Matrix-assisted laser desorption/ionization interfaces

Matrix-Assisted Laser Desorption Ionization Interfaces. 1474... 

Mass spectrometry (MS) is combined to TLC through an ESI (electrospray ionization) interface or a MALDI (matrix-assisted laser desorption ionization) interface. In the first case, E SI-M S is carried out using the extracts of the eluted compounds. The spots/zones need to be scraped oflF from the plate before the compounds are extracted. MALDI can be carried out directly on the plate after adding an appropriate MALDI matrix. 

The introduction and eventual commercialization of matrix-assisted laser desorption/ionization (MALDI) and electrospray (ESI) allowed biomarker status to be extended to proteins in 1996.15"17 With a few exceptions, ESI has been used in conjunction with extractions and high-pressure liquid chromatography (UPLC) interfaced with mass spectrometry. MALDI, on the other hand, has been widely adapted for rapid analysis of intact organisms, supported by bioinformatics.1819... 

T. Miliotis, S. Kjellstrom, P. Onnerfjord, J. Nilsson, T. Laurell, L. E. Edholm, and G. Marko-Varga, Protein identification platform utilizing micro dispensing technology interfaced to matrix-assisted laser desorption ionization time-of-flight mass spectrometry, J. Chromatogr. A, 886 (2000) 99-110. 

C. Ericson, Q. T. Phung, D. M. Horn, E. C. Peters, J. R. Fitchett, S. B. Ficarro, A. R. Salomon, L. M. Brill, and A. Brock, An automated noncontact deposition interface for liquid chromatography matrix-assisted laser desorption/ionization mass spectrometry, Anal. Chem., 75 (2003) 2309-2315. 

Q. Zhan, A. Gusev, and D. M. Hercules, A novel interface for on-line coupling of liquid capillary chromatography with matrix-assisted laser desorption/ionization detection, Rapid Commun. Mass Spectrom., 13 (1999) 2278-2283. 

Mass spectrometers are used not only to detect the masses of proteins and peptides, but also to identify the proteins, to compare patterns of proteins and peptides, and to scan tissue sections for specific masses. MS is able to do this by giving the mass-to-charge ratio of an ionized species as well as its relative abundance. For biological sample analysis, mass spectrometers are connected to an ionizing source, which is usually matrix-assisted laser desorption ionization (MALDI) [14], surface-enhanced laser desorption/ioni-zation (SELDI, a modified form of MALDI) [15], or electrospray ionization [16]. These interfaces enable the transfer of the peptides or proteins from the solid or liquid phase, respectively, to the gas (vacuum) phase inside the mass spectrometer. Both MALDI and electrospray ionization can be connected to different types of mass analyzers, such as quadrupole, quadruple-ion-traps, time of flight (TOF), or hybrid instruments such as quadrupole-TOF or Fourier transform-ion cyclotron resonance. Each of these instruments can... 

Matrix-assisted laser desorption ionization is another ionization mode used for MS analysis. Enzymatically digested peptides have been studied using a 90-well microchip constmcted in a MALDI plate format (see Figure 7.41). Peptide digestion was initiated in the MALDI interface where the peptide hormone, adreno-corticotropin (ACTH) was mixed with the enzyme carboxypeptidase Y. The mixing process was self-activated in the vacuum conditions. Subsequent TOF MS analysis produced kinetic information of the peptide digestion reaction [820]. 

An automated noncontact deposition interface for liquid chromatography matrix-assisted laser desorption/ ionization mass spectrometry. 

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. 

Fung, K.Y.C., Askovic, S., Basile, F. and Duncan, M.W. (2004) A simple and inexpensive approach to interfacing high-performance liquid chromatography and matrix-assisted laser desorption/ionization-time of flight-mass spectrometry. Proteomics 4, 3121-3127. 

Gusev, A.I. (2000) Interfacing matrix-assisted laser desorption/ionization mass spectrometry with column and planar separations. Fresenius J. Anal. Chem. 366, 691-700. 

Preisler, J., Hu, R, Rejtar, T. and Karger, B.L. (2000) Capillary-electrophoresis-matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using a vacuum deposition interface. Anal. Chem. 72,4785 795. 

Tegeler, T.J., Mechref, Y., Boraas, K., Reilly, J.P and Novotny, M.V. (2004) Microdeposition device interfacing capillary electrochromatography and microcolumn liquid chromatography with matrix-assisted laser desorption/ionization mass spectrometry. Anal. Chem. 76, 6698—6706. 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

Currently, API based LC-MS interfaees, i.e., ESI and APCI, are the most widely applied approaches, while other interfaces like TSP and Cf-FAB can be considered obsolete. Despite the successes of these commercially available interfaces, research towards newer and/or advanced interface strategies continues. These research efforts comprise among others the implementation of on-line LC-MS using matrix-assisted laser desorption/ionization (Ch. 5.9), the sonic spray (Ch. 5.7.1), and the laser spray (Ch. 5.7.2) interface. 

In the recent past, a nnmber of laser-based interface approaches were described. The role of the laser was different from providing heat for the mobile-phase nebnlization and snbseqnent solvent evaporation in the laser spray interface (Ch. 5.7.2), via laser-indnced mnltiphoton ioitization, to matrix-assisted laser desorption ionization (MALDl). 

These interface techniques for connecting the HPLC with the MS-system are very sensitive for most of the substances of interest to the flavour industry. Therefore, HPLC-MS coupling techniques have become an increasingly powerful tool for quality control of flavourings, especially for the analysis of complex mixtures like process flavourings or contaminants present in such complex mixtures. New developments in the area of mass detection systems, such as time-of-flight (ToF) mass analysers and tandem mass spectrometry systems or the features of matrix-assisted laser desorption ionization (MALDI) techniques, may enhance the analytical capabilities of these systems in the near future [16, 17, 28-31 ]. 

Foret, R Preisler, J. Liquid Phase Interfacing and Miniaturization in Matrix-assisted Laser Desorption/Ionization Mass Spectrometry, Proteomics 2, 360-372 (2002). 

S.B. Salomon, A.R. Brill, L.M. Brock, A. An Automated Noncontact Deposition Interface for Liquid Chromatography Matrix-Assisted Laser Desorption/ionization Mass Spectrometry, Anal. Chem. 75(10), 2309-2315 (2003). 

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