The quantification of carbon dioxide in humid air and exhaled breath by selected ion flow tube mass spectrometry

The reactions of carbon dioxide, CO₂, with the precursor ions used for selected ion flow tube mass spectrometry, SIFT‐MS, analyses, viz. H₃O⁺, NO⁺ and O, are so slow that the presence of CO₂ in exhaled breath has, until recently, not had to be accounted for in SIFT‐MS analyses of breath. This has, however, to be accounted for in the analysis of acetaldehyde in breath, because an overlap occurs of the monohydrate of protonated acetaldehyde and the weakly bound adduct ion, H₃O⁺CO₂, formed by the slow association reaction of the precursor ion H₃O⁺ with CO₂ molecules. The understanding of the kinetics of formation and the loss rates of the relevant ions gained from experimentation using the new generation of more sensitive SIFT‐MS instruments now allows accurate quantification of CO₂ in breath using the level of the H₃O⁺CO₂ adduct ion. However, this is complicated by the rapid reaction of H₃O⁺CO₂ with water vapour molecules, H₂O, that are in abundance in exhaled breath. Thus, a study has been carried out of the formation of this adduct ion by the slow three‐body association reaction of H₃O⁺ with CO₂ and its rapid loss in the two‐body reaction with H₂O molecules. It is seen that the signal level of the H₃O⁺CO₂ adduct ion is sensitively dependent on the humidity (H₂O concentration) of the sample to be analysed and a functional form of this dependence has been obtained. This has resulted in an appropriate extension of the SIFT‐MS software and kinetics library that allows accurate measurement of CO₂ levels in air samples, ranging from very low percentage levels (0.03% typical of tropospheric air) to the 6% level that is about the upper limit in exhaled breath. Thus, the level of CO₂ can be traced through single time exhalation cycles along with that of water vapour, also close to the 6% level, and of trace gas metabolites that are present at only a few parts‐per‐billion. This has added a further dimension to the analysis of major and trace compounds in breath using SIFT‐MS. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantification of methylamine in the headspace of ethanol of agricultural origin by selected ion flow tube mass spectrometry

Selected ion flow tube mass spectrometry, SIFT-MS, has been used to investigate if absolute levels of trace compounds in the headspace of ethanol/water vapour mixture can be quantified. This case study was directed towards the analysis of methylamine in distilled ethanol of agricultural origin because of its relevance to quality control legislation in the distillery industry. This has required a detailed study of the ion chemistry occurring – initiated by H₃O⁺ precursor ions – when ethanol/water vapour mixtures are introduced into the H₃O⁺/helium carrier gas swarm and has resulted in the construction of a full scheme of the complex ionic reactions that occur. It has been found that under the SIFT-MS flow reactor conditions (He pressure 130 Pa and temperature 299 K) the terminating ions of the several parallel and sequential reactions that occur are the proton bound ethanol clusters ions, C₂H₅OH₂⁺(C₂H₅OH)_n, with n = 1,2,3, proton bound trimer (n = 2) being the dominant species. These ethanol cluster ions can be used as precursor (reagent) ions for the chemical ionisation of the methylamine present in the ethanol/water vapour, which produces two characteristic product ions CH₃NH₂H⁺(C₂H₅OH)_1,2 that are used for the methylamine analysis. The ratio of the product ion count rate to the precursor ion count rate is used in an analogous way to the routinely used for SIFT-MS analyses to quantify the methylamine concentration. The results of calibration experiments show that using SIFT-MS it is possible to quantify methylamine in liquid ethanol/water mixtures at levels of 0.1 mg/L or greater.     

Selected ion flow tube mass spectrometry for on-line trace gas analysis in biology and medicine

Selected ion flow tube mass spectrometry, (SIFT-MS), is a technique for simultaneous real-time quantification of several trace gases in air and exhaled breath. It relies on chemical ionization of the trace gas molecules in air/breath samples introduced into helium carrier gas, using H(3)O(+), NO(+) and O(2)(+) reagent (precursor ions). Reactions between the precursor ions and the trace gas molecules proceed for an accurately defined time, the precursor and product ions being detected and counted by a downstream mass spectrometer. Absolute concentrations of trace gases in single breath exhalation can be determined by SIFT-MS down to parts-per-billion (ppb) levels, obviating sample collection into bags or onto traps. Calibration using chemical standards is not required, as the concentrations are calculated using the known reaction rate constants and measured flow rates and pressures. SIFT-MS has been used for many pilot investigations in several areas of research, especially as a non-invasive breath analysis tool to investigate physiological processes in humans and animals, for clinical diagnosis and for therapeutic monitoring. Examples of the results obtained from several such studies are outlined to demonstrate the potential of SIFT-MS for trace gas analysis of air, exhaled breath and the headspace above liquids.     

