DELAYED, GAS-PHASE ION FORMATION IN PLASMA DESORPTION MASS-SPECTROMETRY

The formation mechanisms of secondary ions from organic targets under MeV ion bombardment were studied with a high-resolution time-of-flight (TOF) mass spectrometer, Promptly formed ions H-n(+), C-n(+) and CnH were used for calibrating the TOF scale, Theoretical flight times of other ions were calculated according to the calibration curve and comp ared to experimentally determined values, The TOF values of non-specif ic low mass fragments formed via rearrangement or breaking of several bonds and/or abstraction of several atoms agree well with the theoreti cal values, On the other hand, target-specific organic ions, including molecular ions of peptides, have longer TOF values than predicted by the calibration curve, Time delays of a few hundred picoseconds were f ound for low-mass specific fragments, and a few nanoseconds for peptid e molecular ions, For protonated species and non-covalent clusters, th e delays are larger then for pre-formed and radical molecular ions. Me tals contained in organic samples, as contamination, also give delayed ions, For inorganic targets of LiBF4, significant delays were found f or the clusters (LiF)(n)Li+ with n > 3. A strong correlation was obser ved between the delay of an ion and the tailing of its kinetic energy distribution. The conclusion was made that the majority of target-spec ific ions are formed in the gas phase, at a distance from the target s urface of the order of 1 mu m. (C) 1997 by John Wiley & Sons, Ltd.