Matrix-assisted laser desorption ionization process

Rohlfing, A., Menzel, C., Kukreja, LM., Hillenkamp, F., and Dreisewerd, K. (2003) Photoacoustic analysis of matrix-assisted laser desorption/ ionization processes with pulsed infrared lasers. J. Phys. Chem. B, 107, 12275-12285. 

Gidden, J. Wyttenbach, T. Batka, J. J. Weis, R Jackson, A. T. Scrivens, J. H. Bowers, M. T., Poly (Ethylene terephthalate) oligomers cationized by alkali ions Structures, energetics, and their effect on mass spectra and the matrix-assisted laser desorption/ ionization process , J Am Soc Mass Spectrom 1999, 10, 883-895. 

The focus of this chapter is the development of a technique often called wholecell matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) or whole-cell MALDI-TOF MS. Some groups prefer to use terms such as intact or unprocessed rather than whole, but the intended meaning is the same regardless of which word is used. As noted in the first chapter of this book, there are many different methods for the analysis of bacteria. However, for the analysis of intact or unprocessed bacteria, whole-cell MALDI-TOF MS is the most commonly used approach. This method is very rapid. MALDI-TOF MS analysis of whole cells takes only minutes because the samples can be analyzed directly after collection from a bacterial culture suspension. Direct MALDI MS analysis of fungi or viruses is similar in approach1,2 but is not covered in this chapter. MALDI-TOF MS of whole cells was developed with very rapid identification or differentiation of bacteria in mind. The name (whole cell) should not be taken to imply that the cells are literally intact or whole. Rather, it should be taken to mean that the cells that have not been treated or processed in any way specifically for the removal or isolation of any cellular components from any others. In whole-cell analysis the cells have been manipulated only as necessary to... 

Although electrospray ionization and matrix-assisted laser desorption ionization allow to transfer much larger ions into the gas phase, it is FD that can be regarded the softest ionization method in mass spectrometry. [27,74] This is mainly because the ionization process itself puts no excess energy into the incipient ions. Problems normally arise above 3000 u molecular weight when significant heating of the emitter causes thermal decomposition of the sample. 

This proposal describes the development of a new, systematic approach for qualitatively and quantitatively studying surface-biomolecule interactions by matrix-assisted laser desorption ionization (MALDl) mass spectrometry (MS). This methodology is being developed because of the profound importance that surface-biomolecule interactions play in applications where biomaterials come into contact with complex biological fluids, it can readily be shown that undesired reactions occurring in response to surface-biomolecule contact (protein adsorption, biofouling, immune response activation, etc.) lead to enormous economic and human costs. Thus, the development of analytical methodologies that allow for efficient assessment of the properties of new biomaterials and/or the study of detailed fundamental processes initiated upon surface-biomolecule contact are of critical value ... 

Figure 14.4 Generation of ions by desorption methods. The sample is placed on a target and then hit either by accelerated electrons (secondary ion mass spectrometry), accelerated atoms (fast atom bombardment) or laser light (laser desorption/ ionization, matrix-assisted laser desorption/ionization). In the case of FAB and MALDl, the analyte is additionally embedded in a matrix, which also is desorbed during these processes.

Ehring, H., Karas, M. and Hillenkamp, R, Role of photoionization and photochemistry in ionization processes of organic molecules and relevance for matrix-assisted laser desorption ionization mass spectrometry. Org. Mass Spec., 27, 472, 1992. 

Matrix-assisted laser desorption/ionization (MALDI)-time-of-flight (TOF)-mass spectrometry (MS) is now routinely used in many laboratories for the rapid and sensitive identification of proteins by peptide mass fingerprinting (PMF). We describe a simple protocol that can be performed in a standard biochemistry laboratory, whereby proteins separated by one- or two-dimensional gel electrophoresis can be identified at femtomole levels. The procedure involves excision of the spot or band from the gel, washing and de-stain-ing, reduction and alkylation, in-gel trypsin digestion, MALDI-TOF MS of the tryptic peptides, and database searching of the PMF data. Up to 96 protein samples can easily be manually processed at one time by this method. 

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]. 

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]. 

Kaufmann, R., Kirsch, D., and Spengler, B. (1994). Sequencing of peptides in a time-of flight mass spectrometer Evaluation of post source decay following matrix-assisted laser desorption ionization (MALDI). Int. ]. Mass. Spectrom. Ion Processes 131, 355-385. 

Over the past 10 years, there has been a rapid development of various mass spectrometry techniques in order to sequence, identify, and characterize proteins and peptides. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) has become the most preferably used technique for a fast processing of biological samples. 

Mass spectrometry is one physical technique that does not (at least directly) involve electromagnetic radiation. However, some sample desorption and ionization processes do use high intensity pulses of laser light in techniques such as MALDI (Matrix-Assisted Laser Desorption Ionization) that have proved very useful in mass analysis of proteins and other biologic macromolecules. High resolution mass spectrometry derives from atomic/molecular beam studies in which the trajectories of ionized particles in a vacuum can be manipulated by static... 

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. 

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