Role of radiolytically formed protonating agents in gas phase competitive reactions

Radiolytically formed O₂H⁺, N₂H⁺, and CO₂H⁺ ions were allowed to react with gaseous p-cymene. Dealkylation and isomerization reactions were observed with O₂H⁺ and N₂H⁺ ions, while only the first process occurred when CO₂H⁺ ions were employed. The results show that dealkylation is favored with respect to isomerization as the protonation exothermicity decreases.     

An online ultra-high sensitivity Proton-transfer-reaction mass-spectrometer combined with switchable reagent ion capability (PTR + SRI − MS)

Proton-transfer-reaction mass-spectrometry (PTR-MS) developed in the 1990s is used today in a wide range of scientific and technical fields. PTR-MS allows for real-time, online determination of absolute concentrations of volatile (organic) compounds (VOCs) in air with high sensitivity (into the low pptv range) and a fast response time (in the 40–100 ms time regime). Most PTR-MS instruments employed so far use an ion source consisting of a hollow cathode (HC) discharge in water vapour which provides an intense source of proton donor H₃O⁺ ions. As the use of other ions, e.g. NO⁺ and O₂⁺, can be useful for the identification of VOCs and for the detection of VOCs with proton affinities (PA) below that of H₂O, selected ion flow tube mass spectrometry (SIFT-MS) with mass selected ions has been applied in these instances. SIFT-MS suffers, however, from at least two orders lower reagent ion counts rates and therefore SIFT-MS suffers from lower sensitivity than PTR-MS.Here we report the development of a PTR-MS instrument using a modified HC ion source and drift tube design, which allows for the easy and fast switching between H₃O⁺, NO⁺ and O₂⁺ ions produced in high purity and in large quantities in this source. This instrument is capable of measuring low concentrations (with detection limits approaching the ppqv regime) of VOCs using any of the three reagent ions investigated in this study. Therefore this instrument combines the advantages of the PTR-MS technology (the superior sensitivity) with those of SIFT-MS (detection of VOCs with PAs smaller than that of the water molecule and the capability to distinguish between isomeric compounds).We will first discuss the setup of this new PTR+SRI-MS mass spectrometer instrument, its performance for aromates, aldehydes and ketones (with a sensitivity of up to nearly 1000 cps/ppbv and a detection limit of about several 100 ppqv) and finally give some examples concerning the ability to distinguish structural isomeric compounds.     

The reactions of H3O+, NO+ and O with several flavourant esters studied using selected ion flow tube mass spectrometry

The reactions of H₃O⁺, NO⁺, and O with nineteen ester compounds occurring naturally in plants, and having important flavourant properties, were examined using selected ion flow tube mass spectrometry (SIFT‐MS). The H₃O⁺ reactions primarily generate [R₁COOR₂·H]⁺, and may also produce [R₂]⁺ fragment ions and/or fragmentation within the ester linkage. Collisional association/adduct ions, [R₁COOR₂·NO]⁺, are the main products formed in the NO⁺ reactions, although the carboxyl fragment ion is also detected frequently. The identification of the parent compound may be made more easily in the H₃O⁺ and NO⁺ reactions. The inclusion of O reactions in the analysis provides additional information, which may be applied when the identity of a parent compound cannot be determined solely from the H₃O⁺ and NO⁺ analysis. Consideration of the product ions generated with the three precursors suggests that SIFT‐MS can differentiate between many of the esters investigated, including isomers, although the product ions generated in the reactions with some esters are too similar to allow independent quantification. Our data therefore suggest that SIFT‐MS may be a useful tool to rapidly analyse and quantify flavourant esters in complex gas mixtures. Copyright © 2010 John Wiley & Sons, Ltd.     

