Proton Transfer Reaction Research Articles

Problems with and Improvement of HCHO/NO2 for Diagnosing Ozone Sensitivity—A Case in Beijing

Rfn (formaldehyde/nitrogen dioxide) is a common indicator based on satellite observations used to classify ozone formation sensitivity. However, it may underestimate anthropogenic volatile organic compounds (VOCs) in heavily polluted cities when only formaldehyde (HCHO) is used in Rfn to measure VOCs, since it is mainly derived from natural sources worldwide. In this study, we used multiaxis differential optical absorption spectroscopy to acquire tropospheric observations of nitrogen dioxide (NO2), HCHO and glyoxal (CHOCHO) in Beijing from 1 April 2019 to 31 March 2020. Combined with VOCs detected simultaneously by gas chromatography—mass spectrometry and proton transfer reaction–time-of-flight/mass spectrometry near the ground, we evaluated the representativeness of HCHO column densities on total VOCs (TVOC) in equivalent propylene concentrations, which is called reactivity. The results showed that there were significant seasonal differences in the response of HCHO to TVOC reactivity, with fitting slopes of 2.3 (spring), 2.6 (summer), 2.9 (autumn) and 1.0 (winter) in the four seasons, respectively. Since CHOCHO can be used to partly characterize the contribution of anthropogenic VOC emissions and demonstrated a better response to TVOC reactivity in winter, with fitting slopes of 0.2 (spring), 0.2 (summer), 0.2 (autumn) and 0.5 (winter) in the four seasons, respectively, we introduced CHOCHO to construct a new indicator (HCHO + 6 × CHOCHO). The fitting slopes of the four seasons were more similar, being 3.2 (spring), 3.6 (summer), 4.0 (autumn) and 4.0 (winter). The ratio of the new indicator to NO2, Rmn ((HCHO + 6 × CHOCHO)/NO2), was used to reclassify the ozone formation sensitivity of urban areas in North China, revealing that it is a transition regime before 1300 LST (LST = UST + 8) and an NOx-limited regime afterwards. Rmn improved the sensitivity from the VOC-limited regime to the NOx-limited regime, enhancing the sensitivity of NOx and providing new robust support for ozone pollution prevention and control.

Temporal variations of volatile organic compounds inside the cabin of a new electric vehicle under different operation modes during winter using proton transfer reaction time-of-flight mass spectrometry

Transportation is globally becoming more vehicle-dependent as public awareness towards the health risks caused by cabin-emitted volatile organic compounds (VOCs) increases. Therefore, the need for quantifying their concentration increases as well. This study measured the real-time VOCs in a new mini-truck-type electric vehicle cabin using a proton transfer reaction time-of-flight mass spectrometry under varying cabin heating conditions during winter. A total of 246 ions were detected between m/z 30 and 250, 82 of which were quantified. The total ion count in the cabin was double that of the ambient air. Morning-to-noon concentration of total VOCs increased 2.5 times in the cabin under solar exposure (164.47–405.92 µg·m−3). Additionally, 12 VOCs that either had higher indoor-to-outdoor ratios or globally regulated chosen to investigate the effects of cabin air conditions. Heater operation immediately increased concentrations of some VOCs by 54.62%. Furthermore, blocking solar exposure from windows reduced VOC emissions during heater off and on scenarios by 35.49% and 65.42%, respectively, indicating that window coverage also provided insulation against heat loss. Finally, the fresh air reduced cabin VOCs by 62.83% due to ambient air inflow. However, cabin concentrations remained higher than those of ambient air.

