RATIONALE The detection sensitivity of SESI-MS is seen to be much lower for saturated aldehydes than for unsaturated aldehydes. This needs to be understood in terms of gas phase ion-molecule reaction kinetics and energetics to make SESI-MS analytically more quantitative. METHODS Parallel SESI-MS and SIFT-MS analyses were carried out of air containing variable and accurately determined concentrations of saturated (C5 pentanal; C7 heptanal; C8 octanal) and unsaturated (C5 2-pentenal; C7 2-heptenal; C8 2-octenal) aldehyde vapours. The influence of the source gas humidity and of the ion transfer capillary temperature, 250°C and 300°C, in a commercial SESI-MS instrument is explored. Separate experiments were carried out using SIFT to determine the rate coefficients, k73 , for the ligand switching reactions of the H3 O+ (H2 O)3 ions with the 6 aldehydes. RESULTS The relative slopes of the plots of SESI-MS ion signal against SIFT-MS concentration are interpreted as the relative SESI-MS sensitivities for these six compounds. It is seen that the sensitivities for the unsaturated aldehydes are variously some 20 to 60 times greater than for the corresponding C5, C7 and C8 saturated aldehydes. Additionally, the SIFT experiments revealed that the measured k73 are 3-4 times greater for the unsaturated than for the saturated aldehydes. CONCLUSIONS The trends in SESI-MS sensitivities are rationally explained by differences in the rates of the ligand switching reactions, which are justified by theoretically calculated equilibrium rate constants derived from thermochemical DFT calculations of Gibb's free energy changes. Humidity present in SESI gas thus favours the reverse reactions of the saturated aldehyde analyte ions, effectively suppressing their signals in contrast to their unsaturated counterparts.

Nitrogen carrier gas is now being used more frequently for SIFT-MS analyses than helium for the reasons of cost and supply. Yet the extensive kinetics database required has largely been compiled using data obtained in helium carrier gas. This paper asks the question: can the helium-based kinetics library be used with confidence for analyses in nitrogen carrier gas? To investigate this, the rate coefficients and product ion distributions for the reactions of H3O+, NO+ and O2+● with three monoterpenes, β-pinene, camphene and (R)-(+)-limonene, and the specific reactions (a) H3O+ with 2-propanol, (b) O2+● with acetone, (c) NO+ with acetaldehyde and (d) NO+ with 2,3-butanedione have been explored in both helium and nitrogen carrier gases using a Profile 3 SIFT-MS instrument. These reactions were chosen because several primary reaction mechanisms are involved, including proton transfer (a), charge transfer (b), parallel hydride ion transfer and adduct ion formation (c) and parallel charge transfer and adduct ion formation (d). The detailed results show that for the diverse monoterpene reactions that have multiple product ions and for the pure bimolecular reactions (a) and (b), the reaction kinetics in both helium and nitrogen carrier gases are essentially identical. However, reactions (c) and (d) in which adduct ions are partially formed exhibit a slow carrier gas pressure dependence in helium, but a much greater carrier gas pressure dependence in nitrogen, and different product ion distributions. The conclusion is drawn that for pure bimolecular reactions, e.g. (a) and (b), the helium-obtained kinetics data can be used with confidence for trace gas analysis by SIFT-MS in nitrogen carrier gas, whereas kinetics data for ion-molecule reactions that involve adduct ion formation must be obtained by measurements under the specific pressure (and temperature) of the nitrogen carrier gas at which gas analyses are to be performed.

