Heart-cut Multidimensional Gas Chromatography Research Articles

A two-run heart-cut multidimensional gas chromatography method using flame ionization and mass spectrometry for automated and robust determination of nearly complete wine aroma-volatile profiles

A quantitative analytical method capable of determining the concentrations of 81 aroma-relevant wine volatiles covering nine orders of magnitude was developed and validated in this study. The method is based on stir bar sorptive extraction (SBSE) of 200 μL of wine diluted with 1.8 mL NaCl brine with pH 3.5. Volatiles thermally desorbed from the stir bars were separated in two runs in a heart-cut multidimensional gas chromatographic system and quantified using either a flame ionization detector (FID) in the first dimension (27 aroma compounds) or a mass spectrometer in the second dimension (54 aroma compounds, transferred to 22 cuts). Typical limits of compound detection lay around 0.02 mg/L by FID or ranged from 0.001 to 0.30 μg/L by mass spectrometry detector, liying below the corresponding odor thresholds in all cases. Linearity, reproducibility, and recovery were considered satisfactory for most compounds, with typical R2 values of 0.989–0.999, relative standard deviation below 10 % for 37 compounds and between 10 and 20 % for 44 compounds, and recovery rates of approximately 100 % (85–109 %) for all but acetaldehyde. An analysis of 20 wine samples completed our validation of the method, showing that a single-sample preparation procedure combined with heart-cut multidimensional two-detector gas chromatography can determine wine volatile concentrations ranging from 350 mg/L of isoamyl alcohol to 3.8 ng/L of 3-isobutyl-2-methoxypyrazine.

Identification, Quantitation, and Sensory Evaluation of Thiols in Bordeaux Red Wine with Characteristic Aging Bouquet.

Great Bordeaux red wines are known for their distinctive aging bouquet. However, the nature of volatile chemicals underpinning this sensory expression is not fully understood. This work investigated the empyreumatic aging bouquet of a collection of premium Bordeaux red wines using silver-ion (Ag+) solid-phase extraction, cryogenic heart-cutting multidimensional gas chromatography mass spectrometry/olfactometry, and comprehensive two-dimensional gas chromatography time-of-flight mass spectrometry. In doing so, a substantial number of "meaty" odors were revealed. Three detected "meaty" notes were tentatively or unequivocally attributed to furan thiols. Among them, 2-methyltetrahydrofuran-3-thiol (1) with a pleasant "meaty" aroma was reported in wine for the first time. Its trans isomer (trans-1a) was resolved from its racemate by chemical modification, which confirmed its presence in wine. The odor detection threshold of trans-1a in the model wine was determined at 55 ng/L. Moreover, an additive effect between 1 and literature-known 2-methyl-3-furanthiol was observed. By a new ultra high-performance liquid chromatography quadrupole Orbitrap high-resolution mass spectrometry method, the concentration of trans-1a, in addition to those of 2-methyl-3-furanthiol and 2-furfuryl thiol, was measured in the wines at ng/L levels.

Tijuca forest contribution to the improvement of air quality and wellbeing of citizens in the city of Rio de Janeiro, Brazil

The Tijuca Forest, one of the largest urban forests in the world, is a protected area of the Brazilian Atlantic Forest, one of the world's biodiversity hotspots. The forest and the Metropolitan Region of Rio de Janeiro coexist and interact, but their mutual influence regarding air quality is still not well known and a more detailed study is needed. Here, air samples were collected inside the forest, in Tijuca National Park (TNP) and Grajaú State Park (GSP) and in two representative urban areas (Tijuca and Del Castilho Districts). Sampling was performed using stainless steel canisters, and ozone precursor hydrocarbons (HCs) were analyzed with the aid of heart-cutting multidimensional gas chromatography. The sampling points within the forest are currently visited by hundreds of people. Total HC concentrations within the green area were clearly lower than in the urbanized districts, in spite of the anthropogenic impact of visitors and the proximity of the urban area. Median values were 21.5, 35.5, 57.9 and 148.6 μg m−3 at TNP, GSP, Tijuca and Del Castilho, respectively. Total HC concentrations were Del Castilho > Tijuca > GSP > TNP. The kinetic reactivity and ozone-forming potential of individual HCs were evaluated, as well as the intrinsic reactivity of air masses. The air masses in the urbanized area showed a higher average reactivity in all scales. In fact, in spite of the forest's contribution to isoprene emissions, its net contribution to ozone formation was lower than that of urbanized air masses, owing to a reduction in HC concentration, particularly for alkenes and monoaromatic compounds. It is not clear if the forest plays a role in the adsorption of pollutants or if it acts as a physical natural barrier to air masses carrying pollutants. Nonetheless, improving air quality within Tijuca Forest is essential to the welfare of citizens.

