, Cl or matrix-assisted laser desorption ionization (MALDI see Section 2.4). Alternatively, the sample can be introduced as a liquid stream into an ion source at atmospheric pressure, after which it is subjected to electrospray ionization (ESI see Section 2.3). Direct injection does not offer any form of sample separation. 

Pihlainen, K., Grigoras, K., Franssila, S., Ketola, R., Kotiaho, T., and Kostiainen, R. (2005). Analysis of amphetamines and fentanyls by atmospheric pressure desorption/ionization on silicon mass spectrometry and matrix-assisted laser desorption/ionization mass spectrometry and its application to forensic analysis of drug seizures. J. Mass Spectrom. 40 539-545. 

Technological advances of ion-trap mass spectrometers are the ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and the recently released technique, the Orbitrap Fourier transform mass spectrometry (Hu et al., 2005), which enable the determination of molecular formulae with a high mass resolution and mass accuracy in mixtures. Today these ion-trap mass spectrometers are most frequently coupled with atmospheric pressure ionization (API) techniques such as electrospray ionization (ESI) (e.g., Fievre et al., 1997 Qian et al., 2001 Kujawinski et al., 2002 Llewelyn et al., 2002 Stenson et al., 2002,2003 Fard et al., 2003) or matrix-assisted laser desorption/ionization (MALDI) (e.g., Solouki et al.,... 

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... 

A variety of MS formats are widely accepted and applied in the pharmaceutical industry. The specific MS application is often defined by the sample introduction technique. The pharmaceutical applications highlighted in this article feature two types of sample introduction techniques dynamic and static. Dynamic sample introduction involves the use of high-performance liquid chromatography (HPLC) on-line with MS. The resulting liquid chromatography/mass spectrometry (LC/MS) format provides unique and enabling capabilities for pharmaceutical analysis. The electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI) modes are the most widely used. Static sample introduction techniques primarily use matrix-assisted laser desorption/ionization (MALDI). ... 

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