Combining Thermal Desorption with Selected Ion Flow Tube Mass Spectrometry for Analyses of Breath Volatile Organic Compounds

An instrument integrating thermal desorption (TD) to selected ion flow tube mass spectrometry (SIFT-MS) is presented, and its application to analyze volatile organic compounds (VOCs) in human breath is demonstrated for the first time. The rationale behind this development is the need to analyze breath samples in large-scale multicenter clinical projects involving thousands of patients recruited in different hospitals. Following adapted guidelines for validating analytical techniques, we developed and validated a targeted analytical method for 21 compounds of diverse chemical class, chosen for their clinical and biological relevance. Validation has been carried out by two independent laboratories, using calibration standards and real breath samples from healthy volunteers. The merging of SIFT-MS and TD integrates the rapid analytical capabilities of SIFT-MS with the capacity to collect breath samples across multiple hospitals. Thanks to these features, the novel instrument has the potential to be easily employed in clinical practice.

Robust Automated SIFT-MS Quantitation of Volatile Compounds in Air Using a Multicomponent Gas Standard

Selected ion flow tube mass spectrometry, SIFT-MS, has been widely used in industry and research since its introduction in the mid-1990s. Previously described quantitation methods have been advanced to include a gas standard for a more robust and repeatable analytical performance. The details of this approach to calculate the concentrations from ion-molecule reaction kinetics based on reaction times and instrument calibration functions determined from known concentrations in the standard mix are discussed. Important practical issues such as the overlap of product ions are outlined, and best-practice approaches are presented to enable them to be addressed during method development. This review provides a fundamental basis for a plethora of studies in broad application areas that are possible with SIFT-MS instruments.

Accurate selected ion flow tube mass spectrometry quantification of ethylene oxide contamination in the presence of acetaldehyde

In September 2020, traces of ethylene oxide (a toxic substance used as a pesticide in developing countries but banned for use on food items within the European Union) were found in foodstuffs containing ingredients derived from imported sesame seed products. Vast numbers of foodstuffs were recalled across Europe due to this contamination, leading to expensive market losses and extensive trace exposure of ethylene oxide to consumers. Therefore, a rapid analysis method is needed to ensure food safety by high-throughput screening for ethylene oxide contamination. Selected ion flow tube mass spectrometry (SIFT-MS) is a suitable method for rapid quantification of trace amounts of vapours in the headspace of food samples. It turns out, however, that the presence of acetaldehyde complicates SIFT-MS analyses of its isomer ethylene oxide. It was proposed that a combination of the H3O+ and NO+ reagent ions can be used to analyse ethylene oxide in the presence of acetaldehyde. This method is, however, not robust because of the product ion overlaps and potential interferences from other matrix species. Thus, we studied the kinetics of the reactions of the H3O+, NO+, OH- and O-˙ ions with these two compounds and obtained their rate coefficients and product ion branching ratios. Interpretation of these experimental data revealed that the OH- anions are the most suitable SIFT-MS reagents because the product ions of their reactions with acetaldehyde (CH2CHO- at m/z 43) and ethylene oxide (C2H3O2- at m/z 59) do not overlap.

Gas phase H+, H3O+ and NH4+ affinities of oxygen-bearing volatile organic compounds; DFT calculations for soft chemical ionisation mass spectrometry

Quantum chemistry calculations were performed using the density functional theory, DFT, to understand the structures and energetics of organic ions relevant to gas phase ion chemistry in soft chemical ionisation mass spectrometry analytical methods. Geometries of a range of neutral volatile organic compound molecules and ions resulting from protonation, the addition of H3O+ and the addition of NH4+ were optimised using the B3LYP hybrid DFT method. Then, the total energies and the normal mode vibrational frequencies were determined, and the total enthalpies of the neutral molecules and ions were calculated for the standard temperature and pressure. The calculations were performed for several feasible structures of each of the ions. The proton affinities of several benchmark molecules agree with the accepted values within ±4 kJ mol-1, indicating that B3LYP/6-311++G(d,p) provides chemical accuracy for oxygen-containing volatile organic compounds. It was also found that the binding energies of H3O+ and NH4+ to molecules correlate with their proton affinities. The results contribute to the understanding of ligand switching ion-molecule reactions important for secondary electrospray ionisation, SESI, and selected ion flow tube, SIFT, mass spectrometries.

How to Use Ion-Molecule Reaction Data Previously Obtained in Helium at 300 K in the New Generation of Selected Ion Flow Tube Mass Spectrometry Instruments Operating in Nitrogen at 393 K

Selected ion flow tube mass spectrometry (SIFT-MS) instruments have significantly developed since this technique was introduced more than 20 years ago. Most studies of the ion-molecule reaction kinetics that are essential for accurate analyses of trace gases and vapors in air and breath were conducted in He carrier gas at 300 K, while the new SIFT-MS instruments (optimized to quantify concentrations down to parts per trillion by volume) operate with N₂ carrier gas at 393 K. Thus, we pose the question of how to reuse the data from the extensive body of previous literature using He at room temperature in the new instruments operating with N₂ carrier gas at elevated temperatures. Experimentally, we found the product ions to be qualitatively similar, although there were differences in the branching ratios, and some reaction rate coefficients were lower in the heated N₂ carrier gas. The differences in the reaction kinetics may be attributed to temperature, an electric field in the current flow tubes, and the change from He to N₂ carrier gas. These results highlight the importance of adopting an updated reaction kinetics library that accounts for the new instruments' specific conditions. In conclusion, almost all previous rate coefficients may be used after adjustment for higher temperatures, while some product ion branching ratios need to be updated.