Fingerprinting methods for origin and variety assessment of rice: development, validation and data fusion experiments

Several analytical techniques, i.e. Near Infrared (NIR) and Mid-Infrared (MIR) spectroscopy, Hyper Spectral Imaging (HSI), Gas Chromatography coupled to Mass Spectrometry (GC-MS) and Proton-transfer Reaction Time-of-Flight Mass spectrometry (PTR-TOF-MS), combined with chemometrics, are examined to evaluate their potential for solving food authenticity questions on the case of rice. In total, 237 rice samples were analyzed in this study to examine origin and variety assessment and sourced from producing countries (Italy, Spain, Vietnam, Pakistan and Thailand). The Gaussian Process Latent Variable Model (GP-LVM) was applied as technique to obtain a meaningful compressed representation of the data in a two-dimensional space followed by a classification with a nearest neighbour algorithm. What concerns the origin assessment, GC-MS results score good across all countries, resulting in the most accurate method, with prediction rates ranging from 86% to 94%. Data fusion experiments of combination of GC-MS with NIR, or GC-MS with HSI resulted in prediction rates on origin assessment of more than 90%. Variety assessment was performed with these analytical techniques. Using single techniques, HSI achieved prediction values above 90% for all classes (96%–99%). For data fusion experiments of variety assessment combining GC-MS and NIR all prediction values were equal or higher than 92%; for GC-MS combined with HSI, prediction values were all 98%. The presented methodology can be successfully used for the authentication of rice.

Characterization of Volatile Organic Compound Emissions and CO2 Uptake from Eco-roof Plants.

Vegetation plays an important role in biosphere-atmosphere exchange, including emission of biogenic volatile organic compounds (BVOCs) that influence the formation of secondary pollutants. Gaps exist in our knowledge of BVOC emissions from succulent plants, which are often selected for urban greening on building roofs and walls. In this study, we characterize the CO2 uptake and BVOC emission of eight succulents and one moss using proton transfer reaction - time of flight - mass spectrometry in controlled laboratory experiments. CO2 uptake ranged 0 to 0.16 μmol [g DW (leaf dry weight)]-1 s-1 and net BVOC emission ranges -0.10 to 3.11 μg [g DW]-1 h-1. Specific BVOCs emitted or removed varied across plants studied; methanol was the dominant BVOC emitted, and acetaldehyde had the largest removal. Isoprene and monoterpene emissions of studied plants were generally low compared to other urban trees and shrubs, ranging 0 to 0.092 μg [g DW]-1 h-1 and 0 to 0.44 μg [g DW]-1 h-1, respectively. Calculated ozone formation potentials (OFP) of the succulents and moss range 4×10-7 - 4×10-4 g O3 [g DW]-1 d-1. Results of this study can inform selection of plants used in urban greening. For example, on a per leaf mass basis, Phedimus takesimensis and Crassula ovata have OFP lower than many plants presently classified as low OFP and may be promising candidates for greening in urban areas with ozone exceedances.

Insights into wavelength-mediated excited state intramolecular proton transfer in solution: UV resonance Raman spectroscopy and theoretical calculation

The excited-state intramolecular proton transfer (ESIPT) has been increasingly applied in the fields of chemistry, biology, materials science and so on. Herein, we proposed the efficiency and applicability of Raman spectroscopy (RS) toward the direct observation and characterization of transient species and monitoring of excited-state proton transfer reaction mediated by different excitation wavelength. We hope this help to understand the photochemical reactivity of the organic compounds containing CS and SH groups.

Ionic Liquid Induced p-Nitrophenol prototropism

The manuscript “Ionic Liquid Induced p-Nitrophenol Prototropism” deals with the acid-base properties of p-Nitrophenol molecule in the presence of two different Ionic Liquids (ILs). Prototropic equilibria of p-Nitrophenol (PNP) have been studied in water, ethanol, carbon tetrachloride, chloroform, cyclohexane and IL added aqueous solution with the help of Uv-vis absorbance spectroscopy. It was found that PNP mainly exists in neutral form when dissolved in ethanol, carbon tetrachloride, chloroform and cyclohexane. Interestingly, ground state ionization of PNP takes place in water and results into the formation of p-Nitrophenolate anion as a result of proton transfer to water and hence two absorbing species have been detected in the absorbance spectra. Inter-conversion of neutral to anionic and vice versa can be achieved with the addition of small amount of NaOH/HCl. Furthermore, in water/ethanol binary mixture both neutral and anionic forms exist. Interactions of surfactants sodium dodecyl sulfate (SDS) and cetrimonium bromide (CTAB) with neutral and ionic forms of PNP is also explored. To assess the potential role of ILs as a solubilizing media that facilitates proton-transfer reactions, p-Nitrophenol prototropism is investigated in water in the presence of small amount of added ILs. Interestingly, addition of small amount of hydrophilic IL, 1-butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4] or hydrophobic IL, 1-butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6] into PNP dissolved in high pH water causes protonation of the already present p-Nitrophenolate anion. Protonation of p-Nitrophenolate is attributed due to presence of acidic C2-H protons in [bmim]+ cations associated with both ILs. Furthermore, the extent of protonation induced by addition of [BF4]− based IL is found to be much pronounced as compared to [PF6]− based IL having same cation due to difference in hydrolytic properties of [BF4]− based ILs when compared with [PF6]− based ILs. These findings could contribute to the technological applications for detection, removal and sensing of environmentally hazardous p-Nitrophenol and its ionic forms from water. The results presented in this work lead to a better understanding of prototropic behavior of PNP in IL based media as well as different organic solvents based media.

