Preformed ions, mass spectrometry

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. 

Selected Reaction Monitoring (SRM) Data acquired from specific product ions corresponding to m/z selected precursor ions recorded via two or more stages of mass spectrometry. Selected reaction monitoring can be preformed as tandem mass spectrometry in time or tandem mass spectrometry in space. The term multiple reaction monitoring is deprecated [1],... 

This technique is complementary to the thermospray technique. Relative advances of the particles beam technique over thermospray include library searchable electron impact spectra, improved reproducibility, easier use and increased predictability over a broad range of compounds. But since a particle beam requires same sample volatility, very large and polar compounds such as proteins may not provide satisfactory results using particle beam liquid chromatography-mass spectrometry. Additionally, certain classes of compounds such as preformed ions, azo dyes and complex sugars may not yield satisfactory electron impact spectra, but can be run on thermospray. In other words, both liquid chromatography-mass spectrometry techniques complement each other s limitations and the analyst may want to add both to address a broader range of samples. 

Various molecular and quasi-molecular ions can be formed under MALDI conditions. The formation of protonated analyte (A) molecules, [A -F H]+, is generally most important at least for samples containing slightly basic centres, such as the peptides and proteins, MALDI mass spectrometry of which is known to be most facile and reproducible. Therefore, proton transfer from the electronically excited, neutral or ionized, or protonated matrix species is considered to be crucial in the overall MALDI process . Notably, proton transfer can occur already in the condensed phase, followed by desorption of the preformed ions . However, the generation of the [A -F H]+ ions is believed to take place preferably in the so-called plume , that is, in the energized, short-hved and relatively dense vapour phase generated above the solid matrix upon excitation by the laser pulse. The actual proton donor species (be it one or several) in a given case is still a matter of... 

A comparative study of in- and post-source decays of peptide and preformed ions in matrix-assisted laser desorption ionization time-of-flight mass spectrometry effective temperature and matrix effect./. Am. Soc. Mass Spectrom.,... 

Van Breemen, R.B., Snow, M., Cotter, R.J. (1983) Time-resolved Laser Desorption Mass Spectrometry. 1. Desorption of Preformed Ions. Int. J. Mass Spectrom. Ion Phys. 

Mussel man, B. Watson, J.T. Chang, C.K. Direct Evidmce for Preformed Ions of Porjiiyrins in the Solvent Matrix for Fast Atom Bombardment Mass Spectrometry. Org. Mass Spectrom. 1986,21, 215-219. 

Lehmann, E. Knochenmuss, R. Zenobi, R. Ionization mechanisms in matrix-assisted laser-desorption/ ionization mass-spectrometry—Contribution of preformed ions. Rapid Commun. Mass Spectrom. 

Talroze, V. L. Leipunsky, I. O. Burlingame, A. L. Baldwin, M. A. Modeling nanosecond non-activated thermal decay of MALDI matrices containing preformed analyte ions and partially ionized matrix molecules as the origin of pneumatic assistance. In 51st ASMS Conference on Mass Spectrometry and Allied Topics, American Society of Mass Spectrometry, Santa Fe, NM, 2003. 

---