Comprehensive Two-dimensional Gas Chromatography-time Research Articles

Defining a core breath profile for healthy, non-human primates

Non-human primates remain the most useful and reliable pre-clinical model for many human diseases. Primate breath profiles have previously distinguished healthy animals from diseased, including non-human primates. Breath collection is relatively non-invasive, so this motivated us to define a healthy baseline breath profile that could be used in studies evaluating disease, therapies, and vaccines in non-human primates. A pilot study, which enrolled 30 healthy macaques, was conducted. Macaque breath molecules were sampled into a Tedlar bag, concentrated onto a thermal desorption tube, then desorbed and analyzed by comprehensive two-dimensional gas chromatography-time of flight mass spectrometry. These breath samples contained 2,017 features, of which 113 molecules were present in all breath samples. The core breathprint was dominated by aliphatic hydrocarbons, aromatic compounds, and carbonyl compounds. The data were internally validated with additional breath samples from a subset of 19 of these non-human primates. A critical core consisting of 23 highly abundant and invariant molecules was identified as a pragmatic breathprint set, useful for future validation studies in healthy primates.

Comprehensive Analysis and Environmental Risk Assessment of Benzotriazoles in Airport Stormwater: A HS-SPME-GC × GC-TOF-MS-Based Procedure as a Tool for Sustainable Airport Runoff Water Management

Despite the numerous benefits of intensive air transport development, many activities associated with the operation of airports contribute to environmental pollution. The purpose of this research was the development, optimization, and validation of a headspace–solid-phase microextraction–comprehensive two-dimensional gas chromatography–time of flight–mass spectrometry (HS-SPME-GC × GC-TOF-MS)-based procedure for determining anti-corrosive compounds in airport stormwater. Optimized HS-SPME conditions include: 45 min extraction time, 100 °C temperature, 1.0 g salt addition, and 10 min desorption time at 270 °C. The developed procedure is sensitive, selective, accurate (recoveries ≥ 80.0%), and precise (the coefficient of variation (CV) ≤ 14.9%), making it a highly suitable tool for extensive airport stormwater quality monitoring. The validated analytical protocol was successfully used to detect pollutants, including 1H-BT, 4-MeBT, 5-MeBT, and 5,6-diMe-1H-BT, in stormwater from various European airports with different flight capacities. Throughout the sampling period at the investigated airports, 1H-benzotriazole was found in the highest concentrations, ranging from below the MQL to 467 mg/L. An ecotoxicological risk assessment revealed that 69% of the sites exhibited high risk levels (Risk Quotient ≥ 1). The developed procedure and carried out environmental risk assessments of benzotriazoles in airport stormwater enable an evidence-based approach to sustainable airport stormwater management.

Determination of Nonylphenol in Selected Foods and Identification of Single Isomers in a Coffee Sample by Comprehensive Two-Dimensional Gas Chromatography-Time of Flight Mass Spectrometry

Nonylphenols (NPs) are a degradation product of nonylphenol ethoxylate surfactants, which occur in many samples from the environment, food, and human organism. NPs are persistent and toxic and exhibit estrogenic activity. One of the challenges in understanding the estrogenic effects of NPs is the large number of isomers that may exist in different samples. After analyzing selected foods from different countries by high-performance liquid chromatography coupled with tandem mass spectrometry, NPs were detected in 8 of the 12 food samples. The concentration of NPs, based on fresh weight, ranged from 0.5 to 18.9 μg kg−1. The highest NP contents were found in coffee samples, and therefore an isomer-specific analysis was carried out by comprehensive two-dimensional gas chromatography coupled with time-of-flight mass spectrometry (GCxGC-TOF-MS). Fourteen pure isomers of NP were used as standards for cross-referencing the NP isomers. We identified the two main NP isomers in the coffee samples as 4-[3-ethyl-1,1-dimethylpentyl]-phenol and 4-[1-ethyl-1,3-dimethylpentyl]-phenol or NP128 and NP111 (Juelich nomenclature), respectively. The GCxGC-TOF-MS system with a liquid nitrogen cryogenic modulator used in this study is suitable for the isomer-specific analysis of NP and can be applied and further optimized for future routine analysis in food control.