A SIFT-MS study of positive and negative ion chemistry of the ortho-, meta- and para-isomers of cymene, cresol, and ethylphenol

Selected Ion Flow Tube Mass Spectrometry (SIFT-MS) is a soft ionisation technique based on gas phase ion-molecule reaction kinetics for the quantification of trace amounts of volatile organic compound vapours. One of its previous limitations is difficulty in resolving isomers, although this can now be overcome using different reactivities of several available reagent cations and anions (H₃O⁺, NO⁺, O₂⁺˙, O^-˙, OH^-, O₂^-˙, NO₂^-, NO₃^-). Thus, the ion-molecule reactions of these eight ions with all isomers of the aromatic compounds cymene, cresol and ethylphenol were studied to explore the possibility of their immediate identification and quantification without chromatographic separation. Rate coefficients and product ion branching ratios determined experimentally for the 72 reactions are reported. DFT calculations of their energetics confirmed the feasibility of the suggested reaction pathways. All positive ion reactions proceeded fast but largely did not discriminate between the isomers. The reactivity of the anions was much more varied. In all cases, OH^- reacts by proton transfer forming (M-H); NO₂^- and NO₃^- were unreactive. The differences observed for product ion branching ratios can be used to identify isomers approximately.

Different reactivities of H3 O+ (H2 O)n with unsaturated and saturated aldehydes: ligand-switching reactions govern the quantitative analytical sensitivity of SESI-MS

RATIONALE

The detection sensitivity of secondary electrospray ionisation mass spectrometry (SESI-MS) is 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 selected ion flow tube mass spectrometry (SIFT-MS) analyses were carried out of air containing variable 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 the ion transfer capillary temperature, 250 and 300°C, in a commercial SESI-MS instrument was 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 six aldehydes.

RESULTS

The relative slopes of the plots of SESI-MS ion signal against SIFT-MS concentration were interpreted as the relative SESI-MS sensitivities for these six compounds. The sensitivities for the unsaturated aldehydes were 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 three or four 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 density functional theory (DFT) calculations of Gibb's free energy changes. The humidity of SESI gas thus favours the reverse reactions of the saturated aldehyde analyte ions, effectively suppressing their signals in contrast to their unsaturated counterparts.

Recent developments and applications of selected ion flow tube mass spectrometry (SIFT-MS)

Selected ion flow tube mass spectrometry (SIFT-MS) is now recognized as the most versatile analytical technique for the identification and quantification of trace gases down to the parts-per-trillion by volume, pptv, range. This statement is supported by the wide reach of its applications, from real-time analysis, obviating sample collection of very humid exhaled breath, to its adoption in industrial scenarios for air quality monitoring. This review touches on the recent extensions to the underpinning ion chemistry kinetics library and the alternative challenge of using nitrogen carrier gas instead of helium. The addition of reagent anions in the Voice200 series of SIFT-MS instruments has enhanced the analytical capability, thus allowing analyses of volatile trace compounds in humid air that cannot be analyzed using reagent cations alone, as clarified by outlining the anion chemistry involved. Case studies are reviewed of breath analysis and bacterial culture volatile organic compound (VOC), emissions, environmental applications such as air, water, and soil analysis, workplace safety such as transport container fumigants, airborne contamination in semiconductor fabrication, food flavor and spoilage, drugs contamination and VOC emissions from packaging to demonstrate the stated qualities and uniqueness of the new generation SIFT-MS instrumentation. Finally, some advancements that can be made to improve the analytical capability and reach of SIFT-MS are mentioned.

Kinetics of reactions of NH4 + with some biogenic organic molecules and monoterpenes in helium and nitrogen carrier gases: A potential reagent ion for selected ion flow tube mass spectrometry

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, N₂ , carrier gases.

METHODS

NH₄ ⁺ and H₃ O⁺ were generated in a microwave gas discharge through an NH₃ and H₂ O vapour mixture and, after m/z selection, injected into He and N₂ 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 N₂ than in He but similar in both bath gases for the monoterpenes. Adducts NH₄ ⁺ M were the dominant product ions in He and N₂ for the biogenic molecules, whereas both MH⁺ and NH₄ ⁺ M product ions were observed in the monoterpene reactions; the monoterpene ratio correlating (R²  = 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.

Ternary association reactions of H3 O+ , NO+ and O2 +• with N2 , O2 , CO2 and H2 O; implications for selected ion flow tube mass spectrometry analyses of air and breath

RATIONALE

The reactions of the reagent ions used for trace gas analysis in selected ion flow tube mass spectrometry (SIFT-MS), R⁺ , viz. H₃ O⁺ , NO⁺ and O₂ ⁺ , with the major gases in air and breath samples, M, viz. N₂ , O₂ , CO₂ and H₂ O, 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 (H₃ O⁺ N₂ ) to 76 (O₂ ^+• 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 × 10^-31  cm⁶  s^-1 to 5 × 10^-29  cm⁶  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 H₂ O) react with H₂ O molecules by ligand switching forming the readily recognised monohydrates of the initial reagent cations R⁺ H₂ O. 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⁺ H₂ O 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.

Sensitivity of secondary electrospray ionization mass spectrometry to a range of volatile organic compounds: Ligand switching ion chemistry and the influence of Zspray™ guiding electric fields

RATIONALE

Secondary electrospray ionization (SESI) is currently only semi-quantitative. In the Zspray™ 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, H₃ O⁺ (H₂ O)_n , with volatile organic compound (VOC) molecules, need to be understood.

