Matrix-assisted laser desorption ionization MALDI fragment generation

The ionization source is the mechanical device that allows ionization to occur. Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) are now the most common ionization sources for biomolecular MS, because both of them are soft ionization techniques that is, the sample ionization process generates few or no fragments even for large biomolecules, such as proteins and oligosaccharides, so that the intact molecular ions can be easily observed. 

Despite these demonstrations, our group presented the first systematic calculations to assess the absolute identifying power or phylogenetic classifying utility of matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) when applied to RNase Ti or RNase A-generated fragments from large 16S rRNA sequence data sets [71-72]. 

In matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry, a UV laser is used to generate atoms, M, of the sample [M + H] or [M + Na] ions are typically detected. It is a soft method and there is little fragmentation. 

The detection of ricin directly by instrumental methods has been limited to matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) detection (Darby et al., 2001 Brinkworth et al., 2009). Darby et al. (2001) separated ricin from other material present in samples using size exclusion chromatography and then hydrolyzed the isolated ricin with trypsin. The hydrolysate was analyzed by MALDI-TOF MS and ricin peptides identified by comparison of mass ions with an online database and comparison of identified peptides with those from a reference sample obtained from ricin. Brinkworth et al. (2009) generated and measured the intact ricin ion as opposed to peptide fragments. Since the presence of readily ionized substances such as salts in the sample can overwhelm the detection of ionized ricin, the sample was concentrated and ultrafiltered to eliminate material of MW below 30kDa. The method detected ricin at levels of 4 gg/mL. 

The unimolecular fragmentation of calixarene-derived ions will not be treated here, especially as studies on this topic are much restricted due to the fact that classical El and Cl techniques cannot be applied to these mvolatile and often quite polar polyphenols. Rather, mass spectrometric analysis is limited to the detection of positively or negatively charged quasi-molecular ions, such as [M + H]+ and [M — H], or molecular adduct ions, such as [M + NR4]+ and [M - - metal]+. In general, these ions can be readily generated by using matrix-assisted laser desorption (MALDI) and/or electrospray ionization (ESI) mass spectrometry. 

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