Proton Transfer Reaction Time-of-flight Mass Spectrometry Research Articles

Genomic Diversity of Two Hydrocarbon-Degrading and Plant Growth-Promoting Pseudomonas Species Isolated from the Oil Field of Bóbrka (Poland).

Hydrocarbon-degrading bacteria are important resources for use in phytoremediation applications. Yet, for many hydrocarbonoclastic strains the genetic information regarding pollutant degradation and detoxification has not been thoroughly revealed. In this study, hydrocarbon-degrading bacteria were isolated from a long-term oil-polluted soil in Bóbrka, Poland. Pseudomonas spp. was the most dominant species. Of all 69 isolated strains tested in the laboratory using qualitative biochemical assays, 61% showed the capability to use diesel as sole carbon source, 33% could produce indole, 19% produced siderophores, 36% produced organic acids, and 54% were capable of producing 1-aminocyclopropane-1-carboxylate (ACC)-deaminase. From all morphologically and genetically different strains, two representative Pseudomonas spp., strain VI4.1 and VI4T1, were selected for genome sequencing. Genomic analyses indicated the presence of the full naphthalene dioxygenase operon (plasmid and chromosomal), of genes involved in the degradation of BTEX compounds (Benzene, Toluene, Ethylbenzene, Xylene) and alkanes (alkB gene) as well as the anthranilate degradation pathway (strain VI4T1) and terephthalate dioxygenase protein (strain VI4.1). Proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) analyses confirmed naphthalene and BTEX degradation within seven days. Motility, resistance to abiotic stresses, high and low temperatures, low pH, and salinity were confirmed at the genetic level and experimentally verified. The presence of multiple degradative and plant growth promotion genes, together with the in vitro experimental evidence, indicates the high value of these two strains and their potential use for sustainable site clean-up.

Assessment Wine Aroma Persistence by Using an in Vivo PTR-ToF-MS Approach and Its Relationship with Salivary Parameters.

To better understand wine aroma persistence, the nasal cavity of nine volunteers was monitored by Proton Transfer Reaction-Time of Flight-Mass Spectrometry (PTR-ToF-MS) after they rinsed their mouths with three rosé wines (one control and the same wine supplemented with two tannin extracts) during four minutes. Wines were aromatised with a mixture of five target aroma compounds. Results showed that wine aroma persistence was highly compound-dependent: while esters disappeared very fast, other compounds such as linalool remained in the oral cavity for longer times after wine expectoration. A low effect of tannins (at 50 mg/L) on nasal cavity parameters was observed, with the exception for the compound ethyl decanoate that was significantly higher released in the presence of tannins. Strong interindividual differences on aroma persistence were also found. Significant positive correlations with the salivary total protein content and negative with the salivary flow were observed for specific compounds. This work has studied for the first time in vivo wine aroma persistence in real time from an analytical perspective.

Highly efficient visible-light-driven photocatalytic degradation of VOCs by CO2-assisted synthesized mesoporous carbon confined mixed-phase TiO2 nanocomposites derived from MOFs

Improving the visible light response and efficient separation of electron-hole pairs play vital roles in commonly used TiO2 photocatalyst for VOCs degradation. Herein, N-doped mesoporous carbon encapsulated anatase-rutile phase junction TiO2 (TiO2@C-N(x)) was successfully synthesized via the pyrolysis of a representative amine functionalized Ti-based MOF, NH2-MIL-125, under the atmosphere of Ar and subsequent CO2 treatment. Our synthesis stragety was based on the rational regulation of the formation of TiO2 phase junction and the decomposition of amorphous carbon onto the TiO2@C-N (without subsequent CO2 process) using CO2 as both anatase-rutile phase transformation promoter and mild oxidant. Compared with TiO2@C-N, TiO2@C-N(x) nanocomposites with subsequent CO2 process exhibit significantly improved photocatalytic activity as well as mineralization efficiencies. For example, the mineralization efficiency reached 51.9% at 62.4% of styrene degradation within 240 min of visible-light irradiation by using the optimal TiO2@C-N(30) nanocomposites as compared with only 19.7% mineralization efficiency at 31.0% of styrene degradation under the same conditions of TiO2@C-N. Furthermore, the primary radicals involved in degradation of VOCs was identified by electron paramagnetic resonance spectroscopy, and the possible degradation intermediates were also monitored by means of proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). Finally, the radicals involved degradation reaction mechanism was also tentatively proposed.

