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Gas‐phase chemical ionization of 4‐alkyl branched‐chain carboxylic acids and 3‐methylindole using H 3 O + , NO + , and O 2 + ions
Author(s) -
Castada Hardy Z.,
Barringer Sheryl A.,
Wick Macdonald
Publication year - 2017
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.7944
Subject(s) - chemistry , mass spectrometry , chemical ionization , ion , molecule , analytical chemistry (journal) , alkyl , reagent , ionization , electron ionization , reaction rate constant , organic chemistry , chromatography , kinetics , physics , quantum mechanics
Rationale 4‐Methyloctanoic acid, 4‐ethyloctanoic acid, 4‐methylnonanoic acid, and 3‐methylindole are primary contributors to the distinctive aroma and flavor of lamb meat. The reactions of H 3 O + , NO + , and O 2 + with these compounds, and identification of the product ions and their distribution, are fundamental to their characterization and rapid, real‐time trace analysis using selected ion flow tube mass spectrometry (SIFT‐MS). Methods The chemical ionization of pure standards of 4‐ethyloctanoic acid, 4‐methyloctanoic acid, 4‐ethylnonanoic acid, and 3‐methylindole was carried out using the H 3 O + , NO + , and O 2 + reagent ions of a V200™ SIFT mass spectrometer. Kinetic data were calculated using the Langevin collision rate with parameterized trajectory equations. Identification of product ions, distribution, and interferences was performed by further evaluation of the pertinent ion‐molecule reaction mechanisms, careful spectral analyses, and molecular mass‐molecular structure pairing. Results The collisional capture rate constants of the reaction of the precursor ions H 3 O + , NO + , and O 2 + , their extended hydrates and the analytes, which were assumed to occur at or near the collisional rate, were all of the order of 10 −9 cm 3 molecule s −1 – typical for bimolecular ion‐molecule reactions. Positive identification of the primary and secondary product ions, fragmented ionic species, and potential ion conflicts and interferences, from each reagent ion channel, was determined for each compound. Conclusions We have established the ion chemistry involved in the ionization of the 4‐alkyl branched‐chain fatty acids and 3‐methylindole using the precursor ions, H 3 O + , NO + , and O 2 + in SIFT‐MS. The ion‐molecular chemistry and the associated kinetics serve as a fundamental basis for the accurate characterization of these compounds by SIFT‐MS.