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

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

The role of trained and untrained dogs in the detection and warning of seizures.

Seizure unpredictability plays a major role in disability and decreased quality of life in people with epilepsy. Dogs have been used to assist people with disabilities and have shown promise in detecting seizures. There have been reports of trained seizure-alerting dogs (SADs) successfully detecting when a seizure is occurring or indicating imminent seizures, allowing patients to take preventative measures. Untrained pet dogs have also shown the ability to detect seizures and provide comfort and protection during and after seizures. Dogs' exceptional olfactory abilities and sensitivity to human cues could contribute to their seizure-detection capabilities. This has been supported by studies in which dogs have distinguished between epileptic seizure and non-seizure sweat samples, probably though the detection of volatile organic compounds (VOCs). However, the existing literature has limitations, with a lack of well-controlled, prospective studies and inconsistencies in reported timings of alerting behaviours. More research is needed to standardize reporting and validate the results. Advances in VOC profiling could aid in distinguishing seizure types and developing rapid and unbiased seizure detection methods. In conclusion, using dogs in epilepsy management shows considerable promise, but further research is needed to fully validate their effectiveness and potential as valuable companions for people with epilepsy.

Global Urinary Volatolomics with (GC×)GC-TOF-MS.

Urinary volatolomics offers a noninvasive approach for disease detection and monitoring. Herein we present an improved methodology for global volatolomic profiling. Wide coverage was achieved by utilizing a multiphase sorbent for volatile organic compound (VOC) extraction. A single, midpolar column gas chromatography (GC) assay yielded substantially higher numbers of monitored VOCs compared to our previously reported single-sorbent method. Multidimensional GC (GC×GC) enhanced further biomarker discovery while data analysis was simplified by using a tile-based approach. At the same time, the required urine volume was reduced 5-fold from 2 to 0.4 mL. The applicability of the methodology was demonstrated in a pancreatic ductal adenocarcinoma cohort where previous findings were confirmed while a series of additional VOCs with diagnostic potential were discovered.

A Complete Pipeline for Untargeted Urinary Volatolomic Profiling with Sorptive Extraction and Dual Polar and Nonpolar Column Methodologies Coupled with Gas Chromatography Time-of-Flight Mass Spectrometry.

Volatolomics offers an opportunity for noninvasive detection and monitoring of human disease. While gas chromatography-mass spectrometry (GC-MS) remains the technique of choice for analyzing volatile organic compounds (VOCs), barriers to wider adoption in clinical practice still exist, including: sample preparation and introduction techniques, VOC extraction, throughput, volatolome coverage, biological interpretation, and quality control (QC). Therefore, we developed a complete pipeline for untargeted urinary volatolomic profiling. We optimized a novel extraction technique using HiSorb sorptive extraction, which exhibited high analytical performance and throughput. We achieved a broader VOC coverage by using HiSorb coupled with a set of complementary chromatographic methods and time-of-flight mass spectrometry. Furthermore, we developed a data preprocessing strategy by evaluating internal standard normalization, batch correction, and we adopted strict QC measures including removal of nonlinearly responding, irreproducible, or contaminated metabolic features, ensuring the acquisition of high-quality data. The applicability of this pipeline was evaluated in a clinical cohort consisting of pancreatic ductal adenocarcinoma (PDAC) patients (n = 28) and controls (n = 33), identifying four urinary candidate biomarkers (2-pentanone, hexanal, 3-hexanone, and p-cymene), which can successfully discriminate the cancer and noncancer subjects. This study presents an optimized, high-throughput, and quality-controlled pipeline for untargeted urinary volatolomic profiling. Use of the pipeline to discriminate PDAC from control subjects provides proof of principal of its clinical utility and potential for application in future biomarker discovery studies.

Virus-induced Volatile Organic Compounds Are Detectable in Exhaled Breath during Pulmonary Infection.