Rationale To assess the suitability of NH4+ as a reagent ion for trace gas analysis by selected ion flow tube mass spectrometry, SIFT-MS, its ion chemistry must be understood. Thus, rate coefficients and product ions for its reactions with typical biogenic molecules and monoterpenes need to be experimentally determined in both helium, He, and nitrogen, N-2, carrier gases. Methods NH4+ and H3O+ were generated in a microwave gas discharge through an NH3 and H2O vapour mixture and, after m/z selection, injected into He and N-2 carrier gas. Using the conventional SIFT method, NH4+ reactions were then studied with M, the biogenic molecules acetone, 1-propanol, 2-butenal, trans-2-heptenal, heptanal, 2-heptanone, 2,3-heptanedione and 15 monoterpene isomers to obtain rate coefficients, k, and product ion branching ratios. Polarisabilities and dipole moments of the reactant molecules and the enthalpy changes in proton transfer reactions were calculated using density functional theory. Results The k values for the reactions of the biogenic molecules were invariably faster in N-2 than in He but similar in both bath gases for the monoterpenes. Adducts NH4+M were the dominant product ions in He and N-2 for the biogenic molecules, whereas both MH+ and NH4+M product ions were observed in the monoterpene reactions; the monoterpene ratio correlating (R-2 = 0.7) with the proton affinity, PA, of the monoterpene molecule as calculated. The data indicate that this adduct ion formation is the result of bimolecular rather than termolecular association. Conclusions NH4+ can be a useful reagent ion for SIFT-MS analyses of molecules with PA(M) < PA(NH3) when the dominant single product ion is the adduct NH4+M. For molecules with PA(M) > PA(NH3), such as monoterpenes, both MH+ and NH4+M ions are likely products, which must be determined along with k by experiment.

Atlantic salmon is rich in bioactive proteins, antioxidants, vitamins, and omega-3 fatty acids, making it a rich source of essential nutrients. However, it is highly susceptible to biochemical, physicochemical, and microbial spoilage. The aim of this study was to use natural oregano and dill antioxidant extracts in combination with modified atmosphere packaging (MAP) (CO2:N2 60:40) to retain the quality and delay lipid oxidation in Atlantic salmon (Salmo salar) during storage. Extract-treated samples and controls (both vacuum packed and in MAP) were stored under chilled conditions (4 +/- 1 degrees C) for 0-16 days. Quality changes during storage were assessed in terms of drip loss, microbial growth, color, pH, protein solubility, and lipid oxidation. Primary and secondary lipid oxidation products were determined by means of peroxide value and TBARS. In parallel, volatile compounds were analyzed using GC-MS, SIFT-MS and SESI-MS. GC-MS indicated 29 recognizable VOCs present in the headspace of the salmon samples. SIFT-MS quantification was carried out for ammonia and 33 VOCs, including alcohols, several aldehydes, carboxylic acids, sulfur compounds, trimethylamine, and ammonia. While dill or oregano treatment reduced lipid oxidation after 16 days of storage, it did not affect VOC concentrations significantly. The study has revealed that changes of the measured concentration of volatiles with storage time could be used to monitor freshness and spoilage of the fish while observing the effect of natural antioxidants.

The reaction of oxygen anions with carbon dioxide molecules is considered to be the main channel of CO3- ion formation in the Martian ionosphere. Comprehensive theoretical ion-chemical models of the Martian ionosphere require experimental data on the reactivity of negative ions with neutral gases under relevant ambient temperature and pressure conditions. The temperature dependence of the rate of the ternary reaction of O- with CO2 has been measured at temperatures relevant to the ionosphere of Mars using a VT-SIFT instrument using both helium and nitrogen as the carrier (ternary) gases. The ternary rate coefficient of the reaction was measured equal to (1.24 +/- 0.06) x 10(-28) (300/T)(2.86 +/- 0.28) and (2.29 +/- 0.05) x 10(-28 )(300/T) (2.06 +/- 0.13) cm(6)s(-1) for He and N-2 carrier gas, respectively, in the temperature range of 230-340 K. The k(3) is over two times higher in nitrogen than in helium and has a less strong inverse temperature dependence.

