Abstract
RATIONALEThe reactions of NO+ with volatile organic compounds (VOCs) in Selective Reagent Ionization Time-of-Flight Mass Spectrometry (SRI-TOF-MS) reactors are relatively poorly known, inhibiting their use for trace gas analysis. The rationale for this product ion distribution study was to identify the major product ions of the reactions of NO+ ions with 13 organosulfur compounds and 2 organoselenium compounds in an SRI-TOF-MS instrument and thus to prepare the way for their analysis in exhaled breath, in skin emanations and in the headspace of urine, blood and cell and bacterial cultures.METHODSProduct ion distributions have been investigated by a SRI-TOF-MS instrument at an E/N in the drift tube reactor of 130 Td for both dry air and humid air (4.9% absolute humidity) used as the matrix gas. The investigated species were five monosulfides (dimethyl sulfide, ethyl methyl sulfide, methyl propyl sulfide, allyl methyl sulfide and methyl 5-methyl-2-furyl sulfide), dimethyl disulfide, dimethyl trisulfide, thiophene, 2-methylthiophene, 3-methylthiophene, methanethiol, allyl isothiocyanate, dimethyl sulfoxide, and two selenium compounds – dimethyl selenide and dimethyl diselenide.RESULTSCharge transfer was seen to be the dominant reaction mechanism in all reactions under study forming the M+ cations. For methanethiol and allyl isothiocyanate significant fractions were also observed of the stable adduct ions NO+M, formed by ion-molecule association, and [M–H]+ ions, formed by hydride ion transfer. Several other minor product channels are seen for most reactions indicating that the nascent excited intermediate (NOM)+* adduct ions partially fragment along other channels, most commonly by the elimination of neutral CH3, CH4 and/or C2H4 species that are probably bound to an NO molecule. Humidity had little effect on the product ion distributions.CONCLUSIONSThe findings of this study are of particular importance for data interpretation in studies of volatile organosulfur and volatile organoselenium compounds employing SRI-TOF-MS in the NO+ mode. © 2014 The Authors. Rapid Communications in Mass Spectrometry published by John Wiley & Sons Ltd.
Highlights
There is considerable evidence that volatile organic compounds (VOCs) produced and partially released by the human body have great potential for diagnosis in physiology and medicine.[1,2,3,4,5,6] The emission of such compounds may result
Product ions with abundance greater than 0.5% of the total signal were included in Table 2 unless they clearly originated from the species under study
Under the conditions of this particular SRI-TOF-MS instrument at a drift field intensity of 130 Td, the dominant reaction mechanism of the NO+/Volatile sulfur compounds (VSCs) reactions is charge transfer generating M+ ions. These finding shows that the ionization energies (IEs) of the compounds under study are smaller than the recombination energy of the reactive NO+ nitrosonium ion
Summary
There is considerable evidence that volatile organic compounds (VOCs) produced and partially released by the human body have great potential for diagnosis in physiology and medicine.[1,2,3,4,5,6] The emission of such compounds may result. Family have been found in human urine,[13,15,16] blood,[17] and exhaled breath.[15,17,18,19] Several VSCs have been reported to be released by in vitro human cell lines,[20] including E. coli that releases methanethiol, dimethyl sulfide and hydrogen sulfide (strictly speaking not a VOC) that have been quantified and their emissions followed in real time.[21,22] As members of fluid/tissue-specific chemical fingerprints, sulfur compounds have been proposed to be markers of several disease conditions In particular their elevated breath levels have been attributed to halitosis,[23] liver diseases,[24,25,26] schizophrenia,[27] and lung cancer.[28,29] In therapeutic monitoring, breath OCS has been suggested to be a marker of organ rejection after lung transplantation,[30] whereas breath CS2 has been evidenced as an indicator of disulfiram ingestion.[31]
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