Matrix-assisted laser desorption ionization principle

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

FIGURE 4.16 Principle of matrix-assisted laser desorption ionization. 

The introduction of Matrix-Assisted Laser Desorption/Ionization (MALDI) and Electrospray Ionization (ESI) (Chapter 1) has dramatically increased the mass range for molar mass analyses by mass spectrometry. In principle, both techniques are able to produce intact quasi-molecular ions of polymers with high molar mass (>100,000 Da). 

See also Capillary Electrophoresis Overview. Electrophoresis Principles Isotachophoresis Isoelectric Focusing Polyacrylamide Gels Two-Dimensional Gels Affinity Techniques Blotting Techniques Clinical Applications. Mass Spectrometry Overview Matrix-Assisted Laser Desorption Ionization Time-of-Flight. 

See also Bioassays Overview. Capillary Electrophoresis Overview. Electrophoresis Oven/iew Principles Two-Dimensional Gels. Gas Chromatography Overview Mass Spectrometry. Immunoassays Overview. Immunoassays, Applications Food. Immunoassays, Techniques Radioimmunoassays Enzyme Immunoassays. Liquid Chromatography Reversed Phase Liquid Chromatography-Mass Spectrometry Food Applications. Mass Spectrometry Oven/iew Principles Matrix-Assisted Laser Desorption Ionization Time-of-Flight. Radiochemical Methods Radioreceptor Assays Food and Environmental Applications. Thin-Layer Chromatography Oven/iew. 

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. 

In Chapter 4, titled Principles of Mass Spectrometry Imaging Applicable to Thin-Layer Chromatography, the authors first introduce a budding analytical approach known as imaging mass spectrometry (IMS) strategy and then present some successful examples of its practical applications. Then, they introduce in detail three mass spectrometric techniques as those routinely used within the framework of IMS. These are secondary mass spectrometry (SIMS), matrix-assisted laser desorption/ ionization (MALDI-IMS), and desorption electrospray ionization (DESI). Finally, the authors discuss the advances and bottlenecks of these techniques when applied to TLC. 

Imniunoaffinity separations performed on a matrix-assisted laser desorption ionization-mass spectrometer (MALDI-MS) probe tip have given rise to the term PAMS (probe affinity MS) [134]. Antibiotin covalently bound to the surface of the probe allowed MS discrimination of bovine insulin with different degrees of biotinylation. The method has also been used to detect lysozyme in human tears by binding antilysozyme to the MS probe. Based on the same PAMS principle, modifications aimed to obtain faster and improved retention of Abs have been carried out by immobilizing the Abs on the probe via a nitrocellulose film instead of binding them directly to the MALDI probe surface [135]. The procedure allowed determination of a therapeutic peptide and one metabolite. 

Agapito et al. ° applied fuzzy logic to an array of semiconductor gas sensors to analyze different atmospheres for the different gases and Otto et al. developed a scheme based on the principles of fuzzy logic that makes use of various pieces of information available either from spectroscopic knowledge or from the particular spectrum that requires evaluation. A fuzzy expert system has also been successfully developed for the automated qualitative and semiquantitative interpretation of X-ray diffraction spectra/ automation of matrix-assisted laser desorption-ionization mass spectrometry (MALDI), and for polymer analysis. 

A schematic of the basic principles of a matrix-assisted laser desorption/ion source is shown in Figure 2.35. By the interaction of a focused laser beam with short pulses and a suitable matrix, the energy of the photons is transferred to the matrix molecules. In MALDI mostly pulsed UV (e.g., nitrogen, X = 337 nm, pulse duration 3-10 ns), but also IR lasers (e.g., Er YAG, X = 2.94 (xm or C02, X = 10.6(xm with a higher pulse duration of up to 600 ns) are used. The MALDI mass spectra obtained during soft ionization by UV and IR lasers are identical. The energy density... 

Three popular ionization techniques are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption (MALDI). Electrospray is the most widely used ionization technique when performing LC-MS, and has proved to be a most versatile tool for soft ionization [72] of a large variety of analytes such as them described in paper I. Figure 6 shows the principle of the ESI. 

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