Qualitative Screening of Volatile Thiols in Wine by Selective Silver Ion Solid-Phase Extraction with Heart-Cutting Multidimensional Gas Chromatography Mass Spectrometry/Olfactometry.

The chemical analysis of odorous volatile thiols is intrinsically challenging. Substantial progress has been made in quantitative analysis of targeted thiols at ultra-trace concentrations (ng/L), but lesser analytical attention has been given to the qualitative screening of unknown thiols. This work presents a solid-phase extraction (SPE) method using a silver ion (Ag+)-based sorbent to isolate volatile thiols from red wine. This proposed Ag+ SPE method was effective (recovery: 87-101% for four non-furan thiols and 35-49% for two furan thiols), simple, safe, and greatly reduced artifacts, testifying to its suitability as the sample preparation protocol for a qualitative screening experiment. Separation and detection were conducted using heart-cutting multidimensional gas chromatography coupled to mass spectrometry/olfactometry (H/C MDGC-MS/O). Key parameters including H/C width, main host oven temperature, and cryogenic trapping temperature were investigated for optimal instrument performance. The developed Ag+ SPE H/C MDGC-MS/O strategy was readily applicable for qualitative screening of odorous volatile thiols in wine, as demonstrated by two case studies.

Detection of Sub-Aroma Threshold Concentrations of Wine Methoxypyrazines by Multidimensional GCMS

Complex matrices, such as wine, provide a challenge in the quantification of compounds. There exists a high likelihood of co-elution in these matrices, thereby artificially increasing the observed concentration. This can often lead to confusing data where compounds are above aroma detection thresholds, but are not detected by olfactory analysis. Additionally, the lack of sensitivity in assays can lead to the non-detection of sub-aroma threshold concentrations and contrasting data when olfactory analysis detects these aromas. To eliminate these pitfalls and gain a better understanding of the role that methoxypyrazines impart green character to wine, a quantitative method using headspace solid-phase microextraction coupled to heart-cutting multidimensional gas chromatography mass spectrometry was developed. The method can quantitate the three common methoxypyrazines found in wine at the picogram per liter level while resolving co-eluting compounds. The proposed method was validated using model wine and wine solutions and was ultimately used for the comparative analysis of white, rosé, and red wines.

A multi-location peak parking approach for calculation of second dimensional retention indices for improved volatile compound identification with cryogen-free comprehensive heart-cut two-dimensional gas chromatography.

Comprehensive heart-cut multidimensional gas chromatography (CH/C MDGC) without a cryogenic trapping device was developed with an established approach for calculation of first and second dimensional retention indices (1I and 2I) for improved compound identification. A first dimensional (1D) DB-1MS column (60 m) and a second dimensional (2D) DB-WAX column (60 m) were applied with a Deans switch (DS) using a constant H/C window of 0.2 min and a periodic multiple heartcut strategy comprising 225H/C throughout the CH/C. 1I was calculated based on comparison of the middle of the heartcut time with the alkane retention times on the 1D column. A multi-location peak parking approach using sixteen sets of automated injections of alkane references was also established with the least square curve fitting method for construction of the alkane isovolatility curves which were applied for 2I calculation. The untargeted compound analysis of a perfume sample was then performed according to comparison with the libraries of mass spectra, 1I and 2I. The CH/C MDGC system with a 25 h analysis time showed a peak capacity (nc) of 9198 and 128 separated peaks with 71 compounds successfully identified according to MS, 1I and 2I library match under the established error approximation criteria. Furthermore, relationship between the analysis time and number of separated peaks was proposed based on the set of 84 identifiable compounds. With the compensation of lower separation performance and greater I errors, the analysis time could be reduced by applying a 2.5 min H/C window with a total analysis time of 2 h and nc of 1134.

