Fast Atom Bombardment
Note: We have replaced FAB with ESI. Samples submitted for FAB will be run first by ESI and rerun by FAB if they do not work. If you know you need FAB rather than ESI, submit your sample to the associate director. Placing the form in the ESI tray will delay your spectrum.
FAB is the most popular ionization technique for involatile and/or thermally labile molecules. FAB has provided mass spectra of samples for which no other method has been successful. It works best for polar and higher molecular weight compounds such as peptides and other biomolecules, sulfonic acid derivatives, ionic organometallics, and other complex molecules. FAB is the best technique for ionic species. FAB utilizes an 8 kV beam of neutral Xenon atoms which impinges on a metal target coated with a liquid matrix in which the sample has been dissolved. FAB spectra are complex, showing (M+H)+ quasimolecular ions, fragments, and cluster groups of the matrix and the sample plus the matrix. All FAB spectra in this lab are generally run in Magic Bullet, a 3:1 mixture of dithiothreitol and dithioerythritol, unless otherwise requested. The sample must be soluble either in the matrix or some solvent soluble in the matrix (usually 1 mg in 100 microliter). Other common matrices are glycerol, thioglycerol, and 3-nitrobenzyl alcohol. Trifluoroacetic acid is sometimes added to the matrix to encourage formation of protonated ions. If there are salts present in the sample, (M+Na)+and/or (M+K)+ peaks will appear, and large amounts of salt will degrade the spectrum to uselessness. Some samples give better negative ion FAB spectra than positive ion spectra. In negative FAB spectra, the quasimolecular ion is usually (M-H)-.
Low resolution FAB Instrument: 70-VSE Sample size: 50 microgram-1 mg
The spectrum is obtained at a resolution of 1000.