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 Hn+, Cn+ and CnH+ 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 than for pre-formed and radical molecular ions. Metals contained in organic samples, as contamination, also give delayed ions. For inorganic targets of LiBF4, significant delays were found for the clusters (LiF)nLi+ 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.

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