Real-time monitoring and quantification of organic by-products and mechanism study of acetone decomposition in a dielectric barrier discharge reactor.

Non-thermal plasma (NTP) degradation of low-concentration acetone was investigated in a cylindrical dielectric barrier discharge reactor. The effects of oxygen content and flow rate on the removal efficiency at various discharge powers were examined in real-time. The acetone removal efficiency decreases drastically and then remains stable or increases gradually as the O2 content increases from 0 to 25%, and further to 50%. The organic by-products were characterized and quantified using a real-time proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS) instrument. The observed organic compounds, with concentrations about ppbv/ppmv by volume, were mainly formaldehyde, methanol, ketene, acetaldehyde, formic acid, acetone, and acetic acid. The discharge power was a critical factor affecting the concentration of the organic by-products and the selectivity toward CO2. The mechanism study based on the by-product monitor in real-time showed that acetone firstly fragments into methyl radicals, acetyl radicals, and H; then, the methyl and acetyl radicals are oxidized by O or OH radicals into acetaldehyde, methanol, and other compounds. It seems that acetaldehyde could be an intermediate in acetone decomposition. Firstly, most of the acetone molecules were decomposed into acetaldehyde molecules; then, the acetaldehyde molecules continued to be decomposed and oxidized into other compounds, such as acetic acid and formaldehyde. These investigations not only proposed a detail decomposition mechanism for acetone in dielectric barrier discharge reactor, but also provided a potential way to analyze and evaluate the practicability of NTP removal of VOCs.

Volatile compounds profiling by using proton transfer reaction‐time of flight‐mass spectrometry (PTR‐ToF‐MS). The case study of dark chocolates organoleptic differences

Volatile compounds profiling by using proton transfer reaction‐time of flight‐mass spectrometry (PTR‐ToF‐MS). The case study of dark chocolates organoleptic differences

Volatile compounds profiling by using proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS). The case study of dark chocolates organoleptic differences.

Direct-injection mass spectrometry (DIMS) techniques have evolved into powerful methods to analyse volatile organic compounds (VOCs) without the need of chromatographic separation. Combined to chemometrics, they have been used in many domains to solve sample categorization issues based on volatilome determination. In this paper, different DIMS methods that have largely outperformed conventional electronic noses (e-noses) in classification tasks are briefly reviewed, with an emphasis on food-related applications. A particular attention is paid to proton transfer reaction mass spectrometry (PTR-MS), and many results obtained using the powerful PTR-time of flight-MS (PTR-ToF-MS) instrument are reviewed. Data analysis and feature selection issues are also summarized and discussed. As a case study, a challenging problem of classification of dark chocolates that has been previously assessed by sensory evaluation in four distinct categories is presented. The VOC profiles of a set of 206 chocolate samples classified in the four sensory categories were analysed by PTR-ToF-MS. A supervised multivariate data analysis based on partial least squares regression-discriminant analysis allowed the construction of a classification model that showed excellent prediction capability: 97% of a test set of 62 samples were correctly predicted in the sensory categories. Tentative identification of ions aided characterisation of chocolate classes. Variable selection using dedicated methods pinpointed some volatile compounds important for the discrimination of the chocolates. Among them, the CovSel method was used for the first time on PTR-MS data resulting in a selection of 10 features that allowed a good prediction to be achieved. Finally, challenges and future needs in the field are discussed.