Static SIMS analysis of carbonate on basic alkali‐bearing surfaces

Carbonate is a somewhat enigmatic anion in static secondary ion mass spectrometry (SIMS) because abundant ions containing intact CO₃^2? are not detected when analyzing alkaline‐earth carbonate minerals common to the geochemical environment. In contrast, carbonate can be observed as an adduct ion when it is bound with alkali cations. In this study, carbonate was detected as the adduct Na₂CO₃·Na⁺ in the spectra of sodium carbonate, bicarbonate, hydroxide, oxalate, formate and nitrite and to a lesser extent nitrate. The appearance of the adduct Na₂CO₃·Na⁺ on hydroxide, oxalate, formate and nitrite surfaces was interpreted in terms of these basic surfaces fixing CO₂ from the ambient atmosphere. The low abundance of Na₂CO₃·Na⁺ in the static SIMS spectrum of sodium nitrate, compared with a significantly higher abundance in salts having stronger conjugate bases, suggested that the basicity of the conjugate anions correlated with aggressive CO₂ fixation; however, the appearance of Na₂CO₃·Na⁺ could not be explained simply in terms of solution basicity constants. The oxide molecular ion Na₂O⁺ and adducts NaOH·Na⁺ and Na₂O·Na⁺ also constituted part of the carbonate spectral signature, and were observed in spectra from all the salts studied. In addition to the carbonate and oxide ions, a low‐abundance oxalate ion series was observed that had the general formula Na_2?xH_xC₂O₄·Na⁺, where 0 < x < 2. Oxalate adsorption from the laboratory atmosphere was demonstrated but the oxalate ion series also was likely to be formed from reductive coupling occurring during the static SIMS bombardment event. The remarkable spectral similarity observed when comparing the sodium salts indicated that their surfaces shared common chemical speciation and that the chemistry of the surfaces was very different from the bulk of the particle. Copyright © 2003 John Wiley & Sons, Ltd.     

Atmospheric pressure chemical ionization of explosives induced by soft X‐radiation in ion mobility spectrometry: mass spectrometric investigation of the ionization reactions of drift gasses,dopants and alkyl nitrates

A promising replacement for the radioactive sources commonly encountered in ion mobility spectrometers is a miniaturized, energy‐efficient photoionization source that produce the reactant ions via soft X‐radiation (2.8 keV). In order to successfully apply the photoionization source, it is imperative to know the spectrum of reactant ions and the subsequent ionization reactions leading to the detection of analytes. To that end, an ionization chamber based on the photoionization source that reproduces the ionization processes in the ion mobility spectrometer and facilitates efficient transfer of the product ions into a mass spectrometer was developed. Photoionization of pure gasses and gas mixtures containing air, N₂, CO₂ and N₂O and the dopant CH₂Cl₂ is discussed. The main product ions of photoionization are identified and compared with the spectrum of reactant ions formed by radioactive and corona discharge sources on the basis of literature data. The results suggest that photoionization by soft X‐radiation in the negative mode is more selective than the other sources. In air, adduct ions of O₂^– with H₂O and CO₂ were exclusively detected. Traces of CO₂ impact the formation of adduct ions of O₂^– and Cl^– (upon addition of dopant) and are capable of suppressing them almost completely at high CO₂ concentrations. Additionally, the ionization products of four alkyl nitrates (ethylene glycol dinitrate, nitroglycerin, erythritol tetranitrate and pentaerythritol tetranitrate) formed by atmospheric pressure chemical ionization induced by X‐ray photoionization in different gasses (air, N₂ and N₂O) and dopants (CH₂Cl₂, C₂H₅Br and CH₃I) are investigated. The experimental studies are complemented by density functional theory calculations of the most important adduct ions of the alkyl nitrates (M) used for their spectrometric identification. In addition to the adduct ions [M + NO₃]^– and [M + Cl]^–, adduct ions such as [M + N₂O₂]^–, [M + Br]^– and [M + I]^– were detected, and their gas‐phase structures and energetics are investigated by density functional theory calculations. Copyright © 2016 John Wiley & Sons, Ltd.     

