Ultraviolet Photoionization Research Articles

Revealing the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene sensitized by dimethyl ether

Low to moderate temperature oxidation has been a vital topic in the combustion chemistry due to its critical role in controlling the combustion characteristics such as autoignition and pollutant emissions. Low temperature oxidation kinetics of alkanes and oxygenated biofuels have been widely explored, while that of branched aromatics is poorly understood. This work aims to investigate the low to moderate temperature oxidation kinetics of 1,2,4-trimethylbenzene (TMB, C9H12), which is a multi-branched alkylbenzene with weak low temperature oxidation reactivity. To enhance the low temperature oxidation reactivity of TMB, dimethyl ether (DME) was added to efficiently provide chain initiators. The oxidation experiment of TMB/DME mixture was performed with an atmospheric jet stirred reactor coupled with synchrotron vacuum ultraviolet radiation photoionization mass spectrometry. The negative temperature coefficient (NTC) behavior, as well as featured aromatic intermediates, especially phenyl hydroperoxides and unexpected highly oxygenated molecules (HOMs), were observed in the experiment. The detection of these fingerprint aromatic intermediates provide crucial evidence for revealing the low to moderate oxidation kinetics of TMB. In addition, a kinetic model was developed to provide in-depth kinetic analysis. The results demonstrate that the active chain initiators produced from DME oxidation trigger the low temperature chain-branching reactions of TMB. The chain recycling process is further sustained due to the regeneration of active intermediates such as phenyl hydroperoxides and •OH radicals. Aromatic intermediates with chemical compositions of C9H12Ox and C9H10Ox were measured and identified to be key indicators for the low temperature chemistry of TMB. In particular, the aromatic HOMs (C9H12O6 and C9H10O5) are fingerprint intermediates produced from the third O2 addition and subsequent reactions of TMB. Besides, the sensitivity analysis indicates that the interaction kinetics between TMB and DME by sharing the highly sensitive reactions of hydroperoxides and •OH radicals. The present work extends the conceptual reaction schemes proposed in alkanes towards branched aromatics, especially sequential O2 addition reactions that contributing to the formation of phenyl hydroperoxides and HOMs.

Experimental and theoretical study of cyclopropanated fuel exo,exo-Tetracyclo[3.3.1.02,4.06,8]nonane pyrolysis in a jet-stirred reactor

Exo,exo-Tetracyclo[3.3.1.02,4.06,8]nonane (exo-TCN), a kind of cyclopropanated fuel with high tension three-membered ring, which has the characteristics of high density, high net heat of combustion and good low-temperature performance, is a potential high-energy-density liquid fuel. To deeply understand its combustion behavior, and lay a foundation for practical application, detailed experimental and theoretical study is necessary. The pyrolysis of exo-TCN was investigated in a jet-stirred reactor at a temperature range of 500–1010 K under atmospheric pressure with a residence time of 2 s. 54 products and intermediates ranging from m/z = 2–204 were identified and quantified by synchrotron vacuum ultraviolet photoionization mass spectrometry and online gas chromatography, the major products were hydrogen, ethylene, benzene, methane, propene, toluene and styrene. The potential energy surfaces of exo-TCN unimolecular decomposition reactions were calculated by quantum chemistry at CBS-QB3 level. It is found that the pyrolysis of exo-TCN experiences three stages. In the first stage, exo-TCN isomerizes into Tricyclo[3.3.1.02,4]non-6-ene (TCN-6) by ring expansion of the three-membered ring. In the second stage, TCN-6 generates two main isomers through ring expansion reactions, and decomposes into pyrolysis products. The third stage is the secondary reaction of the intermediate products. Finally, combined with the experimental results and theoretical calculation, the possible reaction mechanism of exo-TCN pyrolysis was proposed, and the related reaction rate constants were calculated.