Field measurements of methylglyoxal using proton transfer reaction time-of-flight mass spectrometry and comparison to the DNPH–HPLC–UV method

Abstract. Methylglyoxal (MGLY) is an important atmospheric α-dicarbonyl species for which photolysis acts as a significant source of peroxy radicals, contributing to the oxidizing capacity of the atmosphere and, as such, the formation of secondary pollutants such as organic aerosols and ozone. However, despite its importance, only a few techniques exhibit time resolutions and detection limits that are suitable for atmospheric measurements. This study presents the first field measurements of MGLY by proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) performed during the ChArMEx SOP2 field campaign. This campaign took place at a Mediterranean site characterized by intense biogenic emissions and low levels of anthropogenic trace gases. Concomitant measurements of MGLY were performed using the 2,4-dinitrophenylhydrazine (DNPH) derivatization technique and high performance liquid chromatography (HPLC) with UV detection. PTR-ToF-MS and DNPH–HPLC measurements were compared to determine whether these techniques can perform reliable measurements of MGLY. Ambient time series revealed levels of MGLY ranging from 28 to 365 pptv, with a clear diurnal cycle due to elevated concentrations of primary biogenic species during the daytime, and its oxidation led to large production rates of MGLY. A scatter plot of the PTR-ToF-MS and DNPH–HPLC measurements indicates a reasonable correlation (R2=0.48) but a slope significantly lower than unity (0.58±0.05) and a significant intercept of 88.3±8.0 pptv. A careful investigation of the differences between the two techniques suggests that this disagreement is not due to spectrometric interferences from H3O+(H2O)3 or methyl ethyl ketone (or butanal) detected at m∕z 73.050 and m∕z 73.065, respectively, which are close to the MGLY m∕z of 73.029. The differences are more likely due to uncorrected sampling artifacts such as overestimated collection efficiency or loss of MGLY into the sampling line for the DNPH–HPLC technique or unknown isobaric interfering compounds such as acrylic acid and propanediol for the PTR-ToF-MS. Calculations of MGLY loss rates with respect to OH oxidation and direct photolysis indicate similar contributions for these two loss pathways.

A Profile of Exhaled Volatile Organic Compounds Measured Using PTR-TOF-MS for Diagnosis of Chronic Obstructive Pulmonary Disease

Background: Chronic obstructive pulmonary disease (COPD) is one of the leading causes of global death. The diagnosis of COPD clinically relying on pulmonary function tests is invasive for the elders.Method: To develop a non-invasive and efficient diagnosis method for COPD, a panel of 102 subjects, including 44 stable COPD patients and 58 healthy control, were recruited in Beijing, China. The exhaled gas of each subject was collected with Tedlar bags and then analyzed with proton-transfer-reaction time-of-flight mass spectrometry (PTR-TOF-MS). Binary logistic regression model was used to evaluate the associations between exhaled VOCs level and the classification of subjects, modified by smoking status.Results: Among 260 VOCs identified as endogenous VOCs, 16 key VOCs had discriminative values. The performance of classification of COPD patients using the 16 VOCs delineates an area under the curve (AUC) of 0.93 in the Receiver Operating Characteristic (ROC) curve. 10-folds cross validation demonstrated that the discriminative model has a predictive accuracy of 0.909. Among the 16 VOCs, the level of [m/z=34.99, 46.03, 47.05, 69.07, 73.03, 101.10, 169.16, 243.08] were not significantly higher or lower in the subjects with asthma, diabetes mellitus, hyperlipemia, or smoke history, which indicates that the method is specific for COPD diagnosis.Tentative conclusion: In conclusion, several specific exhaled VOCs were screened out for the non-invasive diagnosis of COPD using PTR-TOF-MS, which is promising for clinical uses. The screened VOCs should be further characterized and their metabolic pathways require further research.

Unexpectedly High Levels of Organic Compounds Released by Indoor Photocatalytic Paints.