Comprehensive two-dimensional gas chromatography-time of flight mass spectrometry as a tool for tracking roasting-induced changes in the volatilome of cold-pressed rapeseed oil

The aim of this study was to track changes in the volatilome of cold-pressed oil and press cakes obtained from roasted seeds and to combine it with the profile of non-volatile metabolites in a single study, in order to understand pathways of volatile organic compound (VOC) formation caused by thermal processing. Comprehensive two-dimensional gas chromatography-time of flight mass spectrometry was used for the analysis of VOCs in cold-pressed oils and corresponding press cakes obtained after roasting of seeds at 140 and 180 °C prior to pressing. Contents of primary metabolites (amino acids, saccharides, fatty acids) as well as selected secondary metabolites (glucosinolates, polyphenols) were determined, as many of them serve as precursors to volatile compounds formed especially in thermal reactions. After roasting, the formation of Maillard reaction products increased, which corresponded to the reduction of free amino acids and monosaccharides. Moreover, levels of the products of thermal oxidation of fatty acids, such as aldehydes and ketones, increased with the increasing temperature of roasting, although no significant changes were noted for fatty acids. Among sulphur-containing compounds, contents of the products and intermediates of methionine Strecker degradation increased significantly with the increasing temperature of roasting. Degradation of glucosinolates to nitriles occurred after thermal treatment. The results of this study confirmed that seed roasting before cold pressing has a significant effect on the volatiles, but also indicated roasting-induced changes in non-volatile metabolites of oil and press cake. Such an approach helps to understand metabolic changes occurring during rapeseed processing in cold-pressed oil production.Graphical

Optimisation of headspace solid-phase microextraction with comprehensive two-dimensional gas chromatography–time of flight mass spectrometry (HS-SPME–GC × GC–ToFMS) for quantitative analysis of volatile compounds in vegetable oils using statistical experimental design

The analysis of volatile organic compounds (VOCs) influencing the aroma of an oil is essential for evaluating its quality. Although a large number of extraction methods exists, the simple and reliable protocols for quantitative analysis of VOCs in oils are scarce. Therefore, this study aimed to develop an analytical methodology using headspace solid-phase microextraction with comprehensive two-dimensional gas chromatography–time of flight mass spectrometry (HS-SPME–GC × GC–ToFMS) for the quantification of the volatile compounds in rapeseed oil that can influence its aroma. The method optimised using design of experiment (DOE) approach was then applied to track changes in VOCs during roasting and the accelerated storage test of rapeseed and camelina oils. The optimal conditions were established at an equilibration time of 6 min and extraction for 38 min at 58 ºC using a Carboxen™/PDMS fibre. The statistical experimental design effectively selected the optimal values of each factor and analysed interactions between them. This methodology proved to be reliable for the routine analysis of oil samples. The proposed protocol can be easily extended to quantify other VOCs and can be used in future studies on the aroma of vegetable oils.

Effect of final pyrolysis temperature on the composition and structure of shale oil: Synergistic use of multiple analysis and testing methods

As the focus of research on the pyrolysis mechanism of oil shale shifting towards the molecular level, identifying the effect of pyrolysis temperature on the molecular composition and structure becomes all the more important. In this study, the molecular characteristics of shale oils produced at different final pyrolysis temperatures was compared and analyzed through synergistic use of multiple analysis and testing methods. The results showed that compared with the traditional gas chromatography-mass spectrometry (GC-MS), comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC × GC-TOF/MS) has obvious advantages in the characterization of heteroatoms and cycloalkanes. Fourier transform infrared spectrometry (FT-IR) can supplement the deficiency of nuclear magnetic resonance (NMR) in the characterization of oxygen-containing functional groups. With the increase in final temperature, the aromatization reaction of cycloalkanes intensified. Conversely, the degree of alkane branching and carbon chain length decreased. Based upon mutual verification and supplement of various analysis results, a novel method was proposed for the comprehensive molecular characterization of complex liquid fuel oil. According to the advantages and characteristics of different detection methods, synergistic use of multiple methods for different fractions is an important research direction for the molecular characterization of complex liquid fuel oil in the future.

