Desorption/ionization techniques

Soft ionization methods can be classified into different groups (a) those occurring in the gas phase (b) spray ionization techniques (c) desorption ionization techniques. 

Over the years, a lot of desorption ionization techniques have been introduced to MS, such as plasma desorption, field desorption, laser desorption, secondary ion mass spectrometry, fast atom bombardment, matrix assisted laser desorption and desorption electrospray ionization. Most of them are actually no longer used. In the following paragraphs, both matrix assisted laser desorption (MALDI) and desorption electrospray ionization (DESI) will be discussed. 

Electrospray ionization [21] is one of the most widely utilized ionization techniques employed today for the analysis of thermally fragile molecules. As such, it has assumed an important role in the analysis of biologically important molecules. ESI is a desorption ionization technique. This means that ions are formed before or during the transition from the liquid phase and need not be volatilized in advance of the ionization event (as is the case for El, Cl, etc.). Like APCI and APPI, ESI occurs at atmospheric pressure outside the vacuum chamber of the mass spectrometer (Fig. 11.5). A solution of the analyte passes through... 

Fast-atom bombardment ionization [27,28] was introduced in the early 1980s and was the first widely used desorption ionization technique. Its universal appeal stemmed from its ability to ionize polar,... 

Why are desorption ionization techniques well suited to the analysis of biomolecules (they do not depend on thermal energy to volatilize analytes). 

According to a paper by Todd it is a common feature of FAB and LSIMS that they defy any generally acceptable mechanistic description. [48] Unlike El or Cl, where gaseous ions are generated from gaseous molecules, desorption/ionization techniques combine a state transition from liquid or solid to the gas phase and ionization of neutral molecules. Nonetheless, reviews dealing with the processes of desorption and ion formation under FAB and LSIMS conditions offer some in-... 

Desmids, strontium and barium sulfate biomin-eralization, 36 170-171 Desorption ionization techniques, 28 6-7, 21 Desulfoferrodoxin (Dfx), in sulfate-reducing bacteria, 47 366-367 Desulfofnscidin, 47 387 Desulforedoxin, 47 362-363, 424, 425, 448 Desulforubidin, 47 387 Desulfovibrio... 

The purpose of the MS techniques is to detect charged molecular ions and fragments separated according to their molecular masses. Most flavonoid glycosides are polar, nonvolatile, and often thermally labile. Conventional MS ionization methods like electron impact (El) and chemical ionization (Cl) have not been suitable for MS analyses of these compounds because they require the flavonoid to be in the gas phase for ionization. To increase volatility, derivatization of the flavonoids may be performed. However, derivatization often leads to difficulties with respect to interpretation of the fragmentation patterns. Analysis of flavonoid glycosides without derivatization became possible with the introduction of desorption ionization techniques. Field desorption, which was the first technique employed for the direct analysis of polar flavonoid glycosides, has provided molecular mass data and little structural information. The technique has, however, been described as notorious for the transient... 

There are many different ionization techniques available to produce charged molecules in the gas phase, ranging from simple electron (impact) ionization (El) and chemical ionization (Cl) to a variety of desorption ionization techniques with acronyms such as fast atom bombardment (FAB), plasma desorption (PD), electrospray (ES), and matrix-assisted laser desorption ionization (MALDI) (Mano and Goto 2003). 

Desorption Ionization Techniques for Quantitative Analysis of Drug Molecules... 

Gobey, J. S., Janiszewski, J., Credo, G. M., and Bouvier, E. S. P. (2004). A comparison of desorption ionization techniques for small molecule quantitation. In Proceedings of The 52nd ASMS Conference on Mass Spectrometry and Allied Topics, Nashville, TN. 

Continuous-flow FAB (Ch. 4.6, [10]). This was the most successful approach to an on-line combination of LC-MS via a desorption ionization technique. 

B. Matrix-Assisted Laser Desorption Ionization and Other Desorption Ionization Techniques for the Identification of Combinatorial Libraries... 

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