METHODS

A modified Zspray™ 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⁺ H₂ O, 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⁺ H₂ O, but other yet unrecognized factors must play a significant role.

Ligand Switching Ion Chemistry: An SIFDT Case Study of the Primary and Secondary Reactions of Protonated Acetic Acid Hydrates with Acetone

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.

Cross Platform Analysis of Volatile Organic Compounds Using Selected Ion Flow Tube and Proton-Transfer-Reaction Mass Spectrometry

Volatile breath metabolites serve as potential disease biomarkers. Online mass spectrometry (MS) presents real-time quantification of breath volatile organic compounds (VOCs). The study aims to assess the relationship between two online analytical mass spectrometry techniques in the quantification of target breath metabolites: selected ion flow tube mass spectrometry (SIFT-MS) and proton-transfer-reaction time-of-flight mass spectrometry (PTR-ToF-MS). The two following techniques were employed: (i) direct injection with bag sampling using SIFT-MS and PTR-ToF-MS and (ii) direct injection and thermal desorption (TD) tube comparison using PTR-ToF-MS. The concentration of abundant breath metabolites, acetone and isoprene, demonstrated a strong positive linear correlation between both mass spectrometry techniques (r = 0.97, r = 0.89, respectively; p < 0.001) and between direct injection and TD tube (r = 0.97, r = 0.92, respectively; p < 0.001) breath sampling techniques. This was reflected for the majority of short chain fatty acids and alcohols tested (r > 0.80, p < 0.001). Analyte concentrations were notably higher with the direct injection of a sampling bag compared to the TD method. All metabolites produced a high degree of agreement in the detection range of VOCs between SIFT-MS and PTR-ToF-MS, with the majority of compounds falling within 95% of the limits of agreement with Bland-Altman analysis. The cross platform analysis of exhaled breath demonstrates strong positive correlation coefficients, linear regression, and agreement in target metabolite detection rates between both breath sampling techniques. The study demonstrates the transferability of using data outputs between SIFT-MS and PTR-ToF-MS. It supports the implementation of a TD platform in multi-site studies for breath biomarker research in order to facilitate sample transport between clinics and the laboratory.

Reagent and analyte ion hydrates in secondary electrospray ionization mass spectrometry (SESI-MS), their equilibrium distributions and dehydration in an ion transfer capillary: Modelling and experiments

RATIONALE

Secondary electrospray ionization (SESI) in a water spray environment at atmospheric pressure involves the reactions of hydrated hydronium reagent ions, H₃ O⁺ (H₂ O)_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 H₃ O⁺ (H₂ O)_n reagent cluster ion distributions and ligand switching reactions with polar NH₃ molecules leading to equilibrated hydrated ammonium ions NH₄ ⁺ (H₂ O)_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, C₅ H₈ , was used to further probe the nature of the reagent ions.

RESULTS

Equilibrium distributions of H₃ O⁺ (H₂ O)_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 H₂ O ligands from the H₃ O⁺ (H₂ O)_n ions at the collisional rate forming NH₄ ⁺ (H₂ O)_m ions, which travel through the heated ion transfer capillary losing H₂ O molecules. The data for variable NH₃ concentrations match the model predictions and the C₅ H₈ test substantiates the notion of dehydration in the heated capillary.

CONCLUSIONS

Large cluster ions formed in the SESI region are dehydrated to H₃ O⁺ (H₂ O)_1,2,3 and NH₄ ⁺ (H₂ O)_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⁺ (H₂ O)_m analyte ions.

Understanding Gas Phase Ion Chemistry Is the Key to Reliable Selected Ion Flow Tube-Mass Spectrometry Analyses

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.

The development of a fully integrated 3D printed electrochemical platform and its application to investigate the chemical reaction between carbon dioxide and hydrazine

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An integrated electrochemical platform was manufactured by bi-material 3D printing.

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It was applied to investigate the reaction between hydrazine and carbon dioxide.

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Experimental results were supported by finite-element method numerical simulations.

The combination of computer assisted design and 3D printing has recently enabled fast and inexpensive manufacture of customized ‘reactionware’ for broad range of electrochemical applications. In this work bi-material fused deposition modeling 3D printing is utilized to construct an integrated platform based on a polyamide electrochemical cell and electrodes manufactured from a polylactic acid-carbon nanotube conductive composite. The cell contains separated compartments for the reference and counter electrode and enables reactants to be introduced and inspected under oxygen-free conditions. The developed platform was employed in a study investigating the electrochemical oxidation of aqueous hydrazine coupled to its bulk reaction with carbon dioxide. The analysis of cyclic voltammograms obtained in reaction mixtures with systematically varied composition confirmed that the reaction between hydrazine and carbon dioxide follows 1/1 stoichiometry and the corresponding equilibrium constant amounts to (2.8 ± 0.6) × 10³. Experimental characteristics were verified by results of numerical simulations based on the finite-element-method.

Selected ion flow tube mass spectrometry analyses of isobaric compounds methanol and hydrazine in humid air

RATIONALE

The 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.

METHODS

A SIFT study (using a SIFT-MS instrument) was 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 determined for both compounds as a function of their liquid-phase concentrations at 10, 20 and 35°C.

RESULTS

Both compounds react with H₃ O⁺ ions via rapid proton transfer to produce CH₃ OH₂ ⁺ and H₅ N₂ ⁺ ions with the common m/z value of 33. It is revealed that NO⁺ rapidly transfers charge to N₂ H₄ (rate coefficient k = 2.3 × 10^-9 cm³ s^-1 ) but only slowly associates with CH₃ OH (k_2eff  = 7.1 × 10^-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.

