IMPROVED DETECTION LIMITS FOR FAST-ATOM-BOMBARDMENT MASS-SPECTROMETRY - A STUDY OF TIME-DEPENDENT DESORPTION USING A MODEL PULSED BOMBARDMENT IONIZATION METHOD

Certain sample preparations for fast atom bombardment (FAB) yield an intense but brief pulse of sample ions at the onset of ionization. A model system is used to study this phenomenon. This system utilizes a conventional source of a constant flux of fast atoms and a probe that permits mechanical movement of the sample stage. This is used to simulate the effect of pulsing the atom beam. Experiments with model samples and selected ion monitoring detection show that, following bombardment initiation, sample ions are preferentially desorbed with respect to ions from the FAB matrix. Exhibition of preferential sample desorption depends upon the analyte behaving as a surfactant in the selected matrix. When used in conjunction with an array detector that permits data collection in a time-resolved manner, this system allows collection of useful mass spectra with significantly enhanced sensitivity compared to normal bombardment. When applied to the undecapeptide eledoisin (sequence pyro-EPSKDAFIGLM-NH2, MW 1187.6 Da) this novel methodology allows an improvement in detection limit of at least three to four orders of magnitude over that observed when using conventional continuous FAB and a point detector.