Development and Application of a Novel Multiloop Splitter-Based Non-cryogenic Artificial Trapping Modulation System in Comprehensive Two-Dimensional Gas Chromatography.

A multiloop splitter-based non-cryogenic artificial trapping (M-SNAT) modulation technique was established, which applied the first (1D) nonpolar and the second (2D) polar columns, deactivated fused silica (DFS) columns, a microfluidic Deans switch (DS), and splitters located between the 1D column outlet and the DS. The splitters were connected into multiple loops with a progressively doubled perimeter of the next loop. This enabled a duplex splitting mechanism within each loop consisting of splitting of analyte pulses, the pulse delay, and their combination which led to equally split peaks of the same analytes with the number of split peaks (nsplit) equal to 2m (m = number of loops). This system resulted in local profiles of artificially split-and-trapped analytes prior to their selective transfers onto the 2D column by means of periodic multiple heart-cuts (H/C). The developed SNAT approach can be successful, providing that the ratio of modulation period to sampling time (PM/tsamp) is equal to nsplit. The approach with nsplit = 16 was further developed into a single device platform and applied for the modulation of a wide range of compounds in waste tire pyrolysis samples with the RSD of ≤0.01 and <10% for the one-dimensional modulated peak times and peak areas, respectively (n = 50). The method enabled an artificial modulation mechanism without cryogen consumption and enhanced the 2D peak capacity (2nc) and 2D separation by use of a longer 2D column.

Structure Elucidation Using Gas Chromatography-Infrared Spectroscopy/Mass Spectrometry Supported by Quantum Chemical IR Spectrum Simulations.

An improved strategy for compound identification incorporating gas chromatography hyphenated with Fourier transform infrared spectroscopy and mass spectroscopy (GC-FTIR/MS) is reported. (Over)reliance on MS may lead either to ambiguous identity or to incorrect identification of a compound. However, the MS result is useful to provide a cohort of possible compounds. The IR result for each tentative compound match was then simulated using molecular modeling, to provide functional group and isomer differentiation information, and then compared with the experimental FTIR result, offering identification based on both MS and IR. Several basis sets were evaluated for IR simulations; Def2-TZVPP was a suitable basis set and correlated well with experimental data. The approach was applied to industrial applications, confirming the isomers of 2,3-bis(thiosulfanyl)-but-2-enedinitrile, bromination products of 1-bromo-2,3-dimethylbenzene, and autoxidative degradation of phenyl-di-tert-butylphosphine.

Comprehensive Two-Dimensional Gas Chromatography with Mass Spectrometry: Toward a Super-Resolved Separation Technique.

A data interpretation and processing approach for improved compound identification and data presentation in comprehensive two-dimensional gas chromatography (GC×GC) is described. A footprint peak of a compound in 2D space can be represented by a centroid or peak apex, similar to the data-reduced histogram spectra used in mass spectrometry. The workflow was demonstrated on data from GC×GC-TOFMS. Peaks in a modulated chromatogram were initially detected by conventional chromatographic integration, followed by a curve-fitting approach, which interpolated high-precision, absolute retention times for all modulated peaks. First dimension retention time (1tR) was obtained by using an exponentially modified Gaussian (EMG) fitting model for near-Gaussian distributed subpeaks, polynomial fitting for highly asymmetrical peaks, and parabolic fitting for under-sampled peaks, which allows determination of a precise 1tR, considering the dwell-time arising from modulation and 2tR. Area summation of the modulated peaks belonging to the same compound was then performed to yield the total peak area. Each compound in the GC×GC-MS result was then represented by its position at the intersecting coordinates, (1tR, 2tR), in the 2D separation plane, having a height of the same magnitude as the total component summed area. This results in a novel and uncluttered GC×GC output convention based on the scripted total ion chromatogram (TIC) data with precise 1tR, 2tR, and area. Comparison between the contour plots from the scripted and conventional TIC revealed improved data presentation, accompanied by an apparent enhanced resolution. The described approach was applied to the identification of 177 aroma compounds from peaches as indicators of fruit quality.

Gas chromatography-Fourier transform infrared spectroscopy reveals dynamic molecular interconversion of oximes.

