RESEARCH ARTICLE Gas‐phase chemical ionization of 4‐alkyl branched‐chain carboxylic acids and 3‐methylindole using H 3 O + , NO + , and O 2 + ions Hardy Z. Castada 1 | Sheryl A. Barringer 1 | Macdonald Wick 1,2 1 Department of Food Science and Technology, The Ohio State University, Columbus, OH 43210, USA 2 Department of Animal Sciences, The Ohio State University, Columbus, OH 43210, USA Correspondence H. Z. Castada and M. Wick, Department of Food Science and Technology, The Ohio State University, Columbus, OH 43210, USA. Email: castada.1@osu.edu; wick.13@osu.edu Funding information American Lamb Board, Grant/Award Number: 60054445 to MW 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. 1 | INTRODUCTION 4‐Methyloctanoic acid (4‐MOA), 4‐ethyloctanoic acid (4‐EOA), and 4‐methylnonanoic acid (4‐MNA) are saturated 4‐alkyl branched‐chain fatty acids (BCFA) that significantly contribute to the characteristic muttony or goaty flavor in sheep and goat meat. 3‐Methylindole, on the other hand, is closely associated with the pasture‐like aroma in cooked sheep meat and it has become a good indicator of a pasture diet (grass and/or legume) of sheep. However, 3‐methylindole (3‐MI) was also observed to undesirably impart a barnyard and fecal ‐like aroma in sheep meat. 1-12 Although regarded as key flavor compounds commonly deposited in subcutaneous fat of ruminants or ovine adipose tissues, these compounds have been reported to occur in such low amounts that their qualitative and quantitative analytical determination is limited. 2,3,8,9,13 The volatile nature of the branched‐chain fatty acids along with the indole compounds, which are dynamically released from triacylglycerides during cooking and other processes, make these compounds very suitable for a real‐time, trace gas analysis. 3,13-15 Typically, gas chromatography is utilized for the determination of these compounds in lamb fat after a series of time‐consuming sample preparation, extraction, purification and con- centration steps. 2,4,8,16-21 Previously, Kaffarnik and co‐workers described a direct determination of methyl‐esterified branched‐chain fatty acids using gas chromatography/mass spectrometry (GC/MS) designed to produce a fast and easy method which is needed in routine analysis. 13,21 Received: 5 June 2017 Revised: 14 July 2017 Accepted: 21 July 2017 DOI: 10.1002/rcm.7944 Rapid Commun Mass Spectrom. 2017;31:1641–1650. Copyright © 2017 John Wiley & Sons, Ltd. wileyonlinelibrary.com/journal/rcm 1641