Survey of bioavailable PCDDs, PCDFs, dioxin-like PCBs, and PBBs in air, water, and sediment media using semipermeable membrane devices (SPMDs) deployed in the Hartbeespoort Dam area, South Africa.

A survey of bioavailable polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), dioxin-like PCBs (dl-PCBs), and polybrominated biphenyls (PBBs) from ambient air, water and sediment was performed in the Hartbeespoort Dam area in South Africa, a region where data on highly toxic Stockholm Convention persistent organic pollutants (POPs) is scanty. The sampling was designed to simulate POP bioaccumulation in benthic and aquatic dwelling organisms as well as ambient air for estimation of ecological risk. The objective was to survey the spatiotemporal distribution and fate of bioavailable priority persistent organic compounds in the Hartbeespoort Dam in summer, autumn and winter seasons and to validate the utility of a comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GCxGC-TOF) method for PCDD/F, PCB, and PBB analysis. The highest detection rates for bioavailable priority POPs were for PCB 77 and PCB 126 which were detected in 15 and 16 of the 22 samples, though the majority of the detections were < LOQ for PCB 77. Overall, PCB 126, PBB 10 and PBB 49 recorded the highest quantified bioavailable concentrations per site in SPMDs deployed in the Hartbeespoort Dam. The SPMDs deployed in air at the Magalies River site in winter recorded the highest toxic equivalency quotient (TEQ) of 29.77pg TEQ SPMD-1. The highest TEQs recorded for SPMDs deployed in the sediment phase were 10.2, 3.3, and 3.2pg TEQ SPMD-1, recorded at the Harbour site in summer, Dam wall in summer and Harbour in winter respectively. In water, SPMDs deployed at the Crocodile River site recorded the highest TEQ of 0.81pg TEQ SPMD-1 in summer. TEQ data shows that air carries significant bioavailable dl-toxicity compared to the water phase, and sediment generally carries the highest dl-toxicity. Detection rates for bioavailable PBBs were generally very low, with < 3 detections being quantified above the LOQ for the majority of the sites. Statistical analysis of TEQs computed at all sites, using AVOVA shows that the dispersion of TEQs in the Hartbeespoort Dam is largely homogenous as the differences between the TEQs were insignificant (p > 0.05).

Headspace Solid-Phase Microextraction Comprehensive Two-Dimensional Gas Chromatography–Time of Flight Mass Spectrometry (HS-SPME-GC × GC–TOFMS) for Traceability of the Genus Hymenaea Resinite from Various Geographical Origins

Headspace Solid-Phase Microextraction Comprehensive Two-Dimensional Gas Chromatography–Time of Flight Mass Spectrometry (HS-SPME-GC × GC–TOFMS) for Traceability of the Genus Hymenaea Resinite from Various Geographical Origins

Chemical and Sensory Characterization of Vidal Icewines Fermented with Different Yeast Strains

The aim of this study is to comprehensively investigate the aroma composition and sensory attributes of Vidal icewine fermented with four yeast strains (ST, K1, EC1118, and R2). A total of 485 kinds of volatile components were identified by comprehensive two-dimensional gas chromatography-time of flight mass spectrometry, among which 347 kinds of volatile compounds were the same in four kinds of sample. The heat map was conducted with 156 volatile compounds, which have aroma contributions, and the analysis results identified the characteristics of the aroma composition of icewine fermented with different yeasts. Quantitative descriptive analysis was performed with a trained panel to obtain the sensory profiles. The aroma attributes of honey and nut of the icewine fermented by R2 were much higher than others. Partial least squares discriminant analysis further provided 40 compounds that were mainly responsible for the differences of the aroma characteristics of the icewines fermented by four yeasts. This study provides more data on the current status of Vidal icewines by main commercial yeasts.