Salivary Volatile Organic Compound Analysis: An Optimised Methodology and Longitudinal Assessment Using Direct Injection Mass Spectrometry

Analysis of salivary volatile organic compounds (VOCs) may offer a novel noninvasive modality for disease detection. This study aims to optimise saliva headspace VOC analysis and assess longitudinal variation of salivary VOCs. Whole saliva from healthy participants was acquired in order to assess four methodological parameters: saliva collection, volume, dilution, and acidification. Saliva VOCs were analysed using untargeted proton transfer reaction time-of-flight mass spectrometry. Using the optimised method, five saliva samples collected over 3 weeks assessed the longitudinal VOC variability and reproducibility with targeted selected ion flow tube-mass spectrometry analysis. The method of saliva collection influenced VOC detection and was a source of contamination. An amount of 500 µL of whole saliva by passive drool yielded optimal VOCs. Longitudinal variation was negligible with target short chain fatty acids and aldehydes. However, certain compounds showed variability suggesting the influence of potential exogenous factors. Overall, there was an acceptable range of inter- and intraindividual VOC variability. Standardisation with morning sampling after a 6 h fast is recommended demonstrating minimal intersubject variability. Future studies should seek to establish salivary VOC levels in healthy and diseased populations.

Observing Aqueous Proton-Uptake Reactions Triggered by Light.

Proton-transfer reactions in water are essential to chemistry and biology. Earlier studies reported on aqueous proton-transfer mechanisms by observing light-triggered reactions of strong (photo)acids and weak bases. Similar studies on strong (photo)base-weak acid reactions would also be of interest because earlier theoretical works found evidence for mechanistic differences between aqueous H+ and OH- transfer. In this work, we study the reaction of actinoquinol, a water-soluble strong photobase, with the water solvent and the weak acid succinimide. We find that in aqueous solutions containing succinimide, the proton-transfer reaction proceeds via two parallel and competing reaction channels. In the first channel, actinoquinol extracts a proton from water, after which the newly generated hydroxide ion is scavenged by succinimide. In the second channel, succinimide forms a hydrogen-bonded complex with actinoquinol and the proton is transferred directly. Interestingly, we do not observe proton conduction in water-separated actinoquinol-succinimide complexes, which makes the newly studied strong base-weak acid reaction essentially different from previously studied strong acid-weak base reactions.

A DFT evaluation of molecular reactivity of volatile organic compounds in support of chemical ionization mass spectrometry

Gas-phase molecular properties of volatile organic compounds (VOCs) play an important role in the selection of gas-phase reagent ions for chemical ionization mass spectrometry (CI-MS). Chemical ionization-based mass spectrometry techniques such as proton transfer reaction mass spectrometry (PTR-MS) and selected ion flow-tube mass spectrometry (SIFT-MS) provide real-time, rapid, and online detection and quantification of VOCs using thermodynamics and kinetics of ion-molecule gas-phase reactions. We apply hybrid density functional theory (DFT) to compute proton affinity (PA), ionization energy (IE), and global reactivity parameters for VOCs, which are widely regarded as the primary sources of taints and off-flavors in wine. Atomic polar tensor (APT) charges and total energies at the stationary point for neutral and protonated molecules are also computed. PA and IE values determine the CI-MS mode of reactions, either proton transfer or electron transfer from the reagent gas ions to VOCs. Global reactivity parameters, such as chemical potential (μ), chemical hardness (η), softness (σ), and electrophilic nature (ω) as obtained from frontier molecular orbitals, are considered useful in rationalizing the chemical reactivity patterns of the molecules. A benchmark calculation of indole molecule with MP2, B3LYP, and M06-2X DFT methods at thermodynamically and kinetically stable protonation sites further supports the applied DFT method. Since limited data are available on computed parameters, the reported values would support CI-MS quantification of trace-level VOCs not only in wine but also in various food products.