This study reports gas chromatography (GC) combined with Fourier transform infrared (FTIR) spectroscopy to investigate the elution profiles of individual oxime isomers undergoing characteristic interconversion (dynamic chromatography) in GC. The use of a light-pipe FTIR interface enables on-line acquisition of FTIR spectra, which in turn render unambiguous identification of the individual molecules. Here, acetaldehyde oxime and propionaldehyde oxime were chosen for comparison of elution behaviour under varying temperature and carrier flow velocities. The choice of selective responses (wavenumber selectivity), which were relatively stronger for each isomer, enabled display and retracing of the individual isomer over the chromatographic time scale and thus provided characteristic single isomer profiles. Chemometric data analysis using the multivariate curve resolution technique further confirmed this isomer elution profile. Simulation of the spectrum for each isomer allowed comparison with instrument-generated FTIR spectra to confirm the elution order of E and Z isomers. The effect of changing chromatographic parameters (temperature, flow) on interconversion rates and/or extents were studied and the corresponding change in FTIR spectrum intensity was noted. The GC-FID data acquired concurrently with GC-FTIR analyses ratified isomerisation chromatographic profiles.

Gas chromatography-mass spectrometry of sapucainha oil (Carpotroche brasiliensis) triacylglycerols comprising straight chain and cyclic fatty acids.

Sapucainha oil, which may be used to treat leprosy, comprises straight chain and cyclic fatty acids (FA), and triacylglycerols (TAG). The FA and TAG content of the oil sample was analysed using gas chromatography-electron ionisation mass spectrometry (GC-EIMS). FA analysis was performed after derivatisation to fatty acid methyl esters (FAME). For free FA and TAG analysis, the oil sample was dissolved in hexane and injected into a short, high-temperature column, for GC with MS analysis. Free FA and FAME were tentatively identified based on mass spectrum information of their molecular and fragment ions, as well as library matching. Overlapping TAG peaks were deconvoluted based on mass fingerprint data. The FA composition was utilised to predict possible TAG identities. FA residues of TAG were identified based on characteristic fragment ions, such as [M-RCO2]+, [RCO+128]+, [RCO+74]+ and RCO+ where R is the aliphatic hydrocarbon chain. FAME analysis showed that the cyclic FA hydnocarpic (36.1%), chaulmoogric (26.5%) and gorlic (23.6%) acids were the major components. In addition, straight chain FA such as palmitic, palmitoleic, stearic, oleic and linoleic acids were detected. Palmitic, oleic, hydnocarpic, chaulmoogric and gorlic acids were also detected as free FA in the oil sample. Six groups of TAG peaks were eluted from GC at temperatures ≥330 °C. After deconvolution and mass spectrum analysis, each TAG peak group was revealed to comprise 2 to 5 co-eluted TAG molecules; >18 TAG were identified. These TAG consisted of a mix of both cyclic and straight chain FA, but were mostly derived from cyclic FA.

Ionic liquid phases with comprehensive two-dimensional gas chromatography of fatty acid methyl esters.

New generation inert ionic liquid (iIL) GC columns IL60i, IL76i and IL111i, comprising phosphonium or imidazolium cationic species, were investigated for separation of fatty acid methyl esters (FAME). In general, the iIL phases provide comparable retention times to their corresponding conventional columns, with only minor selectivity differences. The average tailing factors and peak widths were noticeably improved (reduced) for IL60i and IL76i, while they were slightly improved for IL111i. Inert IL phase columns were coupled with conventional IL columns in comprehensive two-dimensional GC (GC × GC) with a solid-state modulator which offers variable modulation temperature (TM), programmable TM during analysis and trapping stationary phase material during the trap/release (modulation) process, independent of oven T and column sets. Although IL phases are classified as polar, relative polarity of the two phases comprising individual GC × GC column sets permits combination of less-polar IL/polar IL and polar IL/less-polar IL column sets; it was observed that a polar/less-polar column set provided better separation of FAME. A higher first dimension (1D) phase polarity combined with a lower 2D phase polarity, for instance 1D IL111i with 2D IL59 gave the best result; the greater difference in 1D/2D phase polarity results in increasing occupancy of peak area in the 2D space. The IL111i/IL59 column set was selected for analysis of fatty acids in fat and oil products (butter, margarine, fish oil and canola oil). Compared with the conventional IL111, IL111i showed reduced column bleed which makes this more suited to GC × GC analysis of FAME. The proposed method offers a fast profiling approach with good repeatability of analysis of FAME.

Incorporation of Homochirality into a Zeolitic Imidazolate Framework Membrane for Efficient Chiral Separation.