Rationale: Chronic obstructive pulmonary disease (COPD) is a condition punctuated by acute exacerbations commonly triggered by viral and/or bacterial infection. Early identification of exacerbation triggers is important to guide appropriate therapy, but currently available tests are slow and imprecise. Volatile organic compounds (VOCs) can be detected in exhaled breath and have the potential to be rapid tissue-specific biomarkers of infection etiology. Objectives: To determine whether volatile organic compound measurement could distinguish viral from bacterial infection in COPD. Methods: We used serial sampling within in vitro and in vivo studies to elucidate the dynamic changes that occur in VOC production during acute respiratory viral infection. Highly sensitive gas chromatography-mass spectrometry techniques were used to measure VOC production from infected airway epithelial-cell cultures and in exhaled breath samples from healthy subjects experimentally challenged with rhinovirus (RV)-A16 and from subjects with COPD with naturally occurring exacerbations. Measurements and Main Results: We identified a novel VOC signature comprising decane and other long-chain alkane compounds that is induced during RV infection of cultured airway epithelial cells and is also increased in the exhaled breath from healthy subjects experimentally challenged with RV and from patients with COPD during naturally occurring viral exacerbations. These compounds correlated with the magnitude of antiviral immune responses, viral burden, and exacerbation severity but were not induced by bacterial infection, suggesting that they represent a specific virus-inducible signature. Conclusions: Our study highlights the potential for measurement of exhaled breath VOCs as rapid, noninvasive biomarkers of viral infection. Further studies are needed to determine whether measurement of these signatures could be used to guide more targeted therapy with antibiotic/antiviral agents for COPD exacerbations.

Inverse correlation between plasma 2-arachidonoylglycerol levels and subjective severity of depression.

OBJECTIVE: Endocannabinoids have been implicated in the pathophysiology of Major Depressive Disorder (MDD) and might represent potential targets for therapeutic intervention. Objectives of the study were: (1) to measure plasma levels of endocannabinoids in a group of antidepressant-free depressed outpatients; (2) to explore their relationship with the severity of depressive symptoms as subjectively perceived by the patients; and (3) to investigate the effect of the selective serotonin reuptake inhibitor escitalopram on endocannabinoid levels. METHODS: We measured plasma levels of the two major endocannabinoids, 2-arachidonoylglycerol (2-AG) and N-arachidonoylethanolamine (anadamide), in 12 drug-free outpatients diagnosed with MDD and in 12 matched healthy controls. In the patient group, endocannabinoids plasma levels were assessed at baseline and after 2 months of treatment with escitalopram. RESULTS: Baseline plasma levels of the two endocannabinoids did not differ between depressed patients and healthy controls. However, there was a significant inverse correlation between 2-arachidonoylglycerol levels and the severity of subjectively perceived depressive symptoms. Treatment with escitalopram did not change endocannabinoid levels in depressed patients, although it caused the expected improvement of depressive symptoms. CONCLUSIONS: Our results suggest that 2-arachidonylglycerol, the most abundant endocannabinoid in the central nervous system, might act to mitigate depressive symptoms, and raise the interesting possibility that 2-arachidonylglycerol and anandamide are differentially regulated in patients affected by MDD. Also, our data suggest but do not prove that the endocannabinoid system is not regulated by serotonergic transmission, at least in depressed patients.

Impact of oral cleansing strategies on exhaled volatile organic compound levels.

RATIONALE: The analysis of volatile organic compounds (VOCs) within exhaled breath potentially offers a non-invasive method for the detection and surveillance of human disease. Oral contamination of exhaled breath may influence the detection of systemic VOCs relevant to human disease. This study aims to assess the impact of oral cleansing strategies on exhaled VOC levels in order to standardise practice for breath sampling. METHODS: Ten healthy volunteers consumed a nutrient challenge followed by four oral cleansing methods: (a) water, (b) saltwater, (c) toothbrushing, and (d) alcohol-free mouthwash. Direct breath sampling was performed using selected ion flow tube mass spectrometry after each intervention. RESULTS: Proposed reactions suggest that volatile fatty acid and alcohol levels (butanoic, pentanoic acid, ethanol) declined with oral cleansing interventions, predominantly after an initial oral rinse with water. Concentrations of aldehydes and phenols (acetaldehyde, menthone, p-cresol) declined with oral water rinse; however, they increased after toothbrushing and mouthwash use, secondary to flavoured ingredients within these products. No significant reductions were observed with sulphur compounds. CONCLUSIONS: Findings suggest that oral rinsing with water prior to breath sampling may reduce oral contamination of VOC levels, and further interventions for oral decontamination with flavoured products may compromise results. This intervention may serve as a simple and inexpensive method of standardisation within breath research.

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.

Opposite control of frontocortical 2-arachidonoylglycerol turnover rate by cannabinoid type-1 receptors located on glutamatergic neurons and on astrocytes.