Rapid identification of artificial fragrant rice based on volatile organic compounds: From PTR-MS to FTIR

The volatile organic compounds (VOCs) released from foods can reflect their internal properties. Artificial fragrant rice (AFR) is a fraudulent food product in which the flavor of low-quality rice is artificially enhanced by addition of essence. In this study, proton-transfer reaction mass spectrometry, long optical path gas phase FTIR spectroscopy and fiber optic evanescent wave were used to analyze the characteristic mass-charge ratios signal and infrared fingerprint signal of four essence which may be used to make AFR, and the prepared AFR samples with different essence levels (0.001 %-0.3 %) were used to verify the detection performance of the detection methods. The results show that the three detection methods effectively identified AFR containing the minimum recommended dose of essence (≥0.1 %, w/w). The above detection methods can provide detection results in real time without complex sample pretreatment and provide options as rapid screening methods for food regulatory authorities to identify AFR.

Influence of ozone and humidity on PTR-MS and GC-MS VOC measurements with and without a Na2S2O3 ozone scrubber

Abstract. The measurement of volatile organic compounds (VOCs) can be influenced by ozone (O3), resulting in sampling artefacts that corrupt the data obtained. Published literature reports both positive (false enhancements of signal) and negative (loss of signal) interference in VOC data due to ozonolysis occurring in the sample gas. To assure good data quality it is essential to be aware of such interfering processes, to characterize them and to try to minimize the impact with a suitable sampling setup. Here we present results from experiments with a sodium thiosulfate ozone scrubber (Na2S2O3), which is a cost-effective and easily applied option for O3 scavenging during gas-phase sampling. Simultaneous measurement of selected organic trace gases using gas chromatography–mass spectrometry and proton transfer reaction–mass spectrometry was performed at different ozone levels (0–1 ppm) and different relative humidities (0 %–80 %). In this way both tropospheric and stratospheric conditions were examined. The measured data show that several carbonyl compounds including acetaldehyde, acetone and propanal show artificial signal enhancement when ozone is present at higher concentrations (> 150 ppb) in dry air, while analytes with double bonds like isoprene (measured with GC-MS) and terpenes show lower signals due to reaction with ozone. Both effects can be eliminated or in the case of sesquiterpenes substantially reduced by using Na2S2O3 impregnated quartz filters in the inlet line. With the chosen scrubbing material, relative humidity (RH) substantially improves the scrubbing efficiency. Under surface conditions between 50 %–80 % RH, the filter allows for accurate measurement of all species examined.

Features of photoinduced proton transfer in the presence of a polyelectrolyte

Using the example of a series of sulfo derivatives of 2-naphthol, it was shown that the charge field formed by the polyelectrolyte coil significantly changes the constants k1 and k−1 of the photoinduced proton transfer reaction, but no noticeable shift in the equilibrium constant K * was found. This observation is fundamentally different from the behavior of these substances in micellar media, where K * increases by an order of magnitude. The binding constants of the dyes with the cationic polyelectrolyte were also determined.

Proton Reaction Path in Base Pairs of DNA Molecule According to the Complete Active Space Self-Consistent Field Method

The double proton transfer reaction paths in AT and CG base pairs of DNA molecule are calculated in the Complete Active Space Self-Consistent Field method and compared with the same paths in Density Functional Theory with B3LYP approximation approach. We found that an essential increase of an activation energy, which significantly reduces the probability of spontaneous mutations in DNA via double proton transfer. There exist two transition points on the singlet potential energy surface divided by a flat region for GC base pair. The applicability of various quantum-chemical methods for description of double proton transfer reactions was discussed.