Experimental and modeling study of the N, N-dimethylformamide pyrolysis at atmospheric pressure

As a polar solvent, N, N-Dimethylformamide (DMF) is a potential source of volatile organic compounds during industrial processing. The potential energy surface and pressure-dependent rate coefficients of DMF primary and secondary thermal decomposition were theoretically investigated by automatic ab initio calculations. A singlet aminohydroxycarbene route generating CO is found to be the most energetically favored channel among the unimolecular initiation reactions with a tight transition state. A kinetic model was proposed for DMF pyrolysis prediction and validated against the speciation data newly obtained from synchrotron vacuum ultraviolet photoionization mass spectrometry in a flow reactor within 773 K–1048 K and literature data in a jet stirred reactor within 450 K–900 K at atmospheric pressure. Satisfactory agreements were obtained for the major intermediates and products, but the model failed to capture the formation of some oxygenated compounds, possibly due to the missing reaction pathway for bimolecular addition. During pyrolysis, DMF is mainly consumed by the hydrogen abstraction reaction as the relatively high CN bond energy hinders the barrierless homolytic cleavage. Unlike aldehydes where the carbonyl site is favored, H-abstraction from DMF methyl sites is prevalent, although the acylamino group readily undergoes decarbonylation and β-scission forming oxygenated and nitrogenous parts. Consequently, CO and HCN are the major pyrolysis products. Besides, by incorporating essential high temperature kinetics, extended validation was conducted against the available intermediate temperature autoignition delay times (1000–1350 K, 2 bar) and laminar burning velocity data, and the reason for the discrepancy between the model prediction and experiments was inferred according to the sensitivity analyses.

Co-pyrolysis of Hami coal and mixed plastic: An interaction between primary volatiles study via in-situ Py-TOF-MS

Online detection of pyrolysis volatiles is essential for exploring the mechanism of coal and plastics co-pyrolysis. In-situ pyrolysis time-of-flight mass spectrometry (Py-TOF-MS) equipped with double ionization sources, vacuum ultraviolet photoionization (VUVPI) and electron ionization (EI), was used to analyze the release of primary volatiles during the pyrolysis and co-pyrolysis of Hami coal (HM) with mixed plastic (PP/LDPE and PP/LDPE/PS, with ratios of 3:2 and 12:8:1, respectively). Meanwhile, the interaction mechanism was discussed at the molecular level. According to the thermogravimetric (TG) results and the total ion currents (TIC) of primary volatiles with temperature, the inclusion of plastic shortens the overall reaction time. In-situ Py-TOF-MS results indicate that the volatiles produced from HM pyrolysis at lower temperatures induce the decomposition of PP and promote further dehydrogenation of long-chain C–C. Besides, the addition of PP visibly facilitates the transportation of hydrogen, which reacts with phenoxy from HM pyrolysis to produce diphenol. The further addition of LDPE after PP enhances the inhibitory effect of molten plastic on HM pyrolysis, resulting in a decrease in the content of diphenol, such as benzenediol (from 1.34% to 0.71%), methyl-benzenediol (from 1.12% to 0.68%) and dimethyl benzenediol/ethyl benzenediol (from 0.68% to 0.50%). The co-pyrolysis of HM with PP/LDPE/PS generates more aromatic products compared to that of theoretical value, such as styrene (from 0.29% to 3.16%), 1-ethenyl-3-methyl benzene (from 0.07% to 0.31%) and 1-butene-2,4-diphenyl (from 0.19% to 0.65%). This indicates that co-pyrolysis strengthens the breakage of Car-Cβ bond in the PS backbone. The addition of PS to the mixture plastic also promotes intramolecular and intermolecular hydrogen transfer in the system. For the evolution of inorganic gases, the addition of mixed plastics promotes the decomposition of aliphatic and aromatic carboxylic acids in coal at lower temperatures, resulting in a decrease in CO2.