Rationale The reactions of the reagent ions used for trace gas analysis in selected ion flow tube mass spectrometry (SIFT-MS), R+, viz. H3O+, NO+ and O-2(+), with the major gases in air and breath samples, M, viz. N-2, O-2, CO2 and H2O, are investigated. These reactions are seen to form weakly-bound adduct ions, R+M, by ternary association reactions that must not be mistaken for genuine volatile organic compound (VOC) analyte ions. Methods The ternary association rate coefficients mediated by helium (He) carrier gas atoms, k(3a), have been determined for all combinations of R+ and M, which form R+M adduct ions ranging in m/z from 47 (H3O+N2) to 76 (O2+center dot CO2). This was achieved by adding variable amounts of M (up to 0.5 mbar pressure) into the He carrier gas (pressure of 1.33 mbar) in a SIFT-MS flow tube at 300 K. Parabolic curvature was observed on some of the semi-logarithmic decay curves that allowed the rate coefficients mediated by M molecules, k(3b), to be estimated. Results Values of k(3a) were found to range from 1 x 10(-31) cm(6) s(-1) to 5 x 10(-29) cm(6) s(-1), which form mass spectral R+M "ghost peaks" of significant strength when analysing VOCs at parts-per-billion concentrations. It was seen that the R+M adduct ions (except when M is H2O) react with H2O molecules by ligand switching forming the readily recognised monohydrates of the initial reagent cations R+H2O. Whilst this ligand switching diminishes the R+M adduct ghost peaks, it does not eliminate them entirely. Conclusions The significance of these adduct ions for trace gas analysis by SIFT-MS in the low m/z region is alluded to, and some examples are given of m/z spectral overlaps of the R+M and R+H2O adduct cations with analyte cations of VOCs formed by analysis of complex media like exhaled breath, warning that ghost peaks will be enhanced using nitrogen carrier gas in SIFT-MS.

Rationale Secondary electrospray ionization (SESI) in a water spray environment at atmospheric pressure involves the reactions of hydrated hydronium reagent ions, H3O+(H2O)(n), with trace analyte compounds in air samples. Understanding the formation and dehydration of reagent and analyte ions is the foundation for meaningful quantification of trace compounds by SESI-mass spectrometry (MS). Methods A numerical model based on gas-phase ion thermochemistry is developed that describes equilibria in H3O+(H2O)(n) reagent cluster ion distributions and ligand switching reactions with polar NH3 molecules leading to equilibrated hydrated ammonium ions NH4+(H2O)(m). The model predictions are compared with experimental results obtained using a cylindrical SESI source coupled to an ion-trap mass spectrometer via a heated ion transfer capillary. Non-polar isoprene, C5H8, was used to further probe the nature of the reagent ions. Results Equilibrium distributions of H3O+(H2O)(n) ions and their reactions with NH3 molecules have been characterized by the model in the near-atmospheric pressure SESI source. NH3 analyte molecules displace H2O ligands from the H3O+(H2O)(n) ions at the collisional rate forming NH4+(H2O)(m) ions, which travel through the heated ion transfer capillary losing H2O molecules. The data for variable NH3 concentrations match the model predictions and the C5H8 test substantiates the notion of dehydration in the heated capillary. Conclusions Large cluster ions formed in the SESI region are dehydrated to H3O+(H2O)(1,2,3) and NH4+(H2O)(1,2) while passing through the heated capillary, and considerable diffusion losses also occur. This phenomenon is also predicted for other polar analyte molecules, A, that can undergo similar switching reactions, thus forming AH(+) and AH(+)(H2O)(m) analyte ions.