Rapid Determination of Ethyl Carbamate in Chinese Liquor via a Direct Injection Mass Spectrometry with Time-Resolved Flash-Thermal-Vaporization and Acetone-Assisted High-Pressure Photoionization Strategy.

Ethyl carbamate (EC), a carcinogenic compound, is naturally produced in fermented foods and alcoholic beverages. Rapid and accurate measurement of EC is necessary and important for quality control and safety evaluation of Chinese liquor, a traditionally distilled spirit with the highest consumption in China, but it remains a great challenge. In this work, a direct injection mass spectrometry (DIMS) with time-resolved flash-thermal-vaporization (TRFTV) and acetone-assisted high-pressure photoionization (HPPI) strategy has been developed. EC was rapidly separated from the main matrix components, ethyl acetate (EA) and ethanol, by the TRFTV sampling strategy due to the retention time difference of these three compounds with large boiling point differences on the inner wall of a poly(tetrafluoroethylene) (PTFE) tube. Therefore, the matrix effect of EA and ethanol was effectively eliminated. The acetone-assisted HPPI source was developed for efficient ionization of EC through a photoionization-induced proton transfer reaction between EC molecules and protonated acetone ions. The accurate quantitative analysis of EC in liquor was achieved by introducing an internal standard method (ISM) using deuterated EC (d5-EC). As a result, the limit of detection (LOD) for EC was 8.88 μg/L with the analysis time of only 2 min, and the recoveries ranged from 92.3 to 113.1%. Finally, the prominent capability of the developed system was demonstrated by rapid determination of trace EC in Chinese liquors with different flavor types, exhibiting wide potential applications in online quality control and safety evaluation of not only Chinese liquors but also other liquor and alcoholic beverages.

Tuning the Surface Mn/Al Ratio and Crystal Crystallinity of Mn–Al Oxides by Calcination Temperature for Excellent Acetone Low-Temperature Mineralization

Here, Mn–Al oxides with the strengthened synergistic effect of Mn and Al species were fabricated by facilely adjusting the calcination temperature with the hydrolysis-driven redox-precipitation method. Results demonstrated that the surface Mn/Al ratio and KMn8O16 phase can be effectively tamed under different calcination temperatures, which obviously alter the CO2 selectivity, reaction rate, and stability of Mn–Al oxides for catalytic oxidation of acetone, among which the Mn5Al-350 catalyst exhibits the best catalytic performance (90% of acetone converted at 159 °C) with CO2 selectivity higher than 99.5%, mainly owing to its higher surface Mn/Al ratio and weaker Mn–O bond with more Mn3+ as compared to Mn5Al-250, Mn5Al-450, and Mn5Al-550. Although a decrease in the consumption rate of acetic acid in the presence of 3.0 vol % H2O leads to the slight reduction of acetone conversion and CO2 yield, Mn5Al-350 still exhibits a superior catalytic stability. The reaction intermediates including acetaldehyde, ethanol, acetic acid, and formic acid species before total mineralization are determined by proton transfer reaction–mass spectrometry, theoretical calculations, and in situ DRIFTS. Theoretical calculations also reveal that the p-orbital interaction of C with a certain anisotropy leads to a weak catalytic effect in the process of acetic acid decomposition as the rate-limiting step.

Theoretical Study on the Gas Phase and Gas–Liquid Interface Reaction Mechanism of Criegee Intermediates with Glycolic Acid Sulfate

Criegee intermediates (CIs) are important zwitterionic oxidants in the atmosphere, which affect the budget of OH radicals, amines, alcohols, organic/inorganic acids, etc. In this study, quantum chemical calculation and Born-Oppenheimer molecular dynamic (BOMD) simulation were performed to show the reaction mechanisms of C2 CIs with glycolic acid sulfate (GAS) at the gas-phase and gas-liquid interface, respectively. The results indicate that CIs can react with COOH and OSO3H groups of GAS and generate hydroperoxide products. Intramolecular proton transfer reactions occurred in the simulations. Moreover, GAS acts as a proton donor and participates in the hydration of CIs, during which the intramolecular proton transfer also occurs. As GAS widely exists in atmospheric particulate matter, the reaction with GAS is one of the sink pathways of CIs in areas polluted by particulate matter.