Experimental and Modeling Investigation on the Pyrolysis of n-C10H22 Initiated by C2H5NO2

ABSTRACT To investigate the mechanism of the nitro-alkanes initiation effect on the pyrolysis of hydrocarbons, pyrolysis experiments were conducted on n-C10H22, C2H5NO2, and a binary mixture of both compounds using synchronous vacuum ultraviolet photoionization mass spectrometry. These experiments were performed within a heated flow tube under varying pressure conditions of 30 and 760 Torr. Identifying and measuring pyrolysis species, comprising stable species and radicals, provides insights into the mechanism underlying the pyrolysis and the interaction of n-C10H22 and C2H5NO2. The findings demonstrate that C2H5NO2 can reduce the initial decomposition temperature of n-C10H22, and the acceleration effect on pyrolysis becomes more pronounced with an increase in experimental pressure. The addition of C2H5NO2 also affects the distribution of alkanes, alkenes, dienes, and alkynes. A detailed elementary reaction-dynamics model comprising 223 species and 1730 reactions was established and used to simulate the distribution of C2H5NO2, n-C10H22, major species, and significant intermediates observed in the experiments. The results prove that the developed model exhibits good accuracy in predicting experimental results and enables a detailed analysis of the initiation mechanism of C2H5NO2 on n-C10H22. The impact of C2H5NO2 on the conversion rate and selectivity of pyrolysis products of n-C10H22 is highlighted and elucidated.

Counterintuitive Catalytic Reactivity of the Aluminum Oxide "Passivation" Shell of Aluminum Nanoparticles Facilitating the Thermal Decomposition of exo-Tetrahydrodicyclopentadiene (JP-10).

High energy density aluminum nanoparticles (AlNPs) have been at the center of attention as additives to hydrocarbon jet fuels like exo-tetrahydrodicyclopentadiene (JP-10, C10H16) aiming at the superior performance of volume-limited air-breathing propulsion systems. However, a fundamental understanding of the ignition and combustion chemistry of JP-10 in the presence of AlNPs has been elusive. Exploiting an isomer-selective comprehensive identification of the decomposition products in a newly designed high-temperature chemical microreactor coupled to vacuum ultraviolet photoionization, we reveal an active low-temperature heterogeneous surface chemistry commencing at 650 K involving the alumina (Al2O3) shell. Contrary to textbook knowledge of an "inactive alumina surface", this unconventional reactivity, where oxygen is transferred from alumina to JP-10, leads to generating cyclic, oxygenated organics like phenol (C6H5OH) and 2,4-cyclopentadiene-1-one (C5H4O)─key tracers of an alumina-mediated interfacial chemistry. This counterintuitive reactivity transforms our knowledge of the (catalytic) processes of alumina-coated AlNPs on the molecular level.

A study on the pyrolysis of n-hexane initiated by 1-nitropropane: Molecular dynamics simulations and SVUV-PIMS experiments

To investigate the influence of 1-nitropropane (1-NP) initiator on the pyrolysis of n-hexane (n-C6H14), molecular dynamics (MD) simulations and synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) experiments were used to analyze the pyrolysis processes of n-C6H14 and n-C6H14 with 10 % 1-NP added. ReaxFF MD simulations were used to analyze the pyrolysis rates and product distributions of n-C6H14 and 1-NP/n-C6H14 at different temperatures. The mechanism of action of n-C6H14 pyrolysis initiated by 1-NP was investigated. Additionally, the pyrolysis products of n-C6H14 and 1-NP/n-C6H14 were identified and quantitatively analyzed using the SVUV-PIMS technique. These experimental results were compared with the ReaxFF MD simulation results to validate the reliability of the simulations and to complement each other. The results indicate that the presence of 1-NP promotes the pyrolysis of n-C6H14 and significantly improves the chemical heat sink of the fuel. The chemical heat sink in the 1-NP/n-C6H14 system differs from that in the n-C6H14 system by 28.78 kJ/kg at 2103 K. A significant increase in C2H4 production in the pyrolysis products of n-C6H14 with the addition of 1-NP. The pyrolysis of 1-NP/n-C6H14 yielded approximately twice as much C2H4 as n-C6H14 at 2103 K. The pyrolysis reactions were described by first-order kinetics, and the activation energy for the pyrolysis of 1-NP/n-C6H14 was determined to be 140 kJ/mol, approximately 48 % lower than that of n-C6H14. The composition of the pyrolysis products was confirmed by SVUV-PIMS experiments, which were in good agreement with the ReaxFF MD simulations. The mechanism of pyrolysis of n-C6H14 initiated by 1-NP was elucidated: the NO2 radicals generated by 1-NP react with H radicals of the system to form OH radicals. The primary reaction of n-C6H14 involves the abstraction of H by OH, leading to the formation of C6H13. Subsequently, C6H13 is further decomposed to form 1-C3H7, 1-C4H8, 1-C5H10 and 2-C6H12.

