Mass-selected reagent ion chemical ionization (CI) performed in an ion trap instrument is an efficient tool to investigate gas-phase ion reactivities and therefore to find out new and/or optimized applications for structural analysis. For instance, it was shown that the C 3 H 6 O + . (58 mass units) molecular ion originated from vinyl methyl ether (VME) should necessarily be used alone (i.e. unit-mass selected) to produce significant diagnostic-ions for double bond location in aliphatic alkenes. Regarding the assignment of epoxides, the previous NO + /CI method was adapted for an optimal use in the trap through isolation of NO
+ cation from N 2 O (instead of NO) plasma and production of the acylium diagnostic-ions via CID of [M -H]+ formed by NO + -induced hydride abstraction. New alkylation ion-products, e.g. RCH == O + -alk, were also found to characterize isomeric epoxides as a result of either an initial electrophilic addition of the C 2 H 5 + cation (with saturated epoxides) or a methyl-transfer from [VME]+ . (with α, -unsaturated epoxides). The multiple tandem mass spectrometry (MS n ) capabilities of the ion trap were essential to achieve reagent ion mass-selection, structural assignment of the diagnostic-ions, or to provide further selectivity.
The potential of the ion trap mass spectrometer to characterize isomeric long-chain conjugated mono-or diethylenic epoxides was investigated using the multi-stage tandem mass spectrometric (MS n ) facilities of the instrument. In the proposed approach, mass-selected reagent ions obtained from common protonating agents were prepared and allowed to react with the neutral epoxides for controlled reaction times. Positive-ion chemical ionization using the t-C 4 H 9 + ion (t-C 4 H 9 + /CI) and CH 5 + or C 2 H 5 + /CI were found to provide the most informative spectra (e.g. diagnostic ions generated by the decomposition of the MH + ion through competing pathways) as compared with NH 4 + /CI. An acylium diagnostic ion (A + ) stemming from the epoxide side was produced in both types of epoxides but its relative abundance was optimal using CH 5 + or C 2 H 5 + which are of low proton affinity. In the case of the dienes, an [M + A] + covalent adduction, probably resulting from a [4 + 2 + ] Diels-Alder cycloaddition process and thus specifically assigning such a system was generated (C 2 H 5 + /CI conditions). The MS n capabilities of the instrument were also used for the determination of the origin and structure of the diagnostic ions or for additional selectivity.
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