Cryogen-free comprehensive heartcut multidimensional gas chromatography using a Deans switch for improved analysis of petrochemical products derived from palmitic acid oxidation.

Comprehensive heartcut multidimensional gas chromatography was applied with example application for analysis of a sample obtained from palmitic acid oxidation in a Rancimat instrument. The system utilized a single Deans switch (DS) located between first dimensional semi-standard nonpolar (30 m) and second dimensional polar (60 m) columns. A cyclic multiple heartcut strategy consisting of 150 heartcuts with a 0.2 min window was applied offering comprehensive analysis and injection of a narrow band of compounds onto the second column without use of cryogenic trapping devices. Untargeted compound analysis of the sample prepared by solid phase micro-extraction was performed based on match between the experimental MS spectra and first dimensional retention indices with that from the NIST library. The sample contained the major compounds of 2-octanone, 1-methylcyclohexanol, 2,3,6-trimethylphenol, 3-phenylpropanol and 2-nonanone. This approach was then evaluated based on peak capacity and the number of identified compounds. Compared with one dimensional gas chromatography providing a total peak capacity of 172 and 43 identified compounds, the analysis performance was much more improved with a capacity of 5840 and 235 identified compounds by using comprehensive heartcut multidimensional gas chromatography with the total analysis time of 15.3 h. By comparison within the same set of identified compounds, the one dimensional and multidimensional approaches provided the MS match scores of 769 ± 81 and 836 ± 88, respectively. In addition, the nonlinear relationship between the analysis time and number of identifiable peaks was calculated according to the set of 235 compounds. This revealed that the analysis time could be shortened with the compensation of lower separation performance, where application of a 2.5 min heartcut window with the total analysis time of 1.2 h could result in the total peak capacity of 390 with 150 identifiable compounds.

Multidimensional gas chromatography investigation of concentration and temperature effects of oxime interconversion on ionic liquid and poly(ethylene glycol) stationary phases

This study develops analytical approaches using comprehensive heart-cut multidimensional gas chromatography (H/C MDGC) to experimentally investigate effects of concentration and temperature on peak shapes and on kinetic analysis of E/Z isomerisation of oximes which undergo various extents of interconversion on 1D and 2D separation columns. Different stationary phase combinations were investigated. An SLB-IL76 1D column results in good separation of the isomers but with negligible interconversion (1Dsep). Precise targeted heart-cuts (H/C; 3–15 s windows) and cryogenic trapping of selected sections eluting from this 1D column allows well controlled E/Z isomer ratios to be delivered to a 2D poly(ethyleneglycol) (PEG) phase column. This phase catalyses oxime interconversion, with separation; this is termed 2Dinter. This column set describes a 1Dsep × 2Dinter arrangement. The interconversion forward rate constant (k1UE, calculated by using DCXplorer) was found to be independent of the %E isomer (i.e. first order kinetics); k1UE increased at higher temperature (increased Gibbs activation energies, ΔGapp). In addition, a long 1D SLB-IL111 column gave oxime separation with interconversion (1Dinter). Comprehensive multiple H/C of the zone performed on the 2D PEG (2Dinter) at 40, 60 and 80 °C resulted in a 2D presentation (1Dinter × 2Dinter) with a unique rectangular display of interconversion from each H/C pulse with varied isomer concentration ratios. Data for k1UE from 1DGC analysis suggests underestimation of k1UE on 1Dinter SLB-IL111 from the 2D result. The k1UE value on 2Dinter PEG column is more reliably evaluated by summation of all the 2Dinter profiles rather than that obtained from the average value for the individual H/C pulses.