Photocatalytic paints based on titanium dioxide (TiO2) nanoparticles represent a promising treatment technology for cleaning the air at our dwellings. A few studies have shown that instead of elimination of harmful indoor air pollutants the production of carbonyl compounds occurs from the photocatalytic paints. Herein, we report unexpectedly high concentrations of volatile organic compounds (VOCs) released upon irradiation of photocatalytic paints which are meant to clean the air at our dwellings. The concentrations of the VOCs were measured continuously and online by PTR-ToF-MS (Proton Transfer Reaction-Time of Flight-Mass Spectrometry) connected to a well-established flow tube photoreactor. The PTR-ToF-MS analysis revealed the presence of 52 ions in the mass range between 20 and 490 amu, among which 43 have been identified. In particular very high emission rates were estimated of two relevant indoor air pollutants, formaldehyde and acetaldehyde as 355 μg h-1 and 257 μg h-1 for 1 m2, respectively. We suggest a detailed reaction mechanism responsible for the production of these harmful indoor air pollutants (formaldehyde and acetaldehyde, among the others). The hydroxyl radicals (OH) formed upon activation of TiO2, react with the organic constituent (butyl acrylate and vinyl acetate) of the paint binder lead to generation of an important number of organic compounds. We demonstrate that the TiO2 quantity and the organic content of the binder is of paramount importance with respect to the formation of VOCs, which should be considered for future optimization of this air remediation technology based on TiO2 nanoparticles.

Evaluation of volatile organic compound release in modified atmosphere-packaged minced raw pork in relation to shelf-life

Abstract Volatile organic compounds (VOCs) released from fresh meat during storage have the potential to be reliable parameters for monitoring shelf-life and/or spoilage using simple sensors. The VOC release profiles in raw, minced pork packaged under modified atmosphere (70% O2, 30% CO2) stored at 4 °C were measured on days 0, 3, 6, and 12 of storage by proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS). Additional sensory and solid-phase microextraction gas chromatography mass spectrometry/olfactometry (SPME-GC–MS/O) analyses of sample subsets allowed for a comparison of VOC profile changes with changes in sensory impressions and aided compound identification. Alcohols, aldehydes, ketones and acids were observed to change over time, either increasing continuously or decreasing after reaching maximum concentrations early to mid-way during storage. Marked differences were observed in the concentrations of several compounds between the fresh (day 0) and aged samples (day 12). Sensory analyses indicated strong increases in buttery and greasy odours, moderate increases in cheesy, vomit-like, sweaty and sour-cream-like odours, and decreases in fruity odours. The number of lactic acid bacteria (LAB), Enterobacteriaceae, pseudomonads, psychrotrophs, mesophiles, yeasts and moulds increased throughout storage. Percentage wise, LAB dominated the microbial population at each sampling point and pH decreased during storage. Values of thiobarbituric acid reactive substances (TBARS) indicated marginal lipid oxidation during the first week of storage. The present study provides useful information on the development of characteristic volatile spoilage compounds and offers insights for the potential future implementation of specific sensors to monitor shelf-life.

In situ riboflavin fortification of different kefir-like cereal-based beverages using selected Andean LAB strains

Cereal-based functional beverages represent social, economic, and environmental sustainable opportunities to cope with emerging trends in food consumption and global nutrition. Here we report, for the first time, the polyphasic characterization of three cereal-based kefir-like riboflavin-enriched beverages, obtained from oat, maize and barley flours, and their comparison with classical milk-based kefir. The four matrices were successfully fermented with commercial starters: i) milk-kefir and ii) water-kefir, proving the potential of cereal ingredients in the formulation of dairy-like fermented beverages with milk-kefir starter behavior better in these matrices. In the light of their potentiality, seven riboflavin-producing Andean Lactic Acid Bacteria (LAB) were tested for tolerance to food stresses commonly encountered during food fermentation. Moreover, the LAB strains investigated were screened for spontaneous riboflavin overproducing derivatives. Lactobacillus plantarum M5MA1-B2 with outstanding response to stress, was selected to improve riboflavin content in an in situ fortification approach. The combination of L. plantarum M5MA1-B2 riboflavin overproducing strain with milk kefir starter in oat, lead to cover, for one serving of 100 g, 11.4% of Recommended Dietary Allowance (RDA). Besides, addition of L. plantarum M5MA1-B2 improved performance of water kefir in oat and maize matrices. Proton Transfer Reaction Time-of-Flight Mass Spectrometry (PTR-ToF-MS) analysis provided the on-line Volatile Organic Compounds profiles supporting the best combination of starter, LAB and cereal matrix for novel functional foods development.

High-Throughput Breath Volatile Organic Compound Analysis Using Thermal Desorption Proton Transfer Reaction Time-of-Flight Mass Spectrometry.