Rationale Secondary electrospray ionization (SESI) is currently only semi-quantitative. In the Zspray (TM) arrangement of SESI-MS, the transfer of ions from near atmospheric pressure to a triple quadrupole is achieved by guiding electric fields that partially desolvate both reagent and analyte ions which must be understood. Also, to make SESI-MS more quantitative, the mechanisms and the kinetics of the reaction processes, especially ligand switching reactions of hydrated hydronium reagent ions, H3O+(H2O)(n), with volatile organic compound (VOC) molecules, need to be understood. Methods A modified Zspray (TM) ESI ion source operating at sub-atmospheric pressure with analyte sample gas introduced via an inlet coaxial with the spray was used. Variation of the ion-guiding electric fields was used to reveal the degree of desolvation of both reagent and analyte ions. The instrument sensitivity was determined for several classes of VOCs by introducing bag samples of suitably varying concentrations as quantified on-line using selected ion flow tube MS. Results Electric field desolvation resulted in largely protonated VOCs, MH+, and their monohydrates, MH+H2O, and for some VOCs proton-bound dimer ions, MH+M, were formed. There was a highly linear response of the ion signal to the measured VOC sample concentration, which provided the instrument sensitivities, S, for 25 VOCs. The startling results show very wide variations in S from near 0 to 1 for hydrocarbons, and up to 100, on a relative scale, for polar compounds such as monoketones and unsaturated aldehydes. Conclusions The complex ion chemistry occurring in the SESI ion source, largely involving gas-phase ligand switching, results in widely variable sensitivities for different classes of VOCs. The sensitivity is observed to depend on the dipole moment and proton affinity of the analyte VOC molecule, M, and to decrease with the observed fraction of MH+H2O, but other yet unrecognized factors must play a significant role.

A study was performed of the reactions of protonated acetic acid hydrates, CH3COOHH+(H2O)n, with acetone molecules, CH3COCH3, using a selected ion flow-drift tube (SIFDT). The rationale for this study is that hydrated protonated organic molecules are major product ions in secondary electrospray ionization mass spectrometry (SESI-MS) and ion mobility spectrometry (IMS). Yet the formation and reactivity of these hydrates are only poorly understood, and kinetics data are only sparse. The existing SIFDT instrument in our laboratory was upgraded to include an octupole ion guide and a separate drift tube by which hydrated protonated ions can be selectively injected into the drift tube reactor and their reactions with molecules studied under controlled conditions. This case study shows that, in these hydrated ion reactions with acetone molecules, the dominant reaction process is ligand switching producing mostly proton-bound dimer ions (CH3COCH3)H+(CH3COOH), with minor branching into (CH3COCH3)H+(H2O). This switching reaction was observed to proceed at the collisional rate, while other studied hydrated ions reacted more slowly. An attempt is made to understand the reaction mechanisms and the structures of the reaction intermediate ions at the molecular level. Secondary switching reactions of the asymmetric proton-bound dimer ions lead to a formation of strongly bound symmetrical dimers (CH3COCH3)2H+, the terminating ion in this ion chemistry. These results strongly suggest that, in SESI-MS and IMS, the presence of a polar compound, like acetone in exhaled breath, can suppress the analyte ions of low concentration compounds like acetic acid thus compromising their quantification.

A comparative study has been made of the utility of helium and nitrogen as the carrier gas in selected ion flow tube mass spectrometry, SIFT-MS. A most important parameter is shown to be the injection energy, Elab, into the nitrogen carrier gas of the reagent cations H3O+, NO+ and O2+• produced in the microwave discharge ion source. If Elab is too high, partial fragmentation of the reagent ions occurs in the hard collisions with nitrogen molecules, which does not occur to a significant extent in their collisions with lighter helium atoms. Then, the H3O+ ions fragment producing highly reactive ions, including OH+ and H2O+•, that can ionise and dissociate the N2 molecules, and the fraction of the non-dissociated H3O+ can associate with N2 molecules producing H3O+N2 adduct ions. These events promote the production of hydrated hydronium ions, H3O+(H2O)1,2,3, which become dominant even when the carrier gas and sample gas are only a modest humidity. The NO+ reagent ions become electronically excited and partially fragment to either O+• or N+ ions. These events result in their reactions with the N2 molecules producing excessive O2+• ions in the carrier gas. Similarly, the O2+• reagent ions become electronically excited and fragment to O+• ions that react with the N2 molecules producing large fractions of NO+ ions. These processes result in larger fractions of "impurity ions" than are desirable for gas phase analysis using SIFT-MS, which is especially serious for H3O+ reagent ions, but less so for NO+ and O2+•, which can still be used, with caution, for trace gas analysis. It is shown by reducing Elab that fragmentation can be minimised, and it is suggested that by increasing the carrier gas temperature or exploiting drift tube methods, adduct ion formation can be inhibited and thus partially "purify" the reagent ions to approach the situation that pertains to helium carrier gas.