Observation and explanation of two-dimensional interconversion of oximes with multiple heart-cutting using comprehensive multidimensional gas chromatography

Real-time interconversion processes produce unconventional peak broadening in gas chromatography (GC), and can be used to generate kinetic and thermodynamic data. In this study, an unusual separation situation in comprehensive two dimensional GC where two dimensional interconversion (i.e. a raised plateau in both first and second dimension, 1D and 2D) was observed in analysis of oxime isomers. This resulted in a characteristic and unusual rectangular peak shape in the two dimensional result. A related theoretical approach was introduced to explain the peak shape supported by simulation results which can be varied depending on concentration profiles and kinetics of the process. The simulated results were supported by experimental results obtained by a comprehensive heart-cut multidimensional GC (H/C MDGC) approach which was developed to clearly investigate isomerisation of E/Z oxime molecules in both 1D and 2D separations under different isothermal conditions. The carrier gas flow and oven temperature were selected according to initial results for 1D interconversion on a poly(ethyleneglycol) stationary phase, which was further used in both 1D and 2D separations to result in broad zones of oxime interconversion in both dimensions. The method involved repetitive injections of oxime sample, then sampling contiguous fractions of sample into a long 2D column which is intended to promote considerable interconversion. Comprehensiveness arises from the fact that the whole sample is sampled from the 1D to the 2D column, with the long 2D column replacing the short 2D column used in classical comprehensive two-dimensional gas chromatography, where the latter will not promote sufficient interconversion. Data processing and presentation permits a ‘rectangular’ distribution corresponding to the separated compounds, characteristic of this experiment.

Quantification ethyl carbamate in wines using reaction-assisted-extraction with 9-xanthydrol and detection by heart-cutting multidimensional gas chromatography-mass spectrometry

9-Xanthydrol was introduced as an assisted-extraction-reagent to quantify ethyl carbamate (EC) in wines by heart-cutting multidimensional gas chromatography coupled with mass spectrometry (MDGC-MS). 9-Xanthydrol could help to increase the extraction efficiency, because it could react with ethyl carbamate to form the low-polar product, which facilitated the transfer of ethyl carbamate into organic phase. Then the reaction product was decomposed in high temperature, so ethyl carbamate could be obtained again in injector port. Heart-cutting multidimensional gas chromatography coupled with mass spectrometry was used not only for avoiding 9-xanthydrol interference but also for getting a larger volume injector, higher sensitivity and lower limit of detection. The method was optimized and validated in terms of sample volume, sodium chloride, the acid concentration, the 9-xanthydrol concentration and the reaction time. Good linear relationship (R2 = 0.9998) over the concentration range of 2.00 μg L−1 - 200.00 μg L−1 was obtained. The limit of detection (LOD, 0.02 μg L−1) and quantification (LOQ, 0.10 μg L−1) were lower than previously reported. The RSDs of precision (repeatability and reproducibility) were lower than 8.15%, and the recovery (96.17–99.25%) and accuracy (99.21–110.93%) were validated as well. This methodology was applied to the quantification of ethyl carbamate in several different fermented wines with values ranging from 13.05 to 155.20 μg L−1.

An improved method for the analysis of 2-aminoacetophenone in wine based on headspace solid-phase microextraction and heart-cut multidimensional gas chromatography with selective detection by tandem mass spectrometry

Heart-cut multidimensional gas chromatography coupled to selective mass spectrometric detection (H/C MDGC–MS-MS) was shown to be a suitable combination for analysis of the key component for the wine off-flavor named “atypical aging off-flavor” (german: “Untypische Alterungsnote”, UTA), 2-aminoacetophenone (2-AAP). Headspace solid phase microextraction (HS-SPME) was applied as an easy to automate sample preparation step suiting routine control situations. During method development two critical aspects were found, of which one is the chemical degradation of the neat substance, leading to signal reduction and a false response behavior. A second aspect is the pH stability of the deuterated isotopologue 1-(2-aminophenyl)-2,2,2-trideuterio-ethanone (2-AAP-d3) used for quantification via a stable isotope dilution assay. Despite an earlier suggestion to increase the extractable amount of 2-AAP and its isotopic standard in the headspace, alkalization of 2-AAP-d3 containing aqueous samples were found to be critical. In such a milieu a deuterium exchange can then cause erroneous quantitative results. The method proposed in our study thus uses native wine pH conditions and proved to be suitable for routine control with respect to the detection of the “atypical aging off-flavor” in wine at concentration levels below its sensory threshold of about 1μgL−1. Good linearity was obtained in the calibrated range from 0.1–8μgL−1 (y=1,2338x+0,1029, R=0,9985) with limits of detection or quantification being 0.01 and 0.14μg/L, respectively.