Breath analysis is highly acceptable to patients and health care professionals, but its implementation in clinical practice remains challenging. Clinical trials and routine practice require a robust system for collection, storage, and processing of large numbers of samples. This work describes a platform based upon the hyphenation of thermal desorption (TD) with proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS), coupled by means of an original modification of the TD interface. The performance of TD-PTR-ToF-MS was tested against seven oxygenated volatile organic compounds (VOCs), belonging to three chemical classes (i.e., fatty acids, aldehydes, and phenols), previously identified as possible biomarkers of colorectal and esophago-gastric adenocarcinoma. Limits of detection and quantification were on the order of 0.2-0.9 and 0.3-1.5 parts per billion by volume (ppbV), respectively. Analytical recoveries from TD tubes were 80% or higher, linear response was in the low- to mid-ppbV range ( R2 = 0.98-0.99), and coefficients of variation were within 20% of mean values. The usability of the platform was evaluated in the analysis of a set of breath samples of clinical origin, allowing for a throughput of nearly 100 TD tubes for 24 h of continuous operation. All of these characteristics enhance the implementation of TD-PTR-ToF-MS for large-scale clinical studies.

Analysis of volatile fraction of sweetie (Citrus maxima\u2009\xd7\u2009Citrus paradisi) and its parent fruit using proton transfer reaction mass spectrometry

The quality of the fruit is affected by several main ingredients and the aroma plays a fundamental role during the selection of fruit by consumers. In the case where several fruit have similar aromas and only one of them has specific health properties, it is very important to find the differences in the volatile organic compounds (VOCs) composition to distinguish these samples. Such situations are often found for hybrid fruit. Sweetie is a hybrid of grapefruit and pummelo. Sweetie fruit is characterized by high antioxidant potential and a positive effect on human health. The aim of this study was to verify the unique volatile compositional traits of three species of citrus fruit. Proton transfer reaction Time-of-Flight mass spectrometry (PTR-TOF-MS) was utilized to obtain the mass-resolved fingerprints of VOCs. The chemical formula of these VOC masses was tentatively identified. Principal component analysis was performed to evaluate the differences between the groups.Graphical abstract

Versatile breath sampler for online gas sensor analysis

Breath analysis is an emerging field in gas sensor technology aiming for the next generation of hand-held and non-invasive medical diagnostic and monitoring devices. Often unrecognized, however, remains the importance of correct and standardized breath sampling, a prerequisite for meaningful breath analysis. Here, a simple and modular sampler design is presented that enables monitored breath extraction for reproducible and online gas sensor analysis. It consists of an inlet to monitor airway pressure and CO2 concentration online during exhalation and a tube to capture and buffer (20–40 s) end-tidal breath to meet prolonged exposure requirements of some gas sensors. Demonstrated on the example of acetone, a breath marker for fat burn, this sampler is suitable for continuous (refreshment ∼100 s) and reproducible online breath analysis, as validated by state-of-the-art proton transfer reaction time-of-flight mass spectrometry (PTR-TOF-MS). Most interestingly, even short breath pulses down to 1 s result in reliable determination of end-tidal acetone concentrations while it is severely underestimated when sampling is not done properly. Finally, this sampler is applied to detect breath acetone with solid-state Si-doped WO3 sensors in humans. The proposed sampler can be connected to various types of gas sensors and applied flexibly for other breath markers (including isoprene, ethanol and methanol).

Incomplete synchrony of inflorescence scent and temperature patterns in Arum maculatum L. (Araceae)

In many Araceae both scent and heat production are known to temporally vary throughout anthesis, and in several species strong scents are released for pollinator attraction when thermogenesis is also strong. However, it is not known whether the temporal patterns of both scent emission and temperature are strictly synchronous and, for example, reach their maxima at the same time. We studied Arum maculatum, a brood-site deceptive species attracting its moth fly pollinators with strong fetid scents, to study temporal patterns in scent emission and temperature during anthesis. Inflorescence scents were collected and analysed by dynamic headspace and gas chromatography-mass spectrometry (GC-MS) or by proton-transfer-reaction-time-of-flight mass spectrometry (PTR-TOFMS), and the temperature of the appendix, which is the heating osmophore during pollinator attraction, was recorded by a thermocouple. We overall found that scent emission and temperature patterns were strongly correlated. However, in none of the seven studied individuals was the highest amount of scent released at times with the maximum temperature difference. Thus, patterns of scent emission and temperature are somewhat asynchronous suggesting that high scent emission rates and temporal scent patterns in plants with thermogenesis cannot be solely explained by temperature patterns. This calls for more in-depth studies to better understand the interplay between scent emission and thermogenesis.