RationaleNalophan bags are commonly used to collect breath samples for volatile metabolite analysis. Volatile organic compounds (VOCs) released from the polymer can, however, be mistaken as breath metabolites when analyses are performed by selected ion flow tube mass spectrometry, SIFT‐MS, or techniques that depend on a proper understanding of ion chemistry.MethodsThree analytical techniques were used to analyse the VOCs released into the nitrogen used to expand Nalophan bags, viz. gas chromatography/mass spectrometry (GC/MS), secondary electrospray ionization mass spectrometry (SESI‐MS) and selected ion flow tube mass spectrometry (SIFT‐MS). The most significant VOCs were identified and quantified by SIFT‐MS as a function of storage time, temperature and humidity.ResultsThe consistent results obtained by these three analytical methods identify 1,2‐ethanediol (ethylene glycol) and 2‐methyl‐1,3‐dioxolane as the major VOCs released by the Nalophan. Their concentrations are enhanced by increasing the bag storage temperature and time, reaching 170 parts‐per‐billion by volume (ppbv) for ethylene glycol and 34 ppbv for 2‐methyl‐1,3‐dioxolane in humid nitrogen (absolute humidity of 5%) contained in an 8‐L Nalophan bag stored at 37°C for 160 min.ConclusionsUsing H3O+ reagent ions for SIFT‐MS and SESI‐MS analyses, the following analyte ions (m/z values) are affected by the Nalophan impurities: 45, 63, 81, 89 and 99, which can compromise analyses of acetaldehyde, ethylene glycol, monoterpenes, acetoin, butyric acid, hexanal and heptane.

RATIONALEThe volatile compounds generated by the electrochemical reduction of atmospheric carbon dioxide and nitrogen include isobaric methanol (CH3 OH) and, potentially, hydrazine (N2 H4 ). To achieve quantification of hydrazine molecules by selected ion flow tube mass spectrometry, SIFT-MS, its reactions with H3 O+ , NO+ and O2 + reagent ions must be understood.METHODSA SIFT study (using the SIFT-MS instrument) has been carried out to obtain rate coefficients and product ions for the reactions of H3 O+ , NO+ and O2 + reagent ions with N2 H4 and CH3 OH molecules present in the humid headspace of their aqueous solutions. Using the kinetics data obtained, solution headspace concentrations were obtained for both compounds as a function of their liquid phase concentrations at 10, 20 and 35°C.RESULTSBoth compounds react with H3 O+ ions via rapid proton transfer to produce CH3 OH2 + and H5 N2 + ions with the common m/z of 33. It is revealed that NO+ rapidly transfers charge to N2 H4 (rate coefficient k = 2.3x10-9 cm3 s-1 ) but only slowly associates with CH3 OH (k2eff = 7.1 x10-11 cm3 s-1 ). Thus, selective analysis can be achieved using both H3 O+ and NO+ reagent ions. The headspace methanol vapour concentration was found to increase with increasing solution temperature but that of hydrazine decreased with an associated increase of ammonia (NH3 ) as measured with O2 + reagent ions.CONCLUSIONSThe isobaric compounds methanol and hydrazine can be separately analysed in real time by SIFT-MS using H3 O+ and NO+ reagent ions, even when they co-exist in humid air. The evolution of hydrazine from aqueous solutions can be quantitatively monitored together with its decomposition at elevated temperatures.