Methyl radical chemistry in non-oxidative methane activation over metal single sites

Molybdenum supported on zeolites has been extensively studied as a catalyst for methane dehydroaromatization. Despite significant progress, the actual intermediates and particularly the first C-C bond formation have not yet been elucidated. Herein we report evolution of methyl radicals during non-oxidative methane activation over molybdenum single sites, which leads selectively to value-added chemicals. Operando X-ray absorption spectroscopy and online synchrotron vacuum ultraviolet photoionization mass spectroscopy in combination with electron microscopy and density functional theory calculations reveal the essential role of molybdenum single sites in the generation of methyl radicals and that the formation rate of methyl radicals is linearly correlated with the number of molybdenum single sites. Methyl radicals transform to ethane in the gas phase, which readily dehydrogenates to ethylene in the absence of zeolites. This is essentially similar to the reaction pathway over the previously reported SiO2 lattice-confined single site iron catalyst. However, the availability of a zeolite, either in a physical mixture or as a support, directs the subsequent reaction pathway towards aromatization within the zeolite confined pores, resulting in benzene as the dominant hydrocarbon product. The findings reveal that methyl radical chemistry could be a general feature for metal single site catalysis regardless of the support (either zeolites MCM-22 and ZSM-5 or SiO2) whereas the reaction over aggregated molybdenum carbide nanoparticles likely facilitates carbon deposition through surface C-C coupling. These findings allow furthering the fundamental insights into non-oxidative methane conversion to value-added chemicals.

Effects of 1-nitropropane on pyrolysis of n-hexane at atmospheric pressure

In order to investigate the effects of 1-nitropropane (1-NP) on the pyrolysis of n-hexane, the pyrolysis processes of pure n-hexane and n-hexane/1-NP mixture in a jet-stirred reactor (JSR) were experimentally studied at atmospheric pressure with the technique of synchrotron ultraviolet photoionization mass spectrometry (SVUV-PIMS). The pyrolysis experiments of pure n-hexane and n-hexane/1-NP mixture were performed in the temperature ranges of 673–1143 K and 598–1128 K, respectively. The species pools for the pyrolysis of pure n-hexane and n-hexane/1-NP mixture were comprehensively determined and 25 species for pyrolysis of pure n-hexane versus 18 species for pyrolysis of n-hexane/1-NP mixture were identified. Based on the photoionization mass spectra scanned at different photon energies, the mole fraction profiles of all the pyrolysis species were evaluated. The experimental results showed that the initial pyrolysis temperature of n-hexane was observably reduced by 75 K with the addition of 1-NP, and the initial formation temperatures (IFTs) of some major pyrolysis products and important intermediates were correspondingly reduced. A high degree of conformity in the species pools of the pyrolysis of pure n-hexane and n-hexane/1-NP mixture (at both low and atmospheric pressures) was observed. On the basis of the experimental observations, modeling simulations and mechanisms proposed in previous studies, the low-temperature mechanisms for the 1-NP-initiated pyrolysis of n-hexane were discussed. A mechanism, including unimolecular dissociation reactions of 1-NP, radical reactions to produce highly active radicals, H-abstraction reactions of n-hexane and subsequent C-C bond dissociation reactions of hexyl radicals, was proposed.