A Theoretical Study on Rate Constants of Excited State Proton Transfer Reaction in Anthracene-Urea Derivatives

Abstract We elucidate the origin of the two types of observed fluorescence in a complex consisting of nPUA (1-anthracen-n-yl-3-phenylurea; n = 1, 2, 9) and an acetate ion. By calculating the molecular properties related to the proton transfer reaction in the excited state, we clarified a correlation between the acid dissociation constant in the excited state (pKa) in the urea moiety and the rate constant of the excited state proton transfer reaction. The computed pKa suggests that the proton on the anthracenyl group side is transferred in the case of 1PUA and 2PUA, whereas the proton on the phenyl group side is transferred in 9PUA. Low pKa and activation barriers were calculated for 9PUA, which causes the stability of 9PUA due to the absence of the planarity after the proton transfer reaction.

Discovery of a biomarker for β-Thalassemia by HPLC-MS and improvement from Proton Transfer Reaction – Parallel Ion Parking

β-thalassemia is a quantitative hemoglobin (Hb) disorder resulting in reduced production of Hb A and increased levels of Hb A2. Diagnosis of β-thalassemia can be problematic when combined with other structural Hb variants, so that the separation approaches in routine clinical centers are not sufficiently decisive to obtain accurate results. Here, we separate the intact Hb subunits by high-performance liquid chromatography, followed by top-down tandem mass spectrometry of intact subunits to distinguish Hb variants. Proton transfer reaction-parallel ion parking (PTR-PIP), in which a radical anion removes protons from multiply charged precursor ions and produces charge-reduced ions spanning a limited m/z range, was used to increase the signal-to-noise ratio of the subunits of interest. We demonstrate that the δ/β ratio can act as a biomarker to identify β-thalassemia in normal electrospray ionization MS1 and PTR-PIP MS1. The application of PTR-PIP significantly increases the sensitivity and specificity of the HPLC-MS method to identify δ/β ratio as a thalassemia biomarker.

Evaluation of Workplace Exposures to Volatile Chemicals During COVID-19 Building Disinfection Activities with Proton Transfer Reaction Mass Spectrometry.

We conducted an experimental case study to demonstrate the application of proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) for mobile breathing zone (BZ) monitoring of volatile chemical exposures in workplace environments during COVID-19 disinfection activities. The experiments were conducted in an architectural engineering laboratory-the Purdue zero Energy Design Guidance for Engineers (zEDGE) Tiny House, which served as a simulated workplace environment. Controlled disinfection activities were carried out on impermeable high-touch indoor surfaces, including the entry door, kitchen countertop, toilet bowl, bathroom sink, and shower. Worker inhalation exposure to volatile organic compounds (VOCs) was evaluated by attaching the PTR-TOF-MS sampling line to the researcher's BZ while the disinfection activity was carried out throughout the entire building. The results demonstrate that significant spatiotemporal variations in VOC concentrations can occur in the worker's BZ during multi-surface disinfection events. Application of high-resolution monitoring techniques, such as PTR-TOF-MS, are needed to advance characterization of worker exposures towards the development of appropriate mitigation strategies for volatile disinfectant chemicals.

Evolution of energy landscapes in designer enzymes.

Bottom-up design of biocatalysts provides a unique opportunity to study how the dynamic motions of enzymes change as catalytic efficiency evolves. Using detailed biophysical and molecular dynamics simulations of enzymes designed to promote a simple proton transfer reaction, we have shown that a chemical reaction can be speeded up by modulating protein conformation landscapes to cooperatively preorganize catalytic residues to enhance transition-state stabilization while minimizing competing, unproductive conformations. Moreover, the general tightening of correlated dynamical networks that include the transition state and large parts of the protein was found to induce an activation heat capacity, leading to a non-linear temperature‑activity dependence. These findings suggest that explicit consideration of both static and dynamic computational approaches could aid the design of many de novo enzymes.