Optimization of an online heart-cutting multidimensional gas chromatography clean-up step for isotopic ratio mass spectrometry and simultaneous quadrupole mass spectrometry measurements of endogenous anabolic steroid in urine.

Measuring carbon isotope ratios (CIRs) of urinary analytes represents a cornerstone of doping control analysis and has been particularly optimized for the detection of the misuse of endogenous steroids. Isotope ratio mass spectrometry (IRMS) of appropriate quality, however, necessitates adequate purities of the investigated steroids, which requires extensive pre-analytical sample clean-up steps due to both the natural presence of the target analytes and the high complexity of the matrix. In order to accelerate the sample preparation and increase the automation of the process, the use of multidimensional gas chromatography (MDGC) prior to IRMS experiments, was investigated. A well-established instrumental configuration based on two independent GC ovens and one heart-cutting device was optimized. The first dimension (1D) separation was obtained by a non-polar column which assured high efficiency and good loading capacity, while the second dimension (2D), based on a mid-polar stationary phase, provided good selectivity. A flame ionization detector monitored the 1D, and the 2D was simultaneously recorded by isotope ratio and quadrupole mass spectrometry. The assembled MDGC set-up was applied for measuring testosterone, 5α- and 5β-androstanediol, androsterone, and etiocholanolone as target compounds and pregnanediol as endogenous reference compound. The urine sample were pretreated by conventional sample preparation steps comprising solid-phase extraction, hydrolysis, and liquid-liquid extraction. The extract obtained was acetylated and different aliquots were injected into the MDGC system. Two high performance liquid chromatography steps, conventionally adopted prior to CIR measurements, were replaced by the MDGC approach. The obtained values were consistent with the conventional ones. Copyright © 2016 John Wiley & Sons, Ltd.

Quantitative determination of α-ionone, β-ionone, and β-damascenone and enantiodifferentiation of α-ionone in wine for authenticity control using multidimensional gas chromatography with tandem mass spectrometric detection.

Native concentrations of α-ionone, β-ionone, and β-damascenone were studied in various authentic and commercial wines. In addition, the enantiomeric distribution of α-ionone was determined and its merits as a potential marker for aroma adulteration in wine were discussed. For extractionof volatiles, headspace solid-phase microextraction (HS-SPME) was applied, followed by heart-cut multidimensional gas chromatography coupled to tandem mass spectrometric detection for trace-level analysis. The enantioselective analysis of α-ionone was achieved with octakis(2,3-di-O-pentyl-6-O-methyl)-γ-cyclodextrin as the chiral selector in the separation column for gas chromatography (GC). In all the authentic wines studied, α-ionone showed a high enantiomeric ratio in favor of the (R)-enantiomer. Since an illegal addition of α-ionone in a racemic form changes the enantiomeric ratio, this ratio may serve as an adulteration marker. Concentrations varied between <LOD to 0.081μg/L for α-ionone, <LOD to 1.0μg/L for β-ionone, and 0.03-10.3μg/L for β-damascenone. Commercial wines of suspiciously strong flavor yielded concentrations up to 4.6μg/L for α-ionone, 3.6μg/L for β-ionone, and 4.3μg/L for β-damascenone. Elevated α- and β-ionone concentrations serve as additional indicators for a potential adulteration. In order to classify the concentrations of the analytes in the context of their odor activity in wine, odor thresholds were determined. Graphical abstract Trace-level analysis of authenticity markers in wine.

Enantiodifferentiation of 1,2-propanediol in various wines as phenylboronate ester with multidimensional gas chromatography-mass spectrometry.