An experimental and modeling study on the oxidation of ammonia-methanol mixtures in a jet stirred reactor

Methanol and liquid ammonia can form a mixture with better combustion quality. In order to investigate the ammonia (NH3)/methanol (CH3OH) mixture oxidation process, jet-stirred reactor (JSR) oxidation experiments were conducted at atmospheric pressure and temperature of 700–1200 K. The concentration of NH3 in the NH3/CH3OH mixture was 80%, 60%, and 40%, respectively, for equivalent ratios of 0.8, 1.0, and 1.2. By using synchronous vacuum ultraviolet photoionization mass spectrometry, important species (such as CH2O, CO, NO, NO2, N2O) involved in the oxidation reaction were identified and quantified. The original chemical kinetic model was validated and optimized based on experimental data, which improved the model's ability to predict NH3/CH3OH oxidation process (the overall improvement is approximately 81.3% compared to the original mechanism). The results demonstrated that methanol accelerated the rate of ammonia oxidation,and methanol was rapidly oxidized at high methanol content. The sensitivity analysis of reaction pathways with varying equivalent ratios revealed that HO2 generated a substantial amount of OH radicals and facilitated the oxidation of methanol and ammonia. During the oxidation of ammonia, nearly half of the NO produced NO2 through NO + HO2 = NO2 + OH, and about 60% of the NO came from the reduction reaction of NO2. More than 80% of the N2O was produced by NH2 + NO2 = N2O + H2O, and most of the N2O was converted to N2, while NH2 promoted the conversion of NOx to N2.

A combined sampling approach for simultaneous detection of volatile organic compounds and heavy metal ions in water using vacuum ultraviolet photoionization time-of-flight mass spectrometry

Volatile organic compounds (VOCs) and heavy metal ions significantly threaten human health. Hence, there is an urgent need to develop a faster and simpler method to detect these pollutants in water. Vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUP-TOF-MS) shows an advantage in detecting these pollutants. The VUP-TOF-MS offers a wide detection range due to its ability to ionize analytes using photons and energetic photoelectrons. In this study, we report the first-ever simultaneous detection of VOCs and heavy metal ions in water using a single TOF-MS. We designed a novel combined sampling, which extracted the analytes from water into the gas phase. This was followed by photoionization by the VUP ion source with switched ionization modes. We also investigated ionization mechanisms to explain the characteristic peaks. The detection limits of these analytes ranged from 5.4 × 10−2 to 36.8 μg L−1. We monitored the river water and observed recoveries in the range of 94–118%. Thus, the proposed method provides a new possibility for the simultaneous online detection of VOCs and heavy metal ions by mass spectrometry.

Manipulation of Ion Conversion in Dichloromethane-Enhanced Vacuum Ultraviolet Photoionization Mass Spectrometry of Oxygenated Volatile Organic Compounds.

The ion conversion processes in CH2Cl2-enhanced vacuum ultraviolet photoionization of oxygenated volatile organic compounds (OVOCs) have been systematically studied by regulating the pressure, humidity, and reaction time in the ionization source of a time-of-flight mass spectrometer. As the ionization source pressure increased from 100 to 1100 Pa, the main characteristic ions changed from CH2Cl+ to CH2Cl+(H2O), CH2OH+, and C2H4OH+ and then to the hydrated hydronium ions H3O+(H2O)n (n = 1, 2, 3). The total ion current (TIC) almost remained unchanged even if the humidity increased from 44 to 3120 ppmv, indicating interconversion between ions through ion-molecule reactions. The intensity of protonated methanol/ethanol (sample S) ion was almost linearly correlated with the intensity of H3O+(H2O)n, which pointed to the proton transfer reaction (PTR) mechanism. The reaction time was regulated by the electric field strength in the ionization region. The intensity variation trends of different ions with the reaction time indicated that a series of step-by-step ion-molecule reactions occurred in the ionization source, i.e., the primary ion CH2Cl+ reacted with H2O and converted to the intermediate product ions CH2OH+ and C2H4OH+, which then further reacted with H2O and led to the production of H3O+, and finally, the protonated sample ion SH+ was obtained through PTR with H3O+, as the ion-molecule reactions progressed. This study provides valuable insights into understanding the formation mechanism of some unexpected intermediate product ions and hydrated hydronium ions in dopant-enhanced VUV photoionization and also helps to optimize experimental conditions to enhance the sensitivity of OVOCs.