RationaleThe volatile compounds generated by the electrochemical reduction of atmospheric carbon dioxide and nitrogen include isobaric methanol (CH3OH) and, potentially, hydrazine (N2H4). To achieve quantification of hydrazine molecules by selected ion flow tube mass spectrometry (SIFT‐MS), its reactions with H3O+, NO+ and O2+ reagent ions must be understood.MethodsA SIFT study (using a SIFT‐MS instrument) was carried out to obtain rate coefficients and product ions for the reactions of H3O+, NO+ and O2+ reagent ions with N2H4 and CH3OH molecules present in the humid headspace of their aqueous solutions. Using the kinetics data obtained, solution headspace concentrations were determined for both compounds as a function of their liquid‐phase concentrations at 10, 20 and 35°C.ResultsBoth compounds react with H3O+ ions via rapid proton transfer to produce CH3OH2+ and H5N2+ ions with the common m/z value of 33. It is revealed that NO+ rapidly transfers charge to N2H4 (rate coefficient k = 2.3 × 10−9 cm3 s−1) but only slowly associates with CH3OH (k2eff = 7.1 × 10−11 cm3 s−1). Thus, selective analysis can be achieved using both H3O+ and NO+ reagent ions. The headspace methanol vapour concentration was found to increase with increasing solution temperature, but that of hydrazine decreased with an associated increase of ammonia (NH3) as measured with O2+ reagent ions.ConclusionsThe isobaric compounds methanol and hydrazine can be separately analysed in real time by SIFT‐MS using H3O+ and NO+ reagent ions, even when they co‐occur in humid air. The evolution of hydrazine from aqueous solutions can be quantitatively monitored together with its decomposition at elevated temperatures.

Selected ion flow tube mass spectrometry, SIFT-MS, is a non-separative method for direct quantitative analyses of volatile compounds, VOCs, in air and humid breath based on chemical ionization. Selected reagent ions, either H3O+, NO+ or O2+ (non-reactive with major components of air), ionize analyte molecules during a defined time in a flow tube by ion-molecule reactions thus producing analyte ions that are characteristic of the neutral analyte VOCs. Concentrations can be calculated in real-time from the ion count rates. Direct on-line analysis of single or multiple breath exhalations or off-line analysis of breath samples collected into bags can be performed. Several volatile breath metabolites have been quantified by SIFT-MS, including ammonia, acetone, hydrogen cyanide, alcohols, pentane, acetic acid, methane, and sulphur compounds. Their potential as biomarkers is discussed.

Comprehensive theoretical models of the Martian ionosphere require reliable kinetic data for ion - molecule reactions over a wide range of temperatures. In situ measurements of the negative ions have not yet been performed and so ion chemical modelling is relied upon, which indicate that nitrite anions, NO2-, are important. Thus, in the present work the gas phase reaction of NO2- with carbon dioxide molecules, CO2, has been studied at the temperatures and energies relevant to the Martian atmosphere. The rate coefficient was measured over the temperature range 220 +/- 2 K to 334 +/- 2 K using a variable temperature selected ion flow tube (VT-SIFT) instrument. Additionally, a commercial Tandem Quadrupole Mass Spectrometer (TQMS) has been exploited to explore the reaction occurring during NO2-/CO2 collisions over the centre-of-mass energies from near-thermal to 6 eV. Two different channels of the reaction were studied. Under the thermal conditions of the VT-SIFT, the reaction is seen to proceed slowly via ternary association ultimately producing CO3- ions, the measured effective ternary rate coefficient in helium being k(3) = (3.8 +/- 1.5) x 10(-30) cm(6) s(-1) at 220 +/- 2 K and k(3) = (0.5 +/- 0.2) x 10(-3) cm(6) s(-1) at 334 +/- 2K. The effective ternary rate coefficient can be greater in the Martian atmosphere where CO2 is the stabilizing gas. In the TQMS experiment, CO3- production occurs via a binary reaction above a threshold energy of 1.8 +/- 0.2 eV. These experimental data can be used to refine the existing ion-chemical models of the Martian atmosphere.