Eddy-covariance flux measurements in an Italian deciduous forest using PTR-ToF-MS, PTR-QMS and FIS

ABSTRACTA field campaign in a deciduous forest in Ispra, Italy, was performed by the Joint Research Centre, Ispra, Italy, together with University of Innsbruck, Austria, in 2013, using PTR-ToF-MS (proton transfer reaction – time-of-flight – mass spectrometry), PTR-QMS (proton transfer reaction – quadrupole mass spectrometry), FIS (fast isoprene sensor) and other instrumentations. The main purpose of the campaign was to measure volatile organic carbon (VOC). One of the outcomes of this investigation was the comparison between the three VOC measurement techniques, focusing on the VOC-flux values that could be measured, VOC-volume-mixing-ratio (VOC-VMR) values that could be measured, the number of VOCs identified/measured, and finally if any oxidation products from the primary emitted VOC could be identified/measured. To the best of our knowledge, it is the first time that these three techniques have been used simultaneously during a campaign in a forest, and the comparison of the three techniques is presented in details (PTR-ToF-MS, PTR-QMS and FIS). It was concluded that the isoprene data generated by the three instruments were in good agreement. With PTR-ToF-MS, the following main results were achieved during the summer campaign 2013: The VMR of isoprene was measured up to 16 ppb; large fluxes of isoprene with values up to 53 nmol m−2 s−1 by PTR-ToF-MS; mean diurnal (24 h) averages of VMR of isoprene of (1.07 ± 0.53) ppb and mean diurnal (24 h) flux of isoprene of (5.20 ± 2.42) nmol m−2 s−1. For the isoprene oxidation products methyl-vinyl-ketone/methacrolein (MVK/MACR), VMRs of (0.42 ± 0.22) ppb were measured as a mean diurnal (24 h) average. Mean diurnal (24 h) flux of MVK/MACR were measured to be (0.06 ± 0.04) nmol m−2 s−1; for the monoterpenes, the VMR was measured to be (0.10 ± 0.09) ppb mean diurnal (24 h) and mean diurnal (24 h) flux of monoterpenes to be (0.08 ± 0.05) nmol m−2 s−1. Detailed descriptions of the fluxes/VMRs of other VOCs, acetone/methanol/acetaldehyde, are also presented. The results obtained here have also been compared to several other similar studies and campaigns.

Animal science meets agricultural practice: Preliminary results of an innovative technical approach for exhaled breath analysis in cattle under field conditions

In human and veterinary medicine, the search for exhaled volatiles as potential markers for disease detection has gained increasing attention because exhaled breath analysis bears the advantages of being continuous, non-invasive and reveals instantly available results. The aim of this study was to introduce a technical set-up for breath analysis in bovines. The technical set-up allows (i) CO2-controlled sampling and preconcentration of alveolar gas and (ii) direct breath gas analysis via proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) in parallel. While alveolar gas sampling enables the analysis by gas chromatography-mass spectrometry for unequivocal substance identification, PTR-ToF-MS ensures real-time identification of confounding signals within one measurement. Applicability of the system to conscious cattle and reliability of the signals obtained under field conditions were tested at eight dairy farms. This first testing phase in veterinary practice confirmed that the set-up was well tolerated by each of the cows and that reliable measured values (i.e., raw data) were gained at different respiratory rates. Inspiration could be clearly distinguished from expiration, thus providing the basis for excluding ambient air contamination. Consecutive measurements under standardised conditions in two bovines aged 5–6 months revealed effects of time of day, food intake and handling that need to be evaluated in-depth in future studies with larger groups of animals. As result of these proof-of-concept trials, the introduced technical set-up can be recommended for future studies evaluating exhaled markers in cattle. Breath gas analysis offers a new research area in veterinary medicine and is one step forward towards refining large animal studies, even under field conditions.