Native concentrations and enantiomeric distribution of 1,2-propanediol in various wines were studied in order to evaluate its merits as a potential marker for aroma adulteration in wine. Heart-cut multidimensional gas chromatography coupled to mass spectrometry was applied to analyze 1,2-propanediol after salting-out of the polar phase, derivatization with phenyl boronic acid, and extraction with cyclohexane. The enantiomeric separation of the derivative was achieved with heptakis-(6-O-tert. butyl dimethylsilyl-2,3-di-O-acetyl)-β-cyclodextrin as the chiral selector. In all authentic wines studied, 1,2-propanediol showed a high enantiomeric ratio in favor of the (R)-enantiomer, proving its potential as a marker for the adulteration with flavor extracts based on industrial 1,2-propandiol as solvent. Usually, concentrations varied between 15 and 100mg/L. Higher values (up to 170mg/L) were found in wines made with high amounts of dry berries. However, despite the higher concentrations of 1,2-propanediol in such wines, no apparent influence on the enantiomeric distribution could be detected. Graphical Abstract Detection of fraudulent aromatization of wines by enantiodifferentiation of 1,2-propanediol as its phenylboronate ester.

Quantification of cork off-flavor compounds in natural cork stoppers and wine by multidimensional gas chromatography mass spectrometry

Trace-level analysis of cork off-flavor compounds considered as responsible for typical [2,4,6-trichloroanisole (TCA)] and atypical cork taint [geosmin (GSM); 2-methylisoborneol (MIB); 3-isopropyl-2-methoxypyrazine (IPMP); 3-isobutyl-2-methoxypyrazine (IBMP); 3,5-dimethyl-2-methoxypyrazine (MDMP)] was achieved for cork soaks and wines for concentrations below odor threshold (MIB only in cork soaks). The analytical approach was based on headspace solid-phase microextraction and heart-cut multidimensional gas chromatography with tandem mass spectrometric detection. Quantification was done using a stable isotope dilution assay. Individual cork stoppers with varying sensory off-odor descriptions were analyzed. In particular, IPMP and IBMP correlated with the cork stoppers described with green attributes. MDMP was found in samples described as dusty–musty or nutty-like. In a migration study, transport of off-flavor compounds from affected cork stoppers into the corresponding wine could be observed after a storage period of 13 months. Multivariate statistics on the wines’ sensory analysis and chemical data showed a good correlation of the individual off-flavor compound concentration, its sensory description and the off-flavor perceived in the wine.

Multidimensional Gas Chromatographic Analysis of Low-Temperature Oxidized Jet Fuels: Formation of Alkyldihydrofuranones

Liquid hydrocarbon fuels are complex hydrocarbon mixtures, made even more complex when oxidation reactions produce a variety of oxygenated species. Heart-cut multidimensional gas chromatography coupled with chemical ionization mass spectrometry was applied to the quantification of alkyldihydrofuranones in several fossil and alternatively derived jet fuels. The fuels were then subjected to thermal oxidative stress for extended times using a purpose-designed experimental system. The concentration of furanones in all fuels increased over the course of the experiment, to a maximum of 7.5 mg/L in a hydrotreated fuel. The analytical method is robust with R2 for all analytes above 0.9993 and relative standard deviation below 6%. A mechanism for the formation of these compounds via intramolecular cyclization is also postulated.

Multidimensional gas chromatography methods for bioanalytical research.

Multidimensional gas chromatography (MDGC) methods are high-resolution volatile chemical separation techniques, and comprise classical heart-cutting MDGC and its more recent incarnation, comprehensive 2D GC. Although available for a long period, MDGC approaches are still not widely practiced in the field of bioanalysis, possibly reflecting the general preference for regular GC versus MDGC approaches. With the recent introduction of '-omic' techniques that emphasize global nontargeted profiling of metabolites within living systems, it is evident that MDGC is gaining momentum as a separation tool, since it offers very high resolution. By untangling metabolites within highly complex biological matrices, and expanding the metabolic coverage, MDGC plays a frontline role in '-omics' based studies. This review highlights state-of-the-art MDGC approaches, and summarizes the recent developments in bioanalytics.