Ozone-initiated low-temperature oxidation of two propanol isomers: revealing the effects of Ö/ȮH atoms/radicals

The low-temperature oxidation of propanol isomers (n-propanol and iso-propanol) initiated by ozone addition was carried out in a jet-stirred reactor at an equivalence ratio of 0.5, in a temperature range of 400–800 K and at atmospheric pressure. Synchrotron vacuum ultraviolet photoionization mass spectrometry was used to follow the temperature dependence of key intermediates, including aldehydes, ketones, hydroperoxides, alcohols, alkanes, and alkenes, to explore their low-temperature oxidation reaction pathways. n-Propanol and iso-propanol presented different intermediate distributions during low-temperature oxidation, due to the different position of the hydroxyl group in the molecular structure. The elucidation of the distribution of these intermediate species contributed to the understanding of the low-temperature oxidation reaction network of n-propanol and iso-propanol. Furthermore, a detailed kinetic model of propanol isomers was tentatively developed by systematically updating the sub-mechanism of β-hydroxypropyl radical and by adding the four reaction channels for CO2 formation. These four reactions involve the reactions of the Ö/ȮH atom/radical with formaldehyde/acetaldehyde. The rate of production analysis reveals the crucial roles of Ö/ȮH atoms/radicals on the whole reaction process, highlighting the controlling effects of reactions directly related to Ö/ȮH atoms/radicals. Especially, the reaction between HȮ2 radicals and ozone dominates ȮH radical formation at lower temperatures of ca. 400–475 K, while Waddington reactions, the cyclization reactions of hydroperoxyl hydroxyalkyl radicals and the H-atom abstraction reactions of fuels/intermediates are dominantly responsible for the formation of ȮH radicals at elevated temperatures of ca. > 500 K.

Pyrolysis and kinetic study of dimethyl methylphosphonate (DMMP) by synchrotron photoionization mass spectrometry

Pyrolysis experiments of dimethyl methylphosphonate (DMMP) were carried out in a jet stirred reactor (JSR) coupled to synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS) at 1 atm, from 900 to 1200 K, to quantitatively measure the mole fraction profiles of DMMP and its pyrolysis products. Furthermore, theoretical calculations were carried out. Geometry optimization and vibrational frequency analysis were conducted for DMMP unimolecular and isomerization reactions at the MN15/6-311+G(2df,2p) level of theory, and kinetic calculations were performed using the kinetic code of MESS. The results show that DMMP mainly undergoes isomerization channels to form its isomers before decomposing into pyrolysis products. This is different from the DMMP decomposition channels adopted in the literature models. The kinetic model of DMMP in the literature was modified and improved by adding newly calculated reactions and their rate constants and updating the rate constants of unimolecular reactions for DMMP as well based on computations. Some rate tuning of reactions by 5 or 10 factors has been tried in model development to better describe the experimental measurements. Finally, the model developed by this work showed much better prediction of DMMP consumption and major species production than the prediction by literature models. In addition, the updated model was validated against various previous experimental data (CO mole fraction distributions in the shock tube, the flame speed and species mole fraction distributions in the premixed flame), and the satisfactory validation results indicate the reliability of the DMMP model that was improved in this work. This preliminary work laid a foundation to further study the combustion properties and to develop the kinetic model of organophosphorus compounds.