Ion-molecule reactions (IMR) are at the very core of trace gas analyses in modern chemical ionization (CI) mass spectrometer instruments, which are increasingly being used in diverse areas of research and industry. The focus of this Perspective is on the ion chemistry that underpins gas-phase analytical CI methods. Special attention is given to the soft chemical ionization method known as selected ion flow tube-mass spectrometry (SIFT-MS). The processes involved in the ion chemistry of the reagent cations, H3O+, NO+, and O2+•, and the anions, O-•, O2-•, OH-, and NO2-, are discussed in some detail. Stressed throughout is that an understanding of these processes is mandatory to obtain reliable analyses of humid gaseous media such as ambient air and exhaled breath. It is indicated that further research is needed to understand the consequences of replacing helium in some situations by the more readily available nitrogen as the carrier gas in SIFT-MS.

A study has been made of the volatile organic compounds (VOCs) released into the contained headspace of packaged lamb's lettuce (Valerianella locusta) plants, cut at the roots, and of separated rocket (Eruca sativa) and baby spinach (Spinacia oleracea) leaves. Direct real‐time analyses of the headspace were carried out on each of five consecutive days using selected‐ion flow‐tube mass spectrometry (SIFT‐MS) and secondary electrospray ionization mass spectrometry (SESI‐MS), together with concomitant off‐line analyses using solid‐phase microextraction (SPME) and gas chromatography mass spectrometry (GC‐MS). It was observed that the concentrations of organosulphur compounds, especially dimethyl sulphide (DMS) and dimethyl disulphide (DMDS) were relatively low in the headspace of fresh samples, but increased exponentially over the 5 days for rocket, increased to a lesser extent for spinach, and increased to an even lesser extent for lamb's lettuce. Methanol was detected in the headspace of even fresh samples of lamb's lettuce, and its concentration increased for the subsequent 2–3 days before decreasing. This trend was mirrored in the isoprenoids analysed, which are characteristic of plant tissues. These results show that direct real‐time VOC analyses by SIFT‐MS can be usefully exploited to monitor fresh leaf salad spoilage, and that SESI‐MS spectra show patterns that are characteristic of salad type and storage time.

Soft chemical ionization mass spectrometry techniques, particularly the well-established proton transfer reaction mass spectrometry, PTR-MS, and selected ion flow tube mass spectrometry, SIFT-MS, are widely used for real-time quantification of volatile organic compounds in ambient air and exhaled breath with applications ranging from environmental science to medicine. The most common reagent ions H3O+, NO+, or O2+• can be selected either by quadrupole mass filtering from a discharge ion source, which is relatively inefficient, or by switching the gas/vapor in the ion source, which is relatively slow. The chosen reagent ions are introduced into a flow tube or flow-drift tube reactor where they react with analyte molecules in sample gas. This article describes a new electrostatic reagent ion switching, ERIS, technique by which H3O+, NO+, and O2+• reagent ions, produced simultaneously in three separate gas discharges, can be purified in post-discharge source drift tubes, switched rapidly, and selected for transport into a flow-drift tube reactor. The construction of the device and the ion-molecule chemistry exploited to purify the individual reagent ions are described. The speed and sensitivity of ERIS coupled to a selected ion flow-drift tube mass spectrometry, SIFDT-MS, is demonstrated by the simultaneous quantification of methanol with H3O+, acetone with NO+, and dimethyl sulfide with O2+• reagent ions in single breath exhalations. The present ERIS approach is shown to be preferable to the previously used quadrupole filtering, as it increases analytical sensitivity of the SIFDT-MS instrument while reducing its size and the required number of vacuum pumps.

Dietary restriction together with alteration of the gastrointestinal tract results in major metabolic changes and significant weight loss in patients undergoing bariatric surgery. Current methods of measuring these changes are often inaccurate and lack a molecular basis. The objective of this study was to determine the role of exhaled ketones as non-invasive markers of nutritional status in patients undergoing surgical treatment of obesity.