This study examined the respective influences of cannabinoid type-1 (CB1) receptors expressed either in forebrain GABAergic neurons, in cortical glutamatergic neurons, or in astrocytes on the turnover rates of the endocannabinoids N-arachidonoylethanolamide (AEA) and 2-arachidonoylglycerol (2-AG), and the non-cannabinoid N-acylethanolamides, palmitoylethanolamide (PEA), and oleoylethanolamide (OEA), in mouse forebrain regions. To this end, conditional mutant mice lacking CB1 receptors from either of these cell types were pre-treated systemically with JZL195, a dual inhibitor of fatty acid amide hydrolase, the enzyme degrading AEA, PEA, and OEA, and of monoacylglycerol lipase, the main 2-AG-degrading enzyme. The analyses of frontocortical, hippocampal, and striatal AEA, 2-AG, PEA, and OEA concentrations revealed that their respective baseline concentrations were not influenced by the mouse genotype. On the other hand, the accumulation of frontocortical and/or hippocampal 2-AG levels in JZL195-pre-treated mice was dependent on the mouse genotype. Thus, JZL195-induced 2-AG accumulation rates were diminished in the frontal cortex of mice lacking CB1 receptors in glutamatergic neurons while their respective values were increased in the frontal cortex and hippocampus of mice lacking these receptors in astrocytes. These genotypic differences occurred with parallel and proportionate changes in the fractional rate constants for degradation of 2-AG, thus providing a mechanism whereby the baseline levels of 2-AG remained constant between genotypes. Besides suggesting a cell-type-specific control of frontocortical and/or hippocampal 2-AG synthesis and degradation rates by CB1 receptors, this study highlights the interest of assessing endocannabinoid turnover rates when questioning the status of the endocannabinoid system.

Applied Clinical Tandem Mass Spectrometry-Based Quantification Methods for Lipid-Derived Biomarkers, Steroids and Cannabinoids: Fit-for-Purpose Validation Methods.

The emergence of metabolomics and quantification approaches is revealing new biomarkers applied to drug discovery. In this context, tandem mass spectrometry is the method of choice, requiring a specific validation process for preclinical and clinical applications. Research on the two classes of lipid mediators, steroids and cannabinoids, has revealed a potential interaction in cannabis addiction and metabolism-related disorders. Here we present the development of GC-MS/MS and LC-MS/MS methods for routine quantification of targeted steroids and cannabinoids, respectively. The methods were developed using an isotopic approach, including validation for linearity, selectivity, LLOQ determination, matrix effect, carryover, between- and within-run accuracy and precision, and stability tests to measure 11 steroids and seven cannabinoids in human plasma. These methods were satisfactory for most validity conditions, although not all met the acceptance criteria for all analytes. A comparison of calibration curves in biological and surrogate matrices and in methanol showed that the latter condition was more applicable for our quantification of endogenous compounds. In conclusion, the validation of our methods met the criteria for GLP-qualified rather than GLP-validated methods, which can be used for routine analytical studies for dedicated preclinical and clinical purposes, by combining appropriate system suitability testing, including quality controls in the biological matrix.

The Relationship Between Circulating Endogenous Cannabinoids and the Effects of Smoked Cannabis.

Background: The endogenous cannabinoid system (ECS), including the endocannabinoids (eCBs), anandamide (AEA), and 2-arachidonoylglycerol (2-AG), plays an integral role in psychophysiological functions. Although frequent cannabis use is associated with adaptations in the ECS, the impact of acute smoked cannabis administration on circulating eCBs, and the relationship between cannabis effects and circulating eCBs are poorly understood. Methods: This study measured the plasma levels of AEA, 2-AG, and Δ-9-tetrahydrocannabinol (THC), subjective drug-effects ratings, and cardiovascular measures at baseline and 15-180 min after cannabis users (n=26) smoked 70% of a cannabis cigarette (5.6% THC). Results: Cannabis administration increased the ratings of intoxication, heart rate, and plasma THC levels relative to baseline. Although cannabis administration did not affect eCB levels relative to baseline, there was a significant positive correlation between baseline AEA levels and peak ratings of "High" and "Good Drug Effect." Further, baseline 2-AG levels negatively correlated with frequency of cannabis use (mean days/week) and with baseline THC metabolite levels. Conclusions: In a subset of heavy cannabis smokers: (1) more frequent cannabis use was associated with lower baseline 2-AG, and (2) those with lower AEA got less intoxicated after smoking cannabis. These findings contribute to a sparse literature on the interaction between endo- and phyto-cannabinoids. Future studies in participants with varied cannabis use patterns are needed to clarify the association between circulating eCBs and the abuse-related effects of cannabis, and to test whether baseline eCBs predict the intoxicating effects of cannabis and are a potential biomarker of cannabis tolerance.

Estimation of reference intervals of five endocannabinoids and endocannabinoid related compounds in human plasma by two dimensional-LC/MS/MS.

