Delayed, gas‐phase ion formation in plasma desorption mass spectrometry

Abstract

were used for calibrating the TOF scale. Theoretical flight times of other ions were calculated according to the calibration curve and compared to experimentally determined values. The TOF values of non-specific low mass fragments formed via rearrangement or breaking of several bonds and/or abstraction of several atoms agree well with the theoretical 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 found for low-mass specific fragments, and a few nanoseconds for peptide molecular ions. For protonated species and non-covalent clusters, the delays are larger then for pre-formed and radical molecular ions. Metals contained in organic samples, as contamination, also give delayed ions. For inorganic targets of LiBF 4 , significant delays were found for the clusters (LiF) n Li + with n > 3. A strong correlation was observed between the delay of an ion and the tailing of its kinetic energy distribution. The conclusion was made that the majority of target-specific ions are formed in the gas phase, at a distance from the target surface of the order of 1 µm.

Interaction between explosive and analyte layers in explosive matrix‐assisted plasma desorption mass spectrometry

“…Experimentally, it has been determined that the ion formation process, including desorption into the gas phase, takes 10 À10 to maximum 10 À9 s for large molecular ions. 17…”

“…18 For consistency with the ion track model, 12 this conversion must occur after a very short primary activation of 10 À15 -10 À13 s, and the final product formation time must be within 10 À9 s, the order of time for molecular ion formation in PDMS. 17 The important question that will be considered here is whether this can happen, taking into account rapid (faster than 1 ns) desorption into the gas phase.…”