Enantiodifferentiation of 3-sec-butyl-2-methoxypyrazine in different species using multidimensional and comprehensive two-dimensional gas chromatographic approaches

With respect to the current hypothesis that natural amino acids may serve as starting material for the biosynthesis of alkyl-methoxypyrazines, the enantiomeric distribution of the potent aroma compound 3-sec-butyl-2-methoxypyrazine (SBMP) was determined in various species using heart-cut multidimensional gas chromatography (H/C MDGC) or comprehensive two-dimensional gas chromatography (GC × GC). Complementary to an earlier described separation on octakis-(6-O-methyl-2,3-di-O-pentyl)-γ-cyclodextrin used as chiral stationary phase, we found a reversal of the elution order of SBMP enantiomers on heptakis-(2,3-di-O-methyl-6-O-tert-butyldimethylsilyl)-β-cyclodextrin, providing further confirmation options for that type of analysis. Optimization of the enantioseparation of SBMP in a single-oven H/C enantio-MDGC system involved the use of a dual-jet cryo modulator for trapping of analytes transferred from the achiral (1)D column to the chiral (2)D column before starting the (2)D enantioseparation with an independent temperature ramp. For the enantiodifferentiation by enantio-GC × GC, the modulation period had to be significantly shortened to avoid loss of chiral resolution gained in (1)D. H/C MDGC with mass spectrometric detection (MS) using selected ion monitoring (SIM) was sufficient for parts per billion level analysis, whereas H/C MDGC-MS/MS or GC × GC time-of-flight (TOF) MS were necessary for parts per trillion level analysis. In various vegetables, lady beetles and Vitis vinifera species analyzed, only (S)-SBMP was detected, supporting the hypothesis of natural amino acids serving as starting material for the biosynthesis of alkyl-methoxypyrazines.

Characterization and analysis of structural isomers of dimethyl methoxypyrazines in cork stoppers and ladybugs (Harmonia axyridis and Coccinella septempunctata).

The three constitutional isomers of dimethyl-substituted methoxypyrazines: 3,5-dimethyl-2-methoxypyrazine 1; 2,5-dimethyl-3-methoxypyrazine 2; and 2,3-dimethyl-5-methoxypyrazine 3 are potent flavor compounds with similar mass spectrometric, gas chromatographic, and nuclear magnetic resonance spectroscopic behavior. Therefore, unambiguous analytical determination is critical, particularly in complex matrices. The unequivocal identification of 1-3 could be achieved by homo- and heteronuclear NMR correlation experiments. The observed mass fragmentation for 1-3 is proposed and discussed, benefitting from synthesized partially deuterated 1 and 2. On common polar and apolar stationary phases used in gas chromatography (GC) 1 and 2 show similar behavior whereas 3 can be separated. In our focus on off-flavor analysis with respect to wine aroma, 1 has been described as a "moldy" off-flavor compound in cork and 2 as a constituent in Harmonia axyridis contributing to the so-called "ladybug taint," whereas 3 has not yet been described as a constituent of wine aroma. A successful separation of 1 and 2 could be achieved on octakis-(2,3-di-O-pentyl-6-O-methyl)-γ-cyclodextrin as stationary phase in GC. Applying heart-cut multidimensional GC analysis with tandem mass spectrometric detection we could confirm the presence of 1 as a "moldy" off-flavor compound in cork. However, in the case of Harmonia axyridis, a previous identification of 2 has to be reconsidered. In our experiments we identified the constitutional isomer 1, which was also found in Coccinella septempunctata, another species discussed with respect to the "ladybug taint." The analysis of such structurally related compounds is a demonstrative example for the importance of a chromatographic separation, as mass spectrometric data by itself could not guarantee the unequivocal identification.

Characterisation of the volatile fraction of aromatic caramel using heart-cutting multidimensional gas chromatography

The first aim of our study was to improve characterisation of the volatile fraction of aromatic caramel by applying heart-cutting multidimensional gas chromatography coupled to mass spectrometry and olfactometry (MDGC–MS–O) on targeted odorant fractions. The second aim was to compare the volatile composition of two caramel samples, which differed in terms of their carbohydrate composition and cooking process. MDGC analyses enabled identification of 37 compounds (17 with the addition of pure standard) in the burnt sugar caramel, 20 of which were reported for the first time in caramel. Fifteen compounds were identified as odour-active and described using a range of attributes such as floral, roasted, spicy and almond. Furans, lactones and acids resulting from the thermal breakdown of sugars predominated in the volatile fraction of the burnt sugar caramel, due to the harsher cooking conditions. Finally, these results have enabled a clearer understanding of aromatic caramel as well as the identification of new compounds which might make an important contribution to its aroma.