CONCLUSIONS

The 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-occur in humid air. The evolution of hydrazine from aqueous solutions can be quantitatively monitored together with its decomposition at elevated temperatures.

Impact of oral cleansing strategies on exhaled volatile organic compound levels

RATIONALE

The analysis of volatile organic compounds (VOCs) within exhaled breath potentially offers a non-invasive method for the detection and surveillance of human disease. Oral contamination of exhaled breath may influence the detection of systemic VOCs relevant to human disease. This study aims to assess the impact of oral cleansing strategies on exhaled VOC levels in order to standardise practice for breath sampling.

METHODS

Ten healthy volunteers consumed a nutrient challenge followed by four oral cleansing methods: (a) water, (b) saltwater, (c) toothbrushing, and (d) alcohol-free mouthwash. Direct breath sampling was performed using selected ion flow tube mass spectrometry after each intervention.

RESULTS

Proposed reactions suggest that volatile fatty acid and alcohol levels (butanoic, pentanoic acid, ethanol) declined with oral cleansing interventions, predominantly after an initial oral rinse with water. Concentrations of aldehydes and phenols (acetaldehyde, menthone, p-cresol) declined with oral water rinse; however, they increased after toothbrushing and mouthwash use, secondary to flavoured ingredients within these products. No significant reductions were observed with sulphur compounds.

CONCLUSIONS

Findings suggest that oral rinsing with water prior to breath sampling may reduce oral contamination of VOC levels, and further interventions for oral decontamination with flavoured products may compromise results. This intervention may serve as a simple and inexpensive method of standardisation within breath research.

Volatile compounds released by Nalophan; implications for selected ion flow tube mass spectrometry and other chemical ionisation mass spectrometry analytical methods

Nalophan 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.

METHODS

Three 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.

RESULTS

The 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.

CONCLUSIONS

Using H₃ O⁺ 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.

Ion chemistry of phthalates in selected ion flow tube mass spectrometry: isomeric effects and secondary reactions with water vapour

Reactions of H₃O⁺, O₂⁺ and NO⁺ with phthalates and secondary reactions of product ions with water vapor were studied by SIFT.

Chemical ionization of glyoxal and formaldehyde with H3O+ ions using SIFT-MS under variable system humidity

Protonated glyoxal reacts with water molecules to form protonated formaldehyde, interfering with SIFT-MS analyses of glyoxal in humid samples.

Styrene radical cations for chemical ionization mass spectrometry analyses of monoterpene hydrocarbons

RATIONALE

Monoterpene hydrocarbons play an important role in the formation of secondary aerosol particles and in atmospheric chemistry. Thus, there is a demand to measure their individual concentrations in situ in real time. Currently, only the total concentration of monoterpenes C₁₀ H₁₆ can be determined by chemical ionization mass spectrometry techniques using reagent ions H₃ O⁺ , NO⁺ and (C₆ H₆ )_n ^+• without gas chromatographic separation.

METHODS

The styrene cation C₈ H₈ ^+• was investigated as a reagent for chemical ionization of monoterpenes. The modified selected ion flow drift tube, SIFDT, technique was used to characterize the differences in product ion distributions between α-phellandrene, α-pinene, γ-terpinene, β-pinene, ocimene, sabinene, 3-carene, (R)-limonene, camphene and myrcene.

RESULTS

The monoterpene molecular cation C₁₀ H₁₆ ^+• is the main product (about 90%) for all isomers except (R)-limonene and camphene with an efficient channel of C₈ H₈ ^+• C₁₀ H₁₆ adduct formation and γ-terpinene with unexpectedly significant product ions at m/z 134 and 135 corresponding to losses of H₂ and H.

CONCLUSIONS

Utilization of the styrene cation for the ionization of monoterpenes is beneficial due to the very low fragmentation of the product ions. Specific association product ions for camphene and (R)-limonene and fragment product ions for γ-terpinene allow them to be distinguished from other isomers that produce mostly the molecular cation.

Evaluation of lipid peroxidation by the analysis of volatile aldehydes in the headspace of synthetic membranes using Selected Ion Flow Tube Mass Spectrometry, SIFT-MS

RATIONALE

Oxidative stress of cell membranes leads to a number of pathological processes associated with some diseases and is accompanied by the release of volatile aldehydes, which, potentially, can be used as biomarkers. Thus, the aim was to investigate peroxidation of defined synthetic membranes by direct quantitative analysis of volatile aldehydes.

METHODS

The concentration spectra of volatile compounds present in the headspace of synthetic membranes under peroxidation stress and following mechanical stress due to sonication were obtained using solid phase microextraction (SPME) in combination with Gas Chromatography Mass Spectrometry (SPME/GC/MS) and Selected Ion Flow Tube Mass Spectrometry (SIFT-MS). The focus was on the direct, real time quantification of volatile aldehydes. In addition, the total aldehydes in the aqueous membrane suspensions were quantified using the TBARS method.

RESULTS

Propanal, butanal, pentanal, hexanal, heptanal and malondialdehyde were detected and quantified in the humid headspace of the media containing the synthetic membranes following peroxidation. The composition and concentration of these saturated aldehydes strongly depend on the unsaturated fatty acids representation in the liposomes. Some protective effect of cholesterol was observed especially for membranes peroxidised by Fenton reagents and after application of a mechanical stress.