Homochiral metal-organic frameworks (MOFs) have gained much attention because of their chiral properties and disposition for chiral separation. However, the fabrication of high-quality homochiral MOF membranes remains challenging because of the difficulty in controlling growth of MOF membranes with chiral functionalities. A homochiral zeolitic imidazolate framework-8 (ZIF-8) membrane is reported for efficient chiral separation. The membrane is synthesized by incorporating a natural amino acid, l-histidine (l-His), into the framework of ZIF-8. The homochiral l-His-ZIF-8 membrane exhibits a good selectivity for the R-enantiomer of 1-phenylethanol over the S-enantiomer, showing a high enantiomeric excess value up to 76 %.

In Silico Modeling of Hundred Thousand Experiments for Effective Selection of Ionic Liquid Phase Combinations in Comprehensive Two-Dimensional Gas Chromatography.

The selection of the best column sets is one of the most tedious processes in comprehensive two-dimensional gas chromatography (GC × GC) where a multitude of choices of column sets could be employed for an individual sample analysis. We demonstrate analyte/stationary phase dependent selection approaches based on the linear solvation energy relationship (LSER), which is a reliable concept for the study of interaction mechanisms and retention prediction with a large database pool of columns and compounds. Good correlations between our predicted results, with experimental results reported in the literature, were obtained. The developed approaches were applied to the simulation of 157 920 individual experiments in GC × GC, focusing on the application of 30 nonionic liquid and 111 ionic liquid (IL) stationary phases for separation of some example sets of model compounds present in practical samples. The best column sets for each sample separation could then be extracted according to maximizing orthogonality, which estimates the quality of separation.

Continuum in MDGC Technology: From Classical Multidimensional to Comprehensive Two-Dimensional Gas Chromatography.

Recent advances in multidimensional gas chromatography (MDGC) comprise methods such as multiple heart-cut (H/C) analysis and comprehensive two-dimensional gas chromatography (GC × GC); however, clear approaches to evaluate the MDGC results, choice of the most appropriate method, and optimized separation remain of concern. In order to track the capability of these analytical techniques and select an effective experimental approach, a fundamental approach was developed utilizing a time summation model incorporating temperature-dependent linear solvation energy relationship (LSER). The approach allows prediction of optimized analyte distribution in the 2D space for various MDGC approaches employing different experimental variables such as column lengths, temperature programs, and stationary phase combinations in order to evaluate separation performance (apparent (1)D, (2)D, total number of separated peaks, and orthogonality) for simulated MDGC results. The methodology applied LSER to generate results for nonpolar-polar and polar-nonpolar 2D column configurations for separation of 678 compounds in an oxidized kerosene-based jet fuel sample. Three-dimensional plots were generated in order to illustrate the dependency of separation performance on (2)D column length and number of injections for different stationary phase combinations. With a given limit of analysis time, a MDGC approach to obtain an optimized total separated peak number for a particular column set was proposed depending on (1)D and (2)D analyte peak distribution. This study introduces fundamental concepts and establishes approaches to design effective GC × GC or multiple H/C systems for different column combinations, to provide the best overall separation outcomes with the highest separated peak number and/or orthogonality.

Thermally sensitive behavior explanation for unusual orthogonality observed in comprehensive two-dimensional gas chromatography comprising a single ionic liquid stationary phase.

In this study, a theoretical concept and method to achieve a degree of orthogonality in comprehensive two-dimensional gas chromatography (GC × GC) for separation of fatty acid methyl esters (FAME) by using a single ionic liquid (IL) stationary phase (1-phase-GC × GC) were established. The 1-phase system comprises a long IL column and shorter IL column of the same phase before and after the modulation region, operated under temperature-programmed conditions. Initial isothermal experiments employing six commercial IL columns were conducted at different temperatures. On the basis of the temperature-dependent linear solvation energy relationship (LSER) concept, SLB-IL111 exhibited the greatest thermal sensitivity and degree of difference over the tested temperature (T) range, so it was selected for investigation of the 1-phase-GC × GC mode. With the same temperature program, a significantly high degree of orthogonality was observed for the experiment, varied with column lengths. The switchable separation result, which inverts the retention of saturated and unsaturated FAME on the downstream column ((2)D), was achieved by varying column diameters and surface thicknesses of the IL-coated layers. These results were explained according to the corresponding LSER principles. Also, the time summation model was applied for the simulation of the observed 1-phase-GC × GC results.

Two-dimensional retention indices improve component identification in comprehensive two-dimensional gas chromatography of saffron.

Comprehensive two-dimensional gas chromatography hyphenated with accurate mass time-of-flight mass spectrometry (GC × GC-accTOFMS) was applied for improved analytical accuracy of saffron analysis, by using retention indices in the two-dimensional separation. This constitