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Field ionization mass spectrometry—IV. Isotopically labeled heptanals
Author(s) -
Brown Peter,
Fenselau Catherine
Publication year - 1973
Publication title -
organic mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.475
H-Index - 121
eISSN - 1096-9888
pISSN - 0030-493X
DOI - 10.1002/oms.1210070309
Subject(s) - ion , chemistry , electron ionization , field desorption , mass spectrum , mass spectrometry , analytical chemistry (journal) , ionization , chemical ionization , chromatography , organic chemistry
The field ionization (FI) mass spectra of n ‐heptanal and a series of deuterium labeled analogs have been studied, with the objectives of initiating systematic investigations of reaction mechanisms of FI produced ions and to permit comprison with those found for other ionization processes. It is now recognized that FI ions have: (a) lower average internal energies and (b) shorter residence times than similar ions generated by electron‐impact (EI), and the possibility exists of H/D‐randomization occuring in ions formed by desorption from the emitter, by unimolecular decomposition close to the emitter and by either ‘fast’ or ‘slow’ metastable decompositions. In this study only the peak shifts of normal ions could be utilized; accurate mass measurements of all major ions revealed elemental compositions similar to EI. A site‐specific McLafferty rearrangement gave the base peak at m / e 44 ([C 2 H 4 O] +. ), although the apparently complementary ion at m / e 70 ([C 5 H 10 ] +. ) arose in a less specific process. Ions at m / e 43 ([C 3 H 7 ] + ) and 71 ([C 5 H 11 ] + 80%; [C 4 H 7 O] + 20%) were apparantly generated without significant H/D‐scrambling. Of special interest was the observation of the rearrangement ion at m / e 86 ([C 5 H 10 O] +. ) caused by loss of C‐2 and C‐3 as C 2 H 4 , as found for EI. It is concluded that at least in this system, decomposing molecular ions formed: (a) in the gas phase extremely close to the emitter and/or (b) on the emitter surface, have lifetimes sufficiently short to preclude complete H/D randomization. The results also provide evidence for common fragmentation mechanisms for heptanal molecular ions at both the low end and the high end of the energy distribution.