CONCLUSIONS

This study demonstrates that peroxidation of model synthetic membranes in vitro can be tracked in real time using direct quantification by SIFT-MS of several specific aldehydes in the headspace of the membrane suspensions. Cholesterol plays an important role in retaining membrane structure and can indirectly protect membranes from lipid peroxidation.

Quantification of volatile compounds released by roasted coffee by selected ion flow tube mass spectrometry

RATIONALE

The major objective of this exploratory study was to implement selected ion flow tube mass spectrometry, SIFT-MS, as a method for the on-line quantification of the volatile organic compounds, VOCs, in the headspace of the ground roasted coffee.

METHODS

The optimal precursor ions and characteristic analyte ions were selected for real-time SIFT-MS quantification of those VOCs that are the most abundant in the headspace or known to contribute to aroma. NO⁺ reagent ion reactions were exploited for most of the VOC analyses. VOC identifications were confirmed using gas chromatography/mass spectrometry, GC/MS, coupled with solid-phase microextraction, SPME.

RESULTS

Thirty-one VOCs were quantified, including several alcohols, aldehydes, ketones, carboxylic acids, esters and some heterocyclic compounds. Variations in the concentrations of each VOC in the seven regional coffees were typically less than a factor of 2, yet concentrations patterns characteristic of the different regional coffees were revealed by heat map and principal component analyses. The coefficient of variation in the concentrations across the seven coffees was typically below 24% except for furfural, furan, methylfuran and guaiacol.

CONCLUSIONS

The SIFT-MS analytical method can be used to quantify in real time the most important odoriferous VOCs in ground coffee headspace to sufficient precision to reveal some differences in concentration patterns for coffee produced in different countries.

Spectroscopic investigations of high-energy-density plasma transformations in a simulated early reducing atmosphere containing methane, nitrogen and water

Large-scale plasma was created in gas mixtures containing methane using high-power laser-induced dielectric breakdown (LIDB). The composition of the mixtures corresponded to a cometary and/or meteoritic impact into the early atmosphere of either Titan or Earth. A multiple-centimeter-sized fireball was created by focusing a single 100 J, 450 ps near-infrared laser pulse into the center of a 15 L gas cell. The excited reaction intermediates formed during the various stages of the LIDB plasma chemical evolution were investigated using optical emission spectroscopy (OES) with temporal resolution. The chemical consequences of laser-produced plasma generation in a CH₄-N₂-H₂O mixture were investigated using high resolution Fourier-transform infrared absorption spectroscopy (FTIR) and gas selected ion flow tube spectrometry (SIFT). Several simple inorganic and organic compounds were identified in the reaction mixture exposed to ten laser sparks. Deuterated water (D₂O) in a gas mixture was used to separate several of the produced isotopomers of acetylene, which were then quantified using the FTIR technique.

What is the real utility of breath ammonia concentration measurements in medicine and physiology?

Much effort continues to be devoted to the development of devices to analyse breath ammonia with the anticipation that breath ammonia analyses will be useful in clinical practice. In this perspective we refer to the analytical techniques that have been used to measure breath ammonia, focusing on selected ion flow tube mass spectrometry, SIFT-MS, of which we have special knowledge and understanding. From the collected data obtained using the different techniques, we exam the origins of mouth- and nose-exhaled ammonia and conclude that mouth-exhaled ammonia is always elevated above a concentration that would be equilibrated with blood ammonia and is largely produced by the action of enzymes on salivary urea. Support to this conclusion is given by the reasonable correlation between blood urea concentration and mouth-exhaled ammonia concentration. Further, it is discussed that nose-exhaled ammonia largely originates at the alveolar interface and so its concentration more closely relates to the expected alveolar blood ammonia concentration. Ingestion of proteins results in increased blood/saliva urea and ultimately mouth-exhaled ammonia as does the generation of urease by H. pylori infection. It is also concluded that when mouth-exhaled ammonia is elevated then it may be due to either abnormally high blood urea, a high pH of the saliva/mouth/airways mucosa, poor oral hygiene or a combinations of these.

Pentane and other volatile organic compounds, including carboxylic acids, in the exhaled breath of patients with Crohn's disease and ulcerative colitis

A study has been carried out on the volatile organic compounds (VOCs) in the exhaled breath of patients suffering from inflammatory bowel disease (IBD), comprising 136 with Crohn's disease (CD) and 51 with ulcerative colitis (UC), together with a cohort of 14 healthy persons as controls. Breath samples were collected by requesting the patients to inflate Nalophan bags, which were then quantitatively analysed using selected ion flow tube mass spectrometry (SIFT-MS). Initially, the focus was on n-pentane that had previously been quantified in single exhalations on-line to SIFT-MS for smaller cohorts of IBD patients. It was seen that the median concentration of pentane was elevated in the bag breath samples of the IBD patients compared to those of the healthy controls, in accordance with the previous study. However, the absolute median pentane concentrations in the bag samples were about a factor of two lower than those in the directly analysed single exhalations-a good illustration of the dilution of VOCs in the samples of breath collected into bags. Accounting for this dilution effect, the concentrations of the common breath VOCs, ethanol, propanol, acetone and isoprene, were largely as expected for healthy controls. The concentrations of the much less frequently measured hydrogen sulphide, acetic acid, propanoic acid and butanoic acid were seen to be more widely spread in the exhaled breath of the IBD patients compared to those for the healthy controls. The relative concentrations of pentane and these other VOCs weakly correlate with simple clinical activity indices. It is speculated that, potentially, hydrogen sulphide and these carboxylic acids could be exhaled breath biomarkers of intestinal bacterial overgrowth, which could assist therapeutic intervention and thus alleviate the symptoms of IBD.

