Fast atom bombardment mass spectrometer preparation

CpWIr3(CO)n was prepared as previously described (2). Diphenylacetylene (Aldrich) was used as received. Preparative thin-layer chromatography was carried out on (20 x 20 cm) silica gel Si250F plates (J.T. Baker). Infrared spectra were recorded on a Perkin-Elmer 1750 FT-IR spectrophotometer. H NMR (360 MHz) spectra were obtained on a Nicolet NT-360 spectrometer. Fast atom bombardment (FAB) mass spectra were obtained by the staff of the Mass Spectrometry Laboratory of the School of Chemical Sciences on a VG ZAB-SE mass spectrometer, using a matrix of dithioerythritol/dithiothieitol calculated masses are referenced to 184W and 193Ir. Elemental analyses were performed by the School Microanalytical Laboratory. 

These direct ion sources exist under two types liquid-phase ion sources and solid-state ion sources. In liquid-phase ion sources the analyte is in solution. This solution is introduced, by nebulization, as droplets into the source where ions are produced at atmospheric pressure and focused into the mass spectrometer through some vacuum pumping stages. Electrospray, atmospheric pressure chemical ionization and atmospheric pressure photoionization sources correspond to this type. In solid-state ion sources, the analyte is in an involatile deposit. It is obtained by various preparation methods which frequently involve the introduction of a matrix that can be either a solid or a viscous fluid. This deposit is then irradiated by energetic particles or photons that desorb ions near the surface of the deposit. These ions can be extracted by an electric field and focused towards the analyser. Matrix-assisted laser desorption, secondary ion mass spectrometry, plasma desorption and field desorption sources all use this strategy to produce ions. Fast atom bombardment uses an involatile liquid matrix. 

Three soft ionisation methods are in use for earbohydrates, fast atom bombardment (FAB), eleetrospray ionisation (ESI) and matrix-assisted laser desorption/ionisation (MALDI). FAB is the oldest and involves directing a high-energy beam of Cs" ions or Xe atoms at the sample dissolved in a nonvolatile solvent such as m-nitrobenzyl alcohol. The atoms sputter the sample and matrix [M + H] or [M + Na]" ions are commonly observed. With an upper limit of M of about 2000, FAB is not that soft, and is usually used for small oligosaccharides it has the further disadvantage that the sample is prepared and then directly introduced into the mass spectrometer, so that it cannot be combined with liquid chromatography. 

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