Experimental and theorical study on the pyrolysis behaviors of 2,3,3,3-tetrafluroropropene

The pyrolysis behaviors of 2,3,3,3-tetrafluroropropene (R1234yf) had been first studied in a flow reactor pyrolysis apparatus coupled with the synchrotron vacuum ultraviolet photoionization mass spectrometry and gas chromatography. The pyrolysis temperature, reaction products and intermediates were measured at atmospheric pressure. The experimental results revealed that the initial pyrolysis temperature of R1234yf was 1040K and the thermal decomposition was accelerated with the increase of temperature. It was found that the pyrolysis reaction of R1234yf will be replaced by a self-polymerization reaction at 1160 K. In order to explore the chemical process of R1234yf pyrolysis, a detailed kinetic simulation was performed and compared with experimental results. The calculated results indicated that the unimolecular decomposition paths took a dominant role in the whole pyrolysis processes. The intermediates of C2H2, C2HF, CF2, CHF3, CF4 and CF3CF3 were detected in the pyrolysis experiments. Moreover, the distribution characteristics and sensitivity analysis of the vital fluorinated species of CF2, C2H2, CF4, CF3CF3 and HF were discussed. All of these founding could provide reliable data for the development of pyrolysis models and guidance for the use of R1234yf in the refrigeration system.

Rapid and Online Detection of Foodborne Bacteria via a Novel Ultraviolet Photoionization Time-of-Flight Mass Spectrometry.

Foodborne bacteria are widespread contaminated sources of food; hence, the real-time monitoring of pathogenic bacteria in food production is important for the food industry. In this study, a novel rapid detection method based on microbial volatile organic compounds (MVOCs) emitted from foodborne bacteria was established by using ultraviolet photoionization time-of-flight mass spectrometry (UVP-TOF-MS). The results showed obvious differences of MVOCs among the five species of bacteria, and the characteristic MVOCs for each bacterium were selected by a feature selection algorithm. Online monitoring of MVOCs during bacterial growth displayed distinct metabolomic patterns of the five species. MVOCs were most abundant and varied among species during the logarithmic phase. Finally, MVOC production by bacteria in different food matrixes was explored. The machine learning models for bacteria cultured in different matrixes showed a good classification performance for the five species with an accuracy of over 0.95. This work based on MVOC analysis by online UVP-TOF-MS achieved effective rapid detection of bacteria and showed its great application potential in the food industry for bacterial monitoring.

Assessment of an exhaled breath test using ultraviolet photoionization time-of-flight mass spectrometry for the monitoring of kidney transplant recipients

Continuous monitoring for immunosuppressive status, infection and complications are a must for kidney transplantation (KTx) recipients. Traditional monitoring including blood sampling and kidney biopsy, which caused tremendous medical cost and trauma. Therefore, a cheaper and less invasive approach was urgently needed. We thought that a breath test has the potential to become a feasible tool for KTx monitoring. A prospective-specimen collection, retrospective-blinded assessment strategy was used in this study. Exhaled breath samples from 175 KTx recipients were collected in West China Hospital and tested by online ultraviolet photoionization time-of-flight mass spectrometry (UVP-TOF–MS). The classification models based on breath test performed well in classifying normal and abnormal values of creatinine, estimated glomerular filtration rate (eGFR), blood urea nitrogen (BUN) and tacrolimus, with AUC values of 0.889, 0.850, 0.849 and 0.889, respectively. Regression analysis also demonstrated the predictive ability of breath test for clinical creatinine, eGFR, BUN, tacrolimus level, as the predicted values obtained from the regression model correlated well with the clinical true values (p < 0.05). The findings of this investigation implied that a breath test by using UVP-TOF–MS for KTx recipient monitoring is possible and accurate, which might be useful for future clinical screenings.

Low-Temperature Oxidation Reaction Processes of Cyclopentanone Unraveled by In Situ Mass Spectrometry and Theoretical Study.