On the importance of accurate quantification of individual volatile metabolites in exhaled breath

It is argued that shortcomings of certain approaches to breath analysis research based on superficial interpretation of non-quantitative data are inadvertently inhibiting the progression of non-invasive breath analysis into clinical practice. The objective of this perspective is to suggest more clinically profitable approaches to breath research. Thus, following a discourse on the challenges and expectations in breath research, a brief indication is given of the analytical techniques currently used for the analysis of very humid exhaled breath. The seminal work that has been carried out using GC-MS revealed that exhaled breath comprises large numbers of trace volatile organic compounds, VOCs. Unfortunately, analysis of these valuable GC-MS data is mostly performed using chemometrics to distinguish the VOC content of breath samples collected from patients and healthy controls, and reliable quantification of the VOCs is rarely deemed necessary. This limited approach ignores the requirements of clinically acceptable biomarkers and misses the opportunity to identify relationships between the concentrations of individual VOCs and certain related physiological or metabolic parameters. Therefore, a plea is made for more effort to be directed towards the positive identification and accurate quantification of individual VOCs in exhaled breath, which are more physiologically meaningful as best exemplified by the quantification of breath nitric oxide, NO. Support for the value of individual VOC quantification is illustrated by the SIFT-MS studies of breath hydrogen cyanide, HCN, a biomarker of Pseudomonas aeruginosa infection, breath acetic acid as an indicator of airways acidification in cystic fibrosis patients, and n-pentane as a breath biomarker of inflammation in idiopathic bowel disease patients. These single VOCs could be used as non-invasive monitors of the efficacy of therapeutic intervention. The increase of breath methanol following the ingestion of a known amount of the sweetener aspartame impressively shows that accurate breath analysis is a reliable indicator of blood concentrations. However, using individual VOCs for specific disease diagnosis does have its problems and it is, perhaps, more appropriate to see their concentrations as proxy markers of general underlying physiological change. We dedicate this perspective to Lars Gustafsson for his seminal work on breath research and especially for his pioneering work on nitric oxide measurements in exhaled breath in asthma, which best shows the utility and value of the quantification of individual breath biomarkers on which this perspective focuses.

Evaluation of peroxidative stress of cancer cells in vitro by real-time quantification of volatile aldehydes in culture headspace

RATIONALE

Peroxidation of lipids in cellular membranes results in the release of volatile organic compounds (VOCs), including saturated aldehydes. The real-time quantification of trace VOCs produced by cancer cells during peroxidative stress presents a new challenge to non-invasive clinical diagnostics, which as described here, we have met with some success.

METHODS

A combination of selected ion flow tube mass spectrometry (SIFT-MS), a technique that allows rapid, reliable quantification of VOCs in humid air and liquid headspace, and electrochemistry to generate reactive oxygen species (ROS) in vitro has been used. Thus, VOCs present in the headspace of CALU-1 cancer cell line cultures exposed to ROS have been monitored and quantified in real time using SIFT-MS.

RESULTS

The CALU-1 lung cancer cells were cultured in 3D collagen to mimic in vivo tissue. Real-time SIFT-MS analyses focused on the volatile aldehydes: propanal, butanal, pentanal, hexanal, heptanal and malondialdehyde (propanedial), that are expected to be products of cellular membrane peroxidation. All six aldehydes were identified in the culture headspace, each reaching peak concentrations during the time of exposure to ROS and eventually reducing as the reactants were depleted in the culture. Pentanal and hexanal were the most abundant, reaching concentrations of a few hundred parts-per-billion by volume, ppbv, in the culture headspace.

CONCLUSIONS

The results of these experiments demonstrate that peroxidation of cancer cells in vitro can be monitored and evaluated by direct real-time analysis of the volatile aldehydes produced. The combination of adopted methodology potentially has value for the study of other types of VOCs that may be produced by cellular damage.

Selected ion flow tube study of the reactions of H3 O+ and NO+ with a series of primary alcohols in the presence of water vapour in support of selected ion flow tube mass spectrometry

RATIONALE

Alcohols are often present in foods and other biological media, including exhaled breath, urine and cell culture headspace. For their analysis by selected ion flow tube mass spectrometry (SIFT-MS), the ion chemistry initiated by the reactions of the reagent ions H₃ O⁺ and NO⁺ with alcohol molecules in the presence of water molecules needs to be understood and quantitatively described.

METHODS

The reactions of H₃ O⁺ and NO⁺ ions have been studied with the primary alcohols, methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol and 1-hexanol, under the conditions used for SIFT-MS analyses (1 Torr He; 0.1 Torr air sample; 300 K) and over a range of sample gas humidity from 1% to 5.5%.

RESULTS

The H₃ O⁺ reactions led to the formation of protonated alcohol molecules MH⁺ and their hydrates MH⁺ (H₂ O)_1,2,3 and (MH⁺ -H₂ O) fragment ions. The NO⁺ reactions were observed to proceed mainly via hydride ion transfer, resulting in the formation of [M-H]⁺ product ions. Formation of the NO⁺ M adduct ions was also observed due to ligand switching between the NO⁺ (H₂ O)_1,2 hydrated reagent ions and M, and via direct NO⁺ /M association in the case of ethanol. The variation in the percentages of the hydrated product ions with the air sample humidity is reported.

CONCLUSIONS

This detailed study has provided the kinetics data, including the secondary hydrated ion product distributions, for the reactions of a number of volatile primary alcohols with the SIFT-MS reagent ions H₃ O⁺ and NO⁺ , which allows their analyses by SIFT-MS in humid air and also helps in the interpretation of proton transfer reaction (PTR)-MS data. Copyright © 2016 John Wiley & Sons, Ltd.

