MALDI ionization degree

Selection of an appropriate internal standard can also assist in correcting for ion suppression issues caused by matrix components. However, if the ion suppression is too severe, then inevitably sensitivity will suffer. Ion suppression must be limited to a degree sufficient to avoid sensitivity problems. To reduce ion suppression, a sample cleanup method is necessary. Moreover, proper co-crystallization is directly related to sample composition and therefore, sample cleanup is necessary for successful MALDI ionization. 

Figure 5.3. Sketch of the major processes proposed in cluster models of MALDI ionization. A, analyte m, matrix R, generic counterion. Preformed ions, separated in the preparation solution, are contained in clusters ablated from the initial solid material. Some clusters contain a net excess of positive charge, others net negative (not shown). If analyte is already charged, here by protonation, cluster evaporation may free the ion. In other clusters, charge may need to migrate from its initial location (e.g., on matrix) to the more favorable location on analyte (secondary reaction). For multiply charged analytes, hard and soft desolvation processes may lead to different free ions. Neutralization by electrons or counterions takes place to some degree but is not complete.

For natural poly-APS alkaloids (77), matrix assisted laser desorption ionization-time-of-flight (MALDI-TOF) spectrometric analysis showed that the compounds comprised a mixture of two polymers with molecular weights centred at around 5520 Da and 18900 Da and with a low degree of polydispersity [34]. 

Lasers have been used for many years, with varying degrees of success, to ionize molecules directly. The energy added was usually excessive and resulted in the destruction or extensive fragmentation of the analytes. The significant conceptual differences between direct laser ionization and MALDI are that in the latter there is a matrix present, usually an aromatic acid, in vast molar excess over the analyte and that the matrix absorbs most of the energy supplied by the laser. 

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. 

Measure the MALDI signals of the analyte from the spotted regions under the established instrumental conditions to evaluate the degree of the ionization suppression due to the endogenous materials from the blank tissues ( geNote 12). 

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