Comprehensive two-dimensional gas chromatography–time of flight mass spectrometry (GC×GC-TOFMS) in conventional and reversed column configuration for the investigation of Baijiu aroma types and regional origin

Baijiu is a traditional Chinese spirit with an extraordinarily rich pattern of volatile compounds resulting from the microflora involved in fermentation, as well as the complexity of technological process. Comprehensive two-dimensional gas chromatography in the conventional and reversed column setups (non-polar – polar and polar – non-polar columns) was tested for its ability to differentiate Baijiu samples in terms of their aroma and origin (region). A total of 65 Baijiu samples were used for the study and volatile compounds were extracted by SPME with a subsequent analysis by GC×GC-TOFMS. Orthogonality of both setups was compared, so was the number of compounds identified using each setup. Repeatability of compound groups for the conventional and reversed column setups was compared; moreover, multiblock orthogonal component analysis (MOCA) was used to visualize data sets. OPLS-DA was used for Baijiu classification. Both column setups provided excellent discrimination of the Light, Soy sauce, Feng and Herbal aromas. A better classification result for the Strong and Jian aromas was recorded for the conventional column setup. Within the Strong aroma using OPLS-DA both column setups provided perfect abilities to discriminate Baijiu from the Sichuan, Heilongjiang and Jiangsu regions. Two validation methods were applied in the classification models - all the predictive abilities evaluated by the internal validation were further confirmed by the external validation.

Three Extraction Methods in Combination with GC×GC-TOFMS for the Detailed Investigation of Volatiles in Chinese Herbaceous Aroma-Type Baijiu.

In this study, the detailed volatile compositions of Chinese herbaceous aroma-type Baijiu (HAB) were characterized by comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOFMS). A total of 606 compounds were tentatively identified by similarity, mass spectral data, and retention indices, among which 247 compounds were positively verified by authentic standards. Esters were present in higher numbers (179), followed by aldehydes and ketones (111), and alcohols (81). In addition, there were also many terpenes (82), sulfides (37), furans (29), nitrogenous compounds (29), lactones (17), and so on. Meanwhile, the extraction effects of volatile components from different sample pretreatment methods (headspace solid-phase microextraction (HS-SPME), solid phase extraction (SPE), and stir bar sorptive extraction (SBSE)) for HAB were also revealed. The results indicated that HS-SPME has a better extraction effect on easily volatile compounds, such as alcohols and sulfides, especially for terpenes. SPE was particularly beneficial for the analysis of nitrogen-containing compounds; SBSE showed medium extraction ability for most types of compounds and was more suitable for the target analysis of trace content substances.

A comparison of liquid-liquid extraction and stir bar sorptive extraction for multiclass organic contaminants in wastewater by comprehensive two-dimensional gas chromatography time of flight mass spectrometry

Liquid-liquid extraction (LLE) and stir bar sorptive extraction (SBSE) are extraction methods used for the analysis of contaminants in aqueous samples. In this study, both LLE and SBSE were compared for the extraction of priority pollutants and contaminants of emerging concern (CECs) in wastewater influent and effluent samples, for analysis with comprehensive two-dimensional gas chromatography with time of flight mass spectrometry (GC × GC-TOFMS). The methods were compared for their extraction efficiency of a broad range of compounds, matrix effects, accurate and reliable quantification of targets, and sensitivity. The target analytes studied were semi-volatile organic compounds (SVOC) including polycyclic aromatic hydrocarbons, phenols, phthalate esters, anilines, ethers, aromatic nitro compounds, and nitrosamines. LLE allowed for a higher number of target analytes to be extracted with over 70% recovery and quantified more targets in the influent samples. Matrix interference effects had a negative impact on the recovery of non-polar contaminants, such as polycyclic aromatic hydrocarbons (PAHs), in the influent water samples especially with SBSE. In SBSE, 24 target analytes demonstrated significant matrix interference leading to poor analyte recovery and 13 analytes were negatively affected in the same way in LLE. Generally, polar compounds also demonstrated poor extraction with SBSE in both effluent and influent water samples. However, SBSE effluent chromatograms contained about three times as many total analytes as compared with LLE, suggesting that SBSE is more sensitive for trace contaminants in effluent samples. Based on this research, LLE is recommended for studies seeking to quantify a broad range of target analytes in complex matrices, like wastewater influent. SBSE is an appropriate method for the non-target and survey analysis of trace contaminants in less complex water samples.

