CI uses a reagent ion to react with the analyte molecules to form ions by either a proton or hydride transfer:
MH + C2H5+ --> MH2+ + C2H4
MH + C2H5+ --> M+ + C2H6
The reagent ions are produced by introducing a large excess of methane (relative to the analyte) into an electron impact (EI) ion source. Electron collisions produce CH4+ and CH3+ which further react with methane to form CH5+ and C2H5+:
CH4+ + CH4 --> CH5+ + CH3
CH3+ + CH4 --> C2H5+ + H2
A plasma is a hot, partially-ionized gas that effectively excites and ionizes atoms.
A glow discharge is a low-pressure plasma maintained between two electrodes. It is particularly effective at sputtering and ionizing materials from solid surfaces.
An EI source uses an electron beam, usually generated fron a tungsten filament, to ionize gas-phase atoms or molecules. An electron from the beam knocks an electron off analyte atoms or molecules to create ions.
The ESI source consists of a very fine needle and a series of skimmers. A sample solution is sprayed into the source chamber to form droplets. The droplets carry charge when the exit the capillary and as the solvent vaporizes the droplets disappear leaving highly charged analyte molecules. ESI is particularly useful for large biological molecules that are difficult to vaporize or ionize.
In FAB a high-energy beam of netural atoms, typically Xe or Ar, strikes a solid sample causing desoprtion and ionization. It is used for large biological molecules that are difficult to get into the gas phase. FAB causes little fragmentation and usually gives a large molecular ion peak, making it useful for molecular weight determination.
The atomic beam is produced by accelerating ions from an ion source though a charge-exchange cell. The ions pick up an electron in collisions with netural atoms to form a beam of high energy atoms.
A laser pulse ablates material from the surface of a sample and creates a microplasma that ionizes some of the sample constituents.
A UV laser pulse ablates the matrix which carries some of the large molecules into the gas phase in an ionized form so they can be extracted into a mass spectrometer.
/chem-ed/ms/ionizatn.htm, updated 11/3/96
Copyright © 1996 by Brian M. Tissue, all rights reserved.
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