Combined use of gas chromatography and selected ion flow tube mass spectrometry for absolute trace gas quantification

The value of the gas chromatography (GC) and selected ion flow tube mass spectrometry (SIFT-MS) combination for the analysis of trace gases is demonstrated by the quantification of acetone in air samples using the three precursor ions available to SIFT-MS, viz. H3O+, NO+ and O2+, and by the separation of the isomers 1-propanol and 2-propanol, and their analysis using H3O+ precursor ions. It is shown that the GC/SIFT-MS combination allows for accurate trace gas quantification obviating the regular, time-consuming calibrations that are usually required for the more commonly used detectors of GC systems, and the positive identification of isomers in mixtures that is often challenging using SIFT-MS alone. Thus, the GC/SIFT-MS combination paves the way to more confident analyses of complex mixtures such as exhaled breath.     

Oxirane as a chemical ionization gas: Reactivity towards different classes of compounds

Oxirane chemical ionization (CI) gives numerous ions, including C₂H₃O⁺ and C₂H₅O⁺. These ions react with organic molecules through various specific ion–molecule reactions such as hydride abstraction, protonation, additions or cycloadditions. Oxirane CI allows discrimination between unsaturated compounds with [M + 43]⁺ and [M + 57]⁺ adduct ions and heteroatom functions with [M + 45]⁺ adduct ion. All are diagnostic ions. Oxirane CI permits selectivity during the ionization process of a mixture and discrimination of isomers.     

Ion cyclotron resonance studies of some reactions of C+ ions

Product distributions and rate constants for the reaction of ground state C⁺ ions with O₂, NO, HCl, CO₂, H₂S, H₂O, HCN, NH₃, CH₄, H₂CO, CH₃OH, and CH₃NH₂ have been measured. Rate constants were obtained using ion cyclotron resonance trapped ion methods at JPL, and product distributions were obtained using a tandem (Dempster-ICR) mass spectrometer at the University of Utah. Rapid carbon isotope exchange has also been observed in C⁺-CO collisions.     

Quantification of methane in humid air and exhaled breath using selected ion flow tube mass spectrometry

In selected ion flow tube mass spectrometry, SIFT‐MS, analyses of humid air and breath, it is essential to consider and account for the influence of water vapour in the media, which can be profound for the analysis of some compounds, including H₂CO, H₂S and notably CO₂. To date, the analysis of methane has not been considered, since it is known to be unreactive with H₃O⁺ and NO⁺, the most important precursor ions for SIFT‐MS analyses, and it reacts only slowly with the other available precursor ion, O. However, we have now experimentally investigated methane analysis and report that it can be quantified in both air and exhaled breath by exploiting the slow O/CH₄ reaction that produces CH₃O ions. We show that the ion chemistry is significantly influenced by the presence of water vapour in the sample, which must be quantified if accurate analyses are to be performed. Thus, we have carried out a study of the loss rate of the CH₃O analytical ion as a function of sample humidity and deduced an appropriate kinetics library entry that provides an accurate analysis of methane in air and breath by SIFT‐MS. However, the associated limit of detection is rather high, at 0.2 parts‐per‐million, ppm. We then measured the methane levels, together with acetone levels, in the exhaled breath of 75 volunteers, all within a period of 3 h, which shows the remarkable sample throughput rate possible with SIFT‐MS. The mean methane level in ambient air is seen to be 2 ppm with little spread and that in exhaled breath is 6 ppm, ranging from near‐ambient levels to 30 ppm, with no significant variation with age and gender. Methane can now be included in the wide ranging analyses of exhaled breath that are currently being carried out using SIFT‐MS. Copyright © 2010 John Wiley & Sons, Ltd.     

The effect of vibrational energy content on the neutralization–reionization mass spectra of [H2]+˙ ions

Neutralization-reionization experiments were performed on beams of [H²]⁺˙ ions of different, known vibrational energy content using a variety of neutralization target gases (Xe, H₂, Ne) and reionization gases (He, O₂). The recovery of [H₂]⁺˙ ions was found to be only weakly dependent on the vibrational energy of the original [H₂]⁺˙ ions. The ion kinetic energy spectra of H⁺ fragments from the neutralization-reionization experiments were independent of the collision gas; the processes by which they were generated were identified.     