Although cyclopentanone (CPO) is a promising bio-derived fuel, thermodynamic data of its low-temperature oxidation under high-pressure conditions are lacking. In this work, the low-temperature oxidation mechanism of CPO is investigated in a flow reactor in the temperature range of 500-800 K and at a total pressure of 3 atm by a molecular beam sampling vacuum ultraviolet photoionization time-of-flight mass spectrometer. The electronic structure and pressure-dependent kinetic calculations are carried out at the UCCSD(T)-F12a/aug-cc-pVDZ//B3LYP/6-31+G(d,p) level to explore the combustion mechanism of CPO. Experimental and theoretical observations showed that the dominant product channel in the reaction of CPO radicals with O2 is HO2 elimination, yielding 2-cyclopentenone. The hydroperoxyalkyl radical (•QOOH) generated by 1,5-H-shifting is easily reacted with second O2 and forms ketohydroperoxide (KHP) intermediates. Unfortunately, the third O2 addition products are not detected. In addition, the decomposition pathways of KHP during the low-temperature oxidation of CPO are further assessed, and the unimolecular dissociation pathways of CPO radicals are confirmed. The results of this study can be used for future research on the kinetic combustion mechanisms of CPO under high pressure.

Experimental study on n-hexane pyrolysis initiated by 1-nitropropane in a low-pressure flow reactor

In order to investigate the species pool and primary channels of initiated pyrolysis of n-hexane, the low-pressure (30 Torr, 4.0 kPa) flow reactor pyrolysis of n-hexane initiated by 5% 1-nitropropane (1-NP) was studied experimentally in a wide range of temperature from 623 K to 1223 K. Combined with synchrotron vacuum ultraviolet (SVUV) photoionization mass spectra scanned at photon energies ranging from 9.0 to 14.7 eV and near-threshold photoionization efficiency (PIE) spectra, 29 pyrolysis species, covering C1–C6 hydrocarbons and several pyrolysis species containing O or N atoms, were identified. Based on the ion signal intensities of the visible peaks in the mass spectra, the mole fraction profiles of all the experimental species were evaluated. The experimental results suggested that the initial pyrolysis temperature of n-hexane was reduced by approximately 275–325 K. On the basis of the mole fraction profiles and the initial formation temperatures reported in this work and rate coefficients proposed in previous studies, the 1-NP-initiated pyrolysis mechanisms of n-hexane were discussed. It was suggested that n-hexane was dominantly consumed by the H-abstraction reactions by the attack of OH radical in this work. The 1-NP-initiated pyrolysis of n-hexane mainly proceeded via reactions to produce C3H6 + C3H7, or 1-C4H8 + C2H5, or 1-C5H10 + CH3.

Chemical insight into the ozone-assisted low-temperature oxidation of propane

Ozone addition is not only a promising method for combustion control and enhancement but also provides a reliable platform to benchmark low-temperature oxidation chemistry. Studies of ozone-doped combustion provide additional insight into combustion chemistry at lower temperatures. This paper studies the ozone-assisted low-temperature oxidation of propane in an atmospheric jet-stirred reactor (JSR) from 350 to 770 K. More than twenty species are measured and quantified using synchrotron vacuum ultraviolet photoionization mass spectrometry (SVUV-PIMS). The experimental results show that even at temperatures as low as 450 K, the reaction of propane is initiated by Ö atoms that originate from the thermal decomposition of ozone. The reactivity of propane reaches a maximum at 650 K, decreases in the temperature range of 650 – 740 K, and then slightly increases from 740 to 770 K. The propane oxidation behavior observed in this study is effectively captured by NUIGMech1.3 combined with a Princeton ozone sub-mechanism. However, the mole fractions of certain species such as, CO, H2O, and H2O2 are over-estimated by the mechanism at temperatures above 700 K. Kinetics analyses indicate that fuel oxidation and intermediate species formation are sensitive to the reaction temperature which determines the competing reactions involving propyl-peroxy radicals and the formation of ȮH and HȮ2 radicals. These findings are significant in terms of improving the core mechanism and acquiring a deeper comprehension of the ozone-assisted low-temperature oxidation chemistry of alkanes.