Evaluation of PTR-ToF-MS as a tool to track the behavior of hop-derived compounds during the fermentation of beer

Hop-derived volatile organic compounds (VOCs) play an important role in the flavor and aroma of beer, despite making up a small percentage of the overall profile. To understand the changes happening during fermentation, proton transfer reaction-time of flight-mass spectrometry (PTR-ToF-MS) was applied for the first time in brewing science to directly measure the changes in hop-derived VOCs during the fermentation of four different worts containing one of two aroma hops in combination with one of two yeast biotypes. PTR-ToF-MS successfully detected and tracked mass-to-charge ratios (m/z) arising from interactions between the different yeast strains and the hop cultivars. Differences were observed in the dynamic VOC profiles between different beer treatments for m/z such as m/z 145.121 (ethyl hexanoate) and m/z 173.153 (isoamyl isovalerate or ethyl octanoate). The ability to monitor changes in VOCs during fermentation provides valuable information on the priority of production and transformation reactions by yeast.

Effects of humidity, CO2 and O2 on real-time quantitation of breath biomarkers by means of PTR-ToF-MS

Proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS) represents an attractive tool for the real-time analysis of VOC profiles in human breath. Quantification of breath VOCs by means of direct MS may be affected by the matrix, as human breath not only contains several hundred VOCs at the ppbV–pptV level, but is water saturated and contains percentage levels of CO2. Investigation of breath biomarkers in clinical studies requires quantitative and comparable results. We therefore systematically assessed the effect of humidity, CO2 and O2 on the results of PTR-MS analysis. We investigated more than 20 VOCs, including aldehydes, ketones, aromatic compounds and hydrocarbons with different sample humidity, CO2 and O2 content. The influence of data processing (e.g. normalization to the H3O+ ion count) was also addressed. An increase of the H3O+ count of about 20% was observed when the humidity in the sample was increased to breath levels. Large differences regarding the measured VOC intensities were found between the dry and humid samples. Data normalization to the H3O+ or water-clusters could not fully compensate for the humidity-dependent effects. However, as the determination of most VOCs linearly depends on the humidity over the whole investigated range, factor-based correction seems possible. The effects of CO2 were more pronounced in the dry samples than in the humid samples but only had a minor influence on the results. The same was true for the influence of O2. For the reliable quantification of VOCs in clinical studies and for the standardization of VOC research, well-adapted calibration standards are required for PTR-MS analysis.

Identification and quantification of VOCs by proton transfer reaction time of flight mass spectrometry: An experimental workflow for the optimization of specificity, sensitivity, and accuracy.

Proton transfer reaction time of flight mass spectrometry (PTR‐ToF‐MS) is a direct injection MS technique, allowing for the sensitive and real‐time detection, identification, and quantification of volatile organic compounds. When aiming to employ PTR‐ToF‐MS for targeted volatile organic compound analysis, some methodological questions must be addressed, such as the need to correctly identify product ions, or evaluating the quantitation accuracy. This work proposes a workflow for PTR‐ToF‐MS method development, addressing the main issues affecting the reliable identification and quantification of target compounds. We determined the fragmentation patterns of 13 selected compounds (aldehydes, fatty acids, phenols). Experiments were conducted under breath‐relevant conditions (100% humid air), and within an extended range of reduced electric field values (E/N = 48–144 Td), obtained by changing drift tube voltage. Reactivity was inspected using H3O+, NO+, and O2 + as primary ions. The results show that a relatively low (<90 Td) E/N often permits to reduce fragmentation enhancing sensitivity and identification capabilities, particularly in the case of aldehydes using NO+, where a 4‐fold increase in sensitivity is obtained by means of drift voltage reduction. We developed a novel calibration methodology, relying on diffusion tubes used as gravimetric standards. For each of the tested compounds, it was possible to define suitable conditions whereby experimental error, defined as difference between gravimetric measurements and calculated concentrations, was 8% or lower.