A selected ion flow tube study of the reactions of H3O+, NO+, and O2+ with saturated and unsaturated aldehydes and subsequent hydration of the product ions

We have carried out a selected ion flow tube (SIFT) study of the reactions of H₃O⁺, NO⁺, and O₂⁺ ions with several saturated and unsaturated aldehydes. This study is mainly directed toward providing the essential data for a projected SIFT mass spectrometry (SIFTMS) study of the volatile emissions from cooked meats, which always include aldehydes. Thus, it is necessary to know the rate coefficients and the product ions of the reactions of the above-mentioned ions, used as the precursor ions for SIFTMS analyses, with the aldehydes, if proper identification and quantification of the emitted species are to be achieved. The results of this study show that the reactions of H₃O⁺ with the aldehydes, M, result in the protonated molecules MH⁺ and for the saturated aldehydes also in (M - OH)⁺ ions resulting from the loss of a H₂O molecule from the nascent MH⁺ ion. The NO⁺ reactions invariably proceed via the process of hydride ion, H⁻, transfer producing (M - H)⁺ ions, but parallel minor association product ions NO⁺ · M are observed for some of the unsaturated aldehyde reactions. The O₂⁺ reactions proceed by way of charge transfer producing nascent M⁺ ions that partially dissociate producing fragment ions. Because water vapour is invariably present in real samples analysed by SIFTMS, the current experiments were also carried out following the introduction of humid laboratory air into the helium carrier gas of the SIFT. Thus, the reactions of the product ions that form hydrates were also studied as a prelude to future SIFTMS studies of the (humid) emissions from cooked meats.     

Laboratory investigations of negative ion molecule reactions of formic and acetic acids: implications for atmospheric measurements by ion-molecule reaction mass spectrometry

Laboratory measurements of gas-phase ion-molecule reactions of several negative ion species with formic and acetic acid have been carried out. A flow reactor operating at a temperature of 293 ± 3 K and total gas pressures of either 3 or 9 hPa was used. The negative reagent ion species investigated included OH⁻, O₂⁻, O₃⁻, CO₄⁻, CO₃⁻, CO₃⁻H₂O, HCO₃⁻H₂O, NO₃⁻, NO₃⁻H₂O, NO₂⁻, and NO₂⁻H₂O. The reactions were found to proceed either via proton transfer or clustering. Our measurements of ion-molecule reactions of negative ions with gaseous formic and acetic acids provide a firm base for quantitative detection of these acidic trace gases in the atmosphere by negative ion ion-molecule reaction mass spectrometry.     

Development of Dipolar Proton Transfer Reaction Mass Spectrometer for Real-time Monitoring of Volatile Organic Compounds in Ambient Air

Volatile organic compounds (VOCs) in ambient air can participate in photochemical reactions, which lead to the generation of secondary pollutants such as ozone and aerosol. So real-time and accurate monitoring of atmospheric VOCs plays an important role in the study of the causes of air pollution. On the basis of proton transfer reaction mass spectrometry (PTR-MS) research, a novel dipolar proton transfer reaction mass spectrometer (DP-PTR-MS) for real-time and on-line monitoring of atmospheric VOCs was developed. Compared with conventional PTR-MS with one kind of reagent ion H₃O⁺, DP-PTR-MS had three kinds of reagent ions H₃O⁺, OH^?, (CH₃)₂COH⁺, which could be switched according to the actual detection need. So DP-PTR-MS can improve the qualitative ability and expand the detection range effectively. The reagent ion H₃O⁺ can be used for detecting VOCs whose proton affinities are greater than that of H₂O. The reagent ion OH^? can be used to identify VOCs cooperating with the reagent ion H₃O⁺, and can also be used for detecting some inorganic substances such as CO₂. The reagent ion (CH₃)₂COH⁺ can be used for accurately detecting NH₃ under interference elimination circumstances. The limit of detection (LOD) and sensitivity of DP-PTR-MS were measured by using six kinds of standard gases. The results showed that the LOD for detecting toluene was 7 × 10^?12 (V/V) and the sensitivity for detecting ammonia reached 126 cps/10^?9 (V/V). The ambient air in Hefei city was on-line and real-time monitored for continuous 78 h with DP-PTR-MS. The results showed that the newly developed DP-PTR-MS could be used for long-term and real-time monitoring atmospheric VOCs at the concentration of 10^?12 (V/V) level. DP-PTR-MS is an important tool to the study of the causes of atmospheric pollution and the monitoring of trace VOCs emissions.     

