5.3.4 Fast Ion Bombardment (FAB):
Another technique for ionizing large bio-molecules (up to and greater then 10,000 Daltons) is to bombard them with ions of argon or xenon; this is also referred to as a liquid secondary ion source. First, analytes are embedded in a matrix such as glycerol, thioglycerol, m-nitrobenzyl alcohol, crown ethers, sulfolane, 20-nitrophenyloctyl ether, diethanolamine or triethanolamine. An electronic impact (EI) source similar to that described in the GC ionization section is used to ionize Ar or Xe gas at a pressure of 10-5 torr. Ar and Xe ions are accelerated towards the matrix containing the analytes and their impact sputters off positive and negative analytes ions (mostly molecular ions) that enter a mass spectrometer for mass determination.
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