Ion chemistry at elevated ion–molecule interaction energies in a selected ion flow-drift tube: reactions of H3O+, NO+ and O2+ with saturated aliphatic ketones

Rate coefficients and product ion branching ratios determined for proton transfer, association and charge transfer reactions provide insight into reaction mechanisms.

A Pilot Study of Ion - Molecule Reactions at Temperatures Relevant to the Atmosphere of Titan

Reliable theoretical models of the chemical kinetics of the ionosphere of Saturn's moon, Titan, is highly dependent on the precision of the rates of the reactions of ambient ions with hydrocarbon molecules at relevant temperatures. A Variable Temperature Selected Ions Flow Tube technique, which has been developed primarily to study these reactions at temperatures within the range of 200-330 K, is briefly described. The flow tube temperature regulation system and the thermalisation of ions are also discussed. Preliminary studies of two reactions have been carried out to check the reliability and efficacy of kinetics measurements: (i) Rate constants of the reaction of CH3+ ions with molecular oxygen were measured at different temperatures, which indicate values in agreement with previous ion cyclotron resonance measurements ostensibly made at 300 K. (ii) Formation of CH3+ ions in the reaction of N2+ ions with CH4 molecules were studied at temperatures within the range 240-310 K which showed a small but statistically significant decrease of the ratio of product CH3+ ions to reactant N2+ ions with reaction temperature.

In-tube collision-induced dissociation for selected ion flow-drift tube mass spectrometry, SIFDT-MS: a case study of NO(+) reactions with isomeric monoterpenes

RATIONALE

Soft chemical ionisation techniques including selected ion flow tube mass spectrometry, SIFT-MS, and proton transfer reaction mass spectrometry, PTR-MS, cannot currently quantify individual isomers present simultaneously in samples, a notable example being atmospheric monoterpenes. A possible solution lies in integrating in-tube collision-induced dissociation, CID, into a selected ion flow-drift tube mass spectrometry, SIFDT-MS, instrument.

METHODS

In-tube CID was implemented by applying electrostatic potential difference between the resistive glass flow-drift tube downstream end and the nose cone of a quadrupole mass spectrometer. The resulting inhomogeneous electric field accelerates the product ions along the last 1 mm before the nose cone and causes their dissociation in collisions with molecules of the buffer gas (4% air, 96% helium, 2 mbar). Mass spectra of the product ions of NO(+) reactions with 3-carene, β-pinene, (S)-limonene and their mixture were obtained for variable potential difference.

RESULTS

Potential difference up to 47.7 V resulted in dramatic changes in the mass spectra due to fragmentation of the monoterpene radical molecular cations. The main observed fragments correspond to logical losses from different isomeric structures. Fragmentation increases with the potential difference and can be interpreted as single collision dissociation on air molecules at centre-of-mass energies of several eV. Combination of fragmentation patterns at different CID enables distinction of isomers in the mixture on the basis of pseudoinversion.

CONCLUSIONS

In-tube CID represents a simple and low-cost extension to SIFDT-MS that allows real-time identification of isomeric products of ion-molecule reactions on the basis of their structural differences and corresponding changes in fragmentation patterns with CID energy without significantly changing the net reaction time important for absolute quantification. Copyright © 2016 John Wiley & Sons, Ltd.

Differentiation of pulmonary bacterial pathogens in cystic fibrosis by volatile metabolites emitted by their in vitro cultures: Pseudomonas aeruginosa, Staphylococcus aureus, Stenotrophomonas maltophilia and the Burkholderia cepacia complex

As a contribution to the continuing search for breath biomarkers of lung and airways infection in patients with cystic fibrosis, CF, we have analysed the volatile metabolites released in vitro by Pseudomonas aeruginosa and other bacteria involved in respiratory infections in these patients, i.e. those belonging to the Burkholderia cepacia complex, Staphylococcus aureus or Stenotrophomonas maltophilia. These opportunistic pathogens are generally harmless to healthy people but they may cause serious infections in patients with severe underlying disease or impaired immunity such as CF patients. Volatile organic compounds emitted from the cultures of strains belonging to the above-mentioned four taxa were analysed by selected ion flow tube mass spectrometry. In order to minimize the effect of differences in media composition all strains were cultured in three different liquid media. Multivariate statistical analysis reveals that the four taxa can be well discriminated by the differences in the headspace VOC concentration profiles. The compounds that should be targeted in breath as potential biomarkers of airway infection were identified for each of these taxa of CF pathogens.

Increase of methanol in exhaled breath quantified by SIFT-MS following aspartame ingestion

Aspartame, methyl-L-α-aspartyl-L-phenylalaninate, is used worldwide as a sweetener in foods and drinks and is considered to be safe at an acceptable daily intake (ADI) of 40 mg per kg of body weight. This compound is completely hydrolyzed in the gastrointestinal tract to aspartic acid, phenylalanine and methanol, each being toxic at high levels. The objective of the present study was to quantify the volatile methanol component in the exhaled breath of ten healthy volunteers following the ingestion of a single ADI dose of aspartame. Direct on-line measurements of methanol concentration were made in the mouth and nose breath exhalations using selected ion flow tube mass spectrometry, SIFT-MS, several times before aspartame ingestion in order to establish individual pre-dose (baseline) levels and then during two hours post-ingestion to track their initial increase and subsequent decrease. The results show that breath methanol concentrations increased in all volunteers by 1082   ±   205 parts-per-billion by volume (ppbv) from their pre-ingestion values, which ranged from 193 to 436 ppbv to peak values ranging from 981-1622 ppbv, from which they slowly decreased. These observations agree quantitatively with a predicted increase of 1030 ppbv estimated using a one-compartment model of uniform dilution of the methanol generated from a known amount of aspartame throughout the total body water (including blood). In summary, an ADI dose of aspartame leads to a 3-6 fold increase of blood methanol concentration above the individual baseline values.