The elucidation of the role of endocannabinoids in physiological and pathological conditions and the transferability of the importance of these mediators from basic evidence into clinical practice is still hampered by the indefiniteness of their circulating reference intervals. In this work, we developed and validated a two-dimensional LC/MS/MS method for the simultaneous measurement of plasma endocannabinoids and related compounds such as arachidonoyl-ethanolamide, palmitoyl-ethanolamide, and oleoyl-ethanolamide, belonging to the N-acyl-ethanolamide (NAE) family, and 2-arachidonoyl-glycerol and its inactive isomer 1-arachidonoyl-glycerol from the monoacyl-glycerol (MAG) family. We found that several pitfalls in the endocannabinoid measurement may occur, from blood withdrawal to plasma processing. Plasma extraction with toluene followed by on-line purification was chosen, allowing high-throughput and reliability. We estimated gender-specific reference intervals on 121 healthy normal weight subjects fulfilling rigorous anthropometric and hematic criteria. We observed no gender differences for NAEs, whereas significantly higher MAG levels were found in males compared with females. MAGs also significantly correlated with triglycerides. NAEs increased with age in females, and arachidonoyl-ethanolamide correlated with adiposity and metabolic parameters in females. This work paves the way to the establishment of definitive reference intervals for circulating endocannabinoids to help physicians move from the speculative research field into the clinical field.

Variation of volatile organic compound levels within ambient room air and its impact upon the standardisation of breath sampling.

The interest around analysis of volatile organic compounds (VOCs) within breath has increased in the last two decades. Uncertainty remains around standardisation of sampling and whether VOCs within room air can influence breath VOC profiles. To assess the abundance of VOCs within room air in common breath sampling locations within a hospital setting and whether this influences the composition of breath. A secondary objective is to investigate diurnal variation in room air VOCs. Room air was collected using a sampling pump and thermal desorption (TD) tubes in the morning and afternoon from five locations. Breath samples were collected in the morning only. TD tubes were analysed using gas chromatography coupled with time-of-flight mass spectrometry (GC-TOF-MS). A total of 113 VOCs were identified from the collected samples. Multivariate analysis demonstrated clear separation between breath and room air. Room air composition changed throughout the day and different locations were characterized by specific VOCs, which were not influencing breath profiles. Breath did not demonstrate separation based on location, suggesting that sampling can be performed across different locations without affecting results.

The Human Skin Volatolome: A Systematic Review of Untargeted Mass Spectrometry Analysis.

The analysis of volatile organic compounds (VOCs) can provide important clinical information (entirely non-invasively); however, the exact extent to which VOCs from human skin can be signatures of health and disease is unknown. This systematic review summarises the published literature concerning the methodology, application, and volatile profiles of skin VOC studies. An online literature search was conducted in accordance with the preferred reporting items for systematic reviews and meta-analysis, to identify human skin VOC studies using untargeted mass spectrometry (MS) methods. The principal outcome was chemically verified VOCs detected from the skin. Each VOC was cross-referenced using the CAS number against the Human Metabolome and KEGG databases to evaluate biological origins. A total of 29 studies identified 822 skin VOCs from 935 participants. Skin VOCs were commonly sampled from the hand (n = 9) or forearm (n = 7) using an absorbent patch (n = 15) with analysis by gas chromatography MS (n = 23). Twenty-two studies profiled the skin VOCs of healthy subjects, demonstrating a volatolome consisting of aldehydes (18%), carboxylic acids (12%), alkanes (12%), fatty alcohols (9%), ketones (7%), benzenes and derivatives (6%), alkenes (2%), and menthane monoterpenoids (2%). Of the VOCs identified, 13% had putative endogenous origins, 46% had tentative exogenous origins, and 40% were metabolites from mixed metabolic pathways. This review has comprehensively profiled the human skin volatolome, demonstrating the presence of a distinct VOC signature of healthy skin, which can be used as a reference for future researchers seeking to unlock the clinical potential of skin volatolomics. As significant proportions of identified VOCs have putative exogenous origins, strategies to minimise their presence through methodological refinements and identifying confounding compounds are discussed.

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

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

Feasibility and acceptability of breath research in primary care: a prospective, cross-sectional, observational study.