On-line measurement of the absolute humidity of air, breath and liquid headspace samples by selected ion flow tube mass spectrometry

We describe how selected ion flow tube mass spectrometry (SIFT-MS) can be used to determine the absolute humidity of air, breath and liquid headspace samples. This involves the determination of the relative count rates of the H3O+ ions and those H3O+.(H2O)(1,2,3) hydrate ions that inevitably form in the helium carrier gas when humid samples are being analysed by SIFT-MS using H3O+ precursor ions. This requires an understanding of the kinetics of hydrated hydronium ion formation, the involvement of mass discrimination in the analytical quadrupole mass spectrometer and the decreased diffusive loss of the heavier hydrates along the flow tube. Thus, we show that the humidity of breath and liquid headspace samples, typically at the few percent level, can be directly obtained on-line to the SIFT-MS instrument along with the concentrations of trace gases, which are present at much lower levels. We emphasise the value of parallel humidity measurements in ensuring good real-time sampling of breath and liquid headspace and the value of such measurements to trace gas analysis using SIFT-MS.     

Gas‐phase fragmentation of metal adducts of alkali‐metal oxalate salts

Upon collisional activation, gaseous metal adducts of lithium, sodium and potassium oxalate salts undergo an expulsion of CO₂, followed by an ejection of CO to generate a product ion that retains all three metals atoms of the precursor. Spectra recorded even at very low collision energies (2 eV) showed peaks for a 44‐Da neutral fragment loss. Density functional theory calculations predicted that the ejection of CO₂ requires less energy than an expulsion of a Na⁺ and that the [Na₃CO₂]⁺ product ion formed in this way bears a planar geometry. Furthermore, spectra of [Na₃C₂O₄]⁺ and [³⁹K₃C₂O₄]⁺ recorded at higher collision energies showed additional peaks at m/z 90 and m/z 122 for the radical cations [Na₂CO₂]^+? and [K₂CO₂]^+?, respectively, which represented a loss of an M^? from the precursor ions. Moreover, [Na₃CO₂]⁺, [³⁹K₃CO₂]⁺ and [Li₃CO₂]⁺ ions also undergo a CO loss to form [M₃O]⁺. Furthermore, product‐ion spectra for [Na₃C₂O₄]⁺ and [³⁹K₃C₂O₄]⁺ recorded at low collision energies showed an unexpected peak at m/z 63 for [Na₂OH]⁺ and m/z 95 for [³⁹K₂OH]⁺, respectively. An additional peak observed at m/z 65 for [Na₂¹⁸OH] ⁺ in the spectrum recorded for [Na₃C₂O₄]⁺, after the addition of some H₂¹⁸O to the collision gas, confirmed that the [Na₂OH] ⁺ ion is formed by an ion–molecule reaction with residual water in the collision cell. Copyright © 2014 John Wiley & Sons, Ltd.     

Thermal-energy charge transfer of Ar+ with H2O: Internal and kinetic energy of the product H2O+

The reaction of Ar⁺ with H₂O has been investigated at near-thermal energy. The product ions H₂O⁺ and ArH⁺ account for 90 and 10% of the total reaction rate, respectively. Kinetic energy measurements and emission spectroscopy of the H₂O⁺ product ions are reported. It is concluded that at least 60% of H₂O⁺ ions are in the X? state with ≈2.4 eV vibrational energy while up to 40% are in the à state with a mean vibrational energy of 1.4 eV; the à state vibrational distribution has been determined. It is shown that both H₂O⁺ states are populated via an energetically “non-resonant” charge transfer process.     

Experiments and Calculations on Photoionization and Dissociative Photoionization of CH2CO

The dissociative photoionization of molecular‐beam cooled CH₂CO in a region of ?10–20 eV was investigated with photoionization mass spectrometry using a synchrotron radiation as the light source. Photoionization efficiency curves of CH₂CO⁺ and of observed fragment ions CH₂⁺, CHCO⁺, HCO⁺, C₂O⁺, CO⁺, and C₂H₂⁺ were measured to determine their appearance energies. Relative branching ratios as a function of photon energy were determined. Energies for formation of these observed fragment ions and their neutral counterparts upon ionization of CH₂CO are computed with the Gaussian‐3 method. Dissociative photoionization channels associated with six observed fragment ions are proposed based on comparison of determined appearance energies and predicted energies. The principal dissociative processes are direct breaking of C=C and C‐H bonds to form CH₂⁺ + CO and CHCO⁺ + H, respectively; at greater energies, dissociation involving H migration takes place.