Sorbent coatings for solid-phase microextraction targeted towards the analysis of death-related polar analytes coupled to comprehensive two-dimensional gas chromatography: Comparison of zwitterionic polymeric ionic liquids versus commercial coatings

Decomposition of bodies generates several types of polar volatile organic compounds (VOCs), whose types, patterns and ratios change during the various stages of decomposition and, therefore, their determination has huge potential to provide useful information to disclose events related to the time of death, or body surrounding environment. As sample preparation is a mandatory key-point in a method development, this research aims to develop a simple, accurate and rapid approach to study death-related polar VOCs based on headspace solid-phase microextraction (HS-SPME) combined with comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC × GC-ToFMS) analysis. The performance of zwitterionic PIL-based fibers (containing a [VIm+C9COO−] monomer and a [(VIm)2C122+]-2Br− crosslinker), tailored for polar compounds, was evaluated for a set of 19 analytes associated with the unique odour created by decomposing bodies, and it was compared to the commercially-available fibers: divinylbenzene/carboxen/poly(dimethylsiloxane) – DVB/CAR/PDMS, poly(dimethylsiloxane)/divinylbenzene – PDMS/DVB and polyacrylate (PA). Fibers with absorptive-type mechanism, such as PA and PIL, showed the best results in the balance of the parameters studied, being able to detect analytes at ng level and providing a profile representative of the headspace composition, thus they may represent a useful tool to respond to current challenges in forensic taphonomy. The reproducibility (with relative standard deviation lower than 18%, depending on the analyte) and relative recoveries (higher than 99.1%) were similar and acceptable for both fibers. The zwitterionic PIL, with ca. 4 times smaller film thickness than PA, still has potential to have the best performance, supported by the efforts to obtain thicker sorbent coatings.

Transformation reactivity of parent polycyclic aromatic hydrocarbons and the formation trend of halogenated polycyclic aromatic hydrocarbons in the presence of bromide ion during chlorination

The chlorination of parent polycyclic aromatic hydrocarbons (PAHs) has become a recognized fact during chlorination disinfection. However, bromide ion (Br−), which exists in natural water, may fundamentally affect the reaction mechanism. This study investigated the transformation mechanism of PAHs and the formation trend of halogenated PAHs (HPAHs) in the presence of Br− during chlorination. Twelve PAHs were divided into two groups, active and inactive, containing only hypochlorous acid (HOCl). However, the presence of Br− accelerated the reaction process of active PAHs. At a high Br− concentration, inactive PAHs transformed to become “false active”. Two reaction types, including electrophilic substitution and single electron transfer reactions, occurred in PAHs during chlorination. We screened the transformation products using comprehensive two-dimensional gas chromatography-time of flight-mass spectrometry (GC × GC-TOF MS) and identified the occurrence of two reaction types. The presence of Br− affected the species of transformation products. In general, the number of species of chlorinated PAHs decreased and those of brominated PAHs increased with an increase in the Br− concentration. In addition, we first observed the occurrence of 2 types of brominated and chlorinated acenaphthenes, predicted the formation pathways, and inferred the most probable structure using a quantum chemical computation method. Therefore, the formation mechanism of HPAHs in drinking water during the chlorination treatment process was evaluated. The presence of HPAHs in drinking water requires more attention.

Composition and origin of molecular compounds in the condensate oils of the Dabei gas field, Tarim Basin, NW China

Abstract The Dabei gas field in the Kuqa Depression of the Tarim Basin is the most complex and deep continental condensate gas field in China. Comprehensive two-dimensional gas chromatography-time of flight mass spectrometer (GC×GC-TOFMS) analysis was conducted on five condensate oil samples from this field. The results show that the samples have n-alkane series in complete preservation and rich adamantanes. According to the methyladamantane index, the condensate oil is the product of the source rock with vitrinite reflectance (Ro) of 1.3%–1.6%. According to the gas maturity calculated through carbon isotope and vitrinite reflectance, the natural gas is corresponding to Ro of 1.3%–1.7%, reflecting that the natural gas and condensate oil are basically formed during the same period at the high maturity stage of source rock. The Dabei gas field has favorable geological conditions for hydrocarbon accumulation: thick salt rock in the Paleogene acts as a regional high-quality caprock directly overlying the high-quality sandstone reservoir of the Cretaceous, the coal source rocks have high hydrocarbon generation intensity and provide continuous oil and gas, and the subsalt thrust structures develop in rows with rich faults, providing migration pathways for oil and gas migration. These factors together controlled the formation of the Dabei gas field.