OBJECTIVES: To examine the feasibility and acceptability of breath research in primary care. DESIGN: Non-randomised, prospective, mixed-methods cross-sectional observational study. SETTING: Twenty-six urban primary care practices. PARTICIPANTS: 1002 patients aged 18-90 years with gastrointestinal symptoms. MAIN OUTCOME MEASURES: During the first 6 months of the study (phase 1), feasibility of patient enrolment using face-to-face, telephone or SMS-messaging (Short Message Service) enrolment strategies, as well as processes for breath testing at local primary care practices, were evaluated. A mixed-method iterative study design was adopted and outcomes evaluated using weekly Plan-Do-Study-Act cycles, focus groups and general practitioner (GP) questionnaires.During the second 6 months of the study (phase 2), patient and GP acceptability of the breath test and testing process was assessed using questionnaires. In addition a 'single practice' recruitment model was compared with a 'hub and spoke' centralised recruitment model with regards to enrolment ability and patient acceptability.Throughout the study feasibility of the collection of a large number of breath samples by clinical staff over multiple study sites was evaluated and quantified by the analysis of these samples using mass spectrometry. RESULTS: 1002 patients were recruited within 192 sampling days. Both 'single practice' and 'hub and spoke' recruitment models were effective with an average of 5.3 and 4.3 patients accrued per day, respectively. The 'hub and spoke' model with SMS messaging was the most efficient combined method of patient accrual. Acceptability of the test was high among both patients and GPs. The methodology for collection, handling and analysis of breath samples was effective, with 95% of samples meeting quality criteria. CONCLUSIONS: Large-scale breath testing in primary care was feasible and acceptable. This study provides a practical framework to guide the design of Phase III trials examining the performance of breath testing in primary care.

Selected ion flow tube mass spectrometry for targeted analysis of volatile organic compounds in human breath.

The analysis of volatile organic compounds (VOCs) within breath for noninvasive disease detection and monitoring is an emergent research field that has the potential to reshape current clinical practice. However, adoption of breath testing has been limited by a lack of standardization. This protocol provides a comprehensive workflow for online and offline breath analysis using selected ion flow tube mass spectrometry (SIFT-MS). Following the suggested protocol, 50 human breath samples can be analyzed and interpreted in <3 h. Key advantages of SIFT-MS are exploited, including the acquisition of real-time results and direct compound quantification without need for calibration curves. The protocol includes details of methods developed for targeted analysis of disease-specific VOCs, specifically short-chain fatty acids, aldehydes, phenols, alcohols and alkanes. A procedure to make custom breath collection bags is also described. This standardized protocol for VOC analysis using SIFT-MS is intended to provide a basis for wider application and the use of breath analysis in clinical studies.

A literature survey of all volatiles from healthy human breath and bodily fluids: the human volatilome.

This paper comprises an updated version of the 2014 review which reported 1846 volatile organic compounds (VOCs) identified from healthy humans. In total over 900 additional VOCs have been reported since the 2014 review and the VOCs from semen have been added. The numbers of VOCs found in breath and the other bodily fluids are: blood 379, breath 1488, faeces 443, milk 290, saliva 549, semen 196, skin 623 and urine 444. Compounds were assigned CAS registry numbers and named according to a common convention where possible. The compounds have been included in a single table with the source reference(s) for each VOC, an update on our 2014 paper. VOCs have also been grouped into tables according to their chemical class or functionality to permit easy comparison. Careful use of the database is needed, as a number of the identified VOCs only have level 2-putative assignment, and only a small fraction of the reported VOCs have been validated by standards. Some clear differences are observed, for instance, a lack of esters in urine with a high number in faeces and breath. However, the lack of compounds from matrices such a semen and milk compared to breath for example could be due to the techniques used or reflect the intensity of effort e.g. there are few publications on VOCs from milk and semen compared to a large number for breath. The large number of volatiles reported from skin is partly due to the methodologies used, e.g. by collecting skin sebum (with dissolved VOCs and semi VOCs) onto glass beads or cotton pads and then heating to a high temperature to desorb VOCs. All compounds have been included as reported (unless there was a clear discrepancy between name and chemical structure), but there may be some mistaken assignations arising from the original publications, particularly for isomers. It is the authors' intention that this work will not only be a useful database of VOCs listed in the literature but will stimulate further study of VOCs from healthy individuals; for example more work is required to confirm the identification of these VOCs adhering to the principles outlined in the metabolomics standards initiative. Establishing a list of volatiles emanating from healthy individuals and increased understanding of VOC metabolic pathways is an important step for differentiating between diseases using VOCs.

Auto-deconvolution and molecular networking of gas chromatography-mass spectrometry data.

We engineered a machine learning approach, MSHub, to enable auto-deconvolution of gas chromatography-mass spectrometry (GC-MS) data. We then designed workflows to enable the community to store, process, share, annotate, compare and perform molecular networking of GC-MS data within the Global Natural Product Social (GNPS) Molecular Networking analysis platform. MSHub/GNPS performs auto-deconvolution of compound fragmentation patterns via unsupervised non-negative matrix factorization and quantifies the reproducibility of fragmentation patterns across samples.