Exhaled breath hydrogen cyanide as a marker of early Pseudomonas aeruginosa infection in children with cystic fibrosis

Hydrogen cyanide is readily detected in the headspace above Pseudomonas aeruginosa cultures and in the breath of cystic fibrosis (CF) patients with chronic (P. aeruginosa) infection. We investigated if exhaled breath HCN is an early marker of P. aeruginosa infection. 233 children with CF who were free from P. aeruginosa infection were followed for 2 years. Their median (interquartile range) age was 8.0 (5.0-12.2) years. At each study visit, an exhaled breath sample was collected for hydrogen cyanide analysis. In total, 2055 breath samples were analysed. At the end of the study, the hydrogen cyanide concentrations were compared to the results of routine microbiology surveillance. P. aeruginosa was isolated from 71 children during the study with an incidence (95% CI) of 0.19 (0.15-0.23) cases per patient-year. Using a random-effects logistic model, the estimated odds ratio (95% CI) was 3.1 (2.6-3.6), which showed that for a 1- ppbv increase in exhaled breath hydrogen cyanide, we expected a 212% increase in the odds of P. aeruginosa infection. The sensitivity and specificity were estimated at 33% and 99%, respectively. Exhaled breath hydrogen cyanide is a specific biomarker of new P. aeruginosa infection in children with CF. Its low sensitivity means that at present, hydrogen cyanide cannot be used as a screening test for this infection.

Determination of residence times of ions in a resistive glass selected ion flow-drift tube using the Hadamard transformation

RATIONALE

Selected ion flow tube mass spectrometry, SIFT-MS, used for trace gas analyses has certain fundamental limitations that could be alleviated by adding a facility that allows reaction times and ion interaction energies to be varied. Thus, a selected ion flow-drift tube, SIFDT, has been created to explore the influence of an embedded electric field on these parameters and on reaction processes.

METHODS

The new SIFTD instrument was constructed using a miniature resistive glass drift tube. Arrival times of ions, t, analysed by a downstream quadrupole mass spectrometer over the m/z range 10-100 were studied by modulating the injected ion current using a gate lens. Single pulse modulation was compared with pseudorandom time multiplexing exploiting the Hadamard transformation. A simple model involving analysis of ethanol and water vapour mixture in air was used to explore the advantages of the SIFDT concept to SIFT-MS analysis.

RESULTS

It is shown that the resistive glass drift tube is suitable for SIFDT experiments. The Hadamard transformation can be used to routinely determine reagent ion residence time in the flow-drift tube and also to observe differences in arrival times for different product ions. Two-dimensional data combining arrival time and mass spectra can be obtained rapidly. The calculated ion drift velocities vary with the reduced field strength, E/N, and the calculated ion mobilities agree with theoretical and previous literature values.

CONCLUSIONS

This study has provided evidence that the SIFDT-MS technique can be implemented in a miniature and low-cost instrument and two- or three-dimensional data can be obtained (product ion count rates as functions of m/z, t and E/N) using the Hadamard transformation thus providing exciting possibilities for further analytical additions and extensions of the SIFT-MS technique. Copyright © 2015 John Wiley & Sons, Ltd.

Direct detection and quantification of malondialdehyde vapour in humid air using selected ion flow tube mass spectrometry supported by gas chromatography/mass spectrometry

RATIONALE

It has been proposed that malondialdehyde (MDA) reflects free oxygen-radical lipid peroxidation and can be useful as a biomarker to track this process. For the analysis of MDA molecules in humid air by selected ion flow tube mass spectrometry (SIFT-MS), the rate coefficients and the ion product distributions for the reactions of the SIFT-MS reagent ions with volatile MDA in the presence of water vapour are required.

METHODS

The SIFT technique has been used to determine the rate coefficients and ion product distributions for the reactions of H3O(+), NO(+) and O2 (+•) with gas-phase MDA. In support of the SIFT-MS analysis of MDA, solid-phase microextraction, SPME, coupled with gas chromatography/mass spectrometry, GC/MS, has been used to confirm the identification of MDA.

RESULTS

The primary product ions have been identified for the reactions of H3O(+), NO(+) and O2 (+•) with MDA and the formation of their hydrates formed in humid samples is described. The following combinations of reagent and the analyte ions (given as m/z values) have been adopted for SIFT-MS analyses of MDA in the gas phase: H3O(+): 109; NO(+): 89, 102; O2 (+•): 72, 90, 108, 126. The detection and quantification of MDA released by a cell culture by SIFT-MS are demonstrated.

CONCLUSIONS

This detailed study has provided the kinetics data required for the SIFT-MS analysis of MDA in humid air, including exhaled breath and the headspace of liquid-phase biogenic media. The detection and quantification by SIFT-MS of MDA released by a cell culture are demonstrated.

SIFT-MS and FA-MS methods for ambient gas phase analysis: developments and applications in the UK

The origins of SIFT created to study interstellar chemistry and SIFT-MS developed for ambient gas and exhaled breath analysis and the UK centres in which these techniques are being exploited.