Multimodal chemometric approach for the analysis of human exhaled breath in lung cancer patients by TD-GC × GC-TOFMS

Lung cancer is the deadliest cancer in developed countries. To reduce its mortality rate, it is important to enhance our capability to detect it at earlier stages by developing early diagnostic methods. In that context, the analysis of exhaled breath is an interesting approach because of the simplicity of the medical act and its non-invasiveness. Thermal desorption comprehensive two-dimensional gas chromatography time of flight mass spectrometry (TD-GC × GC-TOFMS) has been used to characterize and compare the volatile content of human breath of lung cancer patients and healthy volunteers. On the sampling side, the contaminations induced by the bags membrane and further environmental migration of VOCs during and after the sampling have also been investigated. Over a realistic period of 6 h, the concentration of contaminants inside the bag can increase from 2 to 3 folds based on simulated breath samples. On the data processing side, Fisher ratio (FR) and random forest (RF) approaches were applied and compared in regards to their ability to reduce the data dimensionality and to extract the significant information. Both approaches allow to efficiently smooth the background signal and extract significant features (27 for FR and 17 for RF). Principal component analysis (PCA) was used to evaluate the clustering capacity of the different models. For both approaches, a separation along PC-1 was obtained with a variance score around 35%. The combined model provides a partial separation with a PC-1 score of 52%. This proof-of-concept study further confirms the potential of breath analysis for cancer detection but also underlines the importance of quality control over the full analytical procedure, including the processing of the data.

Characterization of unusual tetracyclic compounds and possible novel maturity parameters for Brazilian crude oils using comprehensive two-dimensional gas chromatography-time of flight mass spectrometry

Eleven Brazilian oil samples from different sources, biodegradation and maturity levels were analyzed by comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOFMS). Geochemical characterization associated with principal component analysis (PCA) allowed the identification of the three most mature oils. The GC×GC chromatographic conditions used in this study allowed us to perform group-type (or chemical class) separation in the following second dimension (2D) order: (i) tricyclic terpanes; followed by (ii) steranes with 3 rings containing 6 carbon atoms and 1 ring containing 5 carbon atoms; (iii) tetracyclic terpanes with 4 rings containing 6 carbon atoms; (iv) pentacyclic terpanes with 4 rings containing 6 carbon atoms and 1 ring containing 5 carbon atoms; and finally (v) pentacyclic terpanes with 5 rings containing 6 carbon atoms. This information was used to tentatively identify the ring structure of eight unusual compounds found in the saturated fraction of the oils. These compounds had the double bond equivalent (DBE) value four, which was calculated based on molecular ions M+ at m/z 274, 288 and 316, indicating possible tetracyclic structures. Two of them presented a diagnostic peak at m/z 191 and six at m/z 203. A trend between the ratios of two of these unusual compounds and the C24 tetracyclic terpane versus 4,6/1,4 dimethyldibenzothiophene ratio shows promise as a maturation parameter. Thus, these unusual compounds are prone to respond to the maturation on a scale from mature to overmature, when compared with traditional maturation parameters.

Analysis of volatile organic compounds released from the decay of surrogate human models simulating victims of collapsed buildings by thermal desorption–comprehensive two-dimensional gas chromatography–time of flight mass spectrometry

Field experiments were devised to mimic the entrapment conditions under the rubble of collapsed buildings aiming to investigate the evolution of volatile organic compounds (VOCs) during the early dead body decomposition stage. Three pig carcasses were placed inside concrete tunnels of a search and rescue (SAR) operational field terrain for simulating the entrapment environment after a building collapse. The experimental campaign employed both laboratory and on-site analytical methods running in parallel. The current work focuses only on the results of the laboratory method using thermal desorption coupled to comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometry (TD-GC×GC-TOF MS). The flow-modulated TD-GC×GC-TOF MS provided enhanced separation of the VOC profile and served as a reference method for the evaluation of the on-site analytical methods in the current experimental campaign. Bespoke software was used to deconvolve the VOC profile to extract as much information as possible into peak lists. In total, 288 unique VOCs were identified (i.e., not found in blank samples). The majority were aliphatics (172), aromatics (25) and nitrogen compounds (19), followed by ketones (17), esters (13), alcohols (12), aldehydes (11), sulfur (9), miscellaneous (8) and acid compounds (2). The TD-GC×GC-TOF MS proved to be a sensitive and powerful system for resolving the chemical puzzle of above-ground “scent of death”.

Analysis of the volatile components in Minnan oolong tea by headspace solid phase microextraction coupled with comprehensive two-dimensional gas chromatography-time of flight mass spectrometry and the application in its variety identification

A method to analyze the volatile components in Minnan oolong tea was developed based on headspace solid phase microextraction (HS-SPME) coupled with comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC×GC-TOF MS). Volatile compounds of 48 oolong tea samples with different qualities and production seasons from five varieties (Tieguanyin, Huangjingui, Benshan, Maoxie and Meizhan) were extracted by HS-SPME and analyzed by GC×GC-TOF MS. More than 2 000 peaks were obtained from each sample, and 51 common compounds were tentatively identified by comparison with the standard mass spectrum databases, retention indices and structure spectra. The projection score of the common compounds obtained from principal component analysis ( PCA) had presented a straightforward classification trend for different oolong tea varieties. In addition, 9 compounds which had significant impact on the classification were selected by stepwise discriminate analysis, and used as variables to establish four discriminated functions by Fisher's discriminate analysis (FDA). The accuracy for the recognition of 48 samples was 97. 9%. The results had demonstrated the feasibility of the method to be used to discriminate the oolong tea varieties.

N-way partial least squares with variable importance in projection combined to GC × GC-TOFMS as a reliable tool for toxicity identification of fresh and weathered crude oils.

In this study, N-way partial least squares (NPLS) is proposed to correlate comprehensive two-dimensional gas chromatography-time of flight mass spectrometry (GC × GC-TOFMS) data of different aromatic oil fractions (fresh and weathered) to their toxicity values. Before NPLS modeling, since drift and wander of baseline interfere with information of sought analytes in GC × GC-TOFMS data, a novel method called two-dimensional asymmetric least squares is thus developed for comprehensive correction of the baseline contributions in both chromatographic dimensions. The algorithm is termed comprehensive because it functions to correct the entire chromatogram in both dimensions and it preserves the separation information in both dimensions. In this method, a smoother is combined with asymmetric weighting of deviations from the (smooth) trend to get an effective baseline estimator in both chromatographic dimensions. After baseline correction, the NPLS model was calibrated with 20 oil fractions and evaluated by leave-one-out cross-validation. The number of latent variables was chosen on the basis of minimum root mean squares error of cross validation (RMSECV), and it was 7 (RMSECV = 0.073). The developed NPLS model was able to accurately predict the toxicity effects in the five oil fractions as prediction sets which were independent of 20 oil fractions in calibration set (RMSEP = 0.0099 and REP = 11.38%). Finally, the newly developed n-way variable importance in projection (NVIP) was used for identification of the most influential chemical components on the toxicity values of different oil fractions. According to the high NVIP values in both chromatographic dimensions and their corresponding mass spectra, alkyl substituted three- and four-ring aromatic hydrocarbons were identified. It is concluded that multivariate chemometric methods (e.g., NPLS) combined to non-target analysis using GC × GC-TOFMS is a viable strategy to be used for analytical identification in fuel oil studies, with a potential to reduce the number of fractionation steps needed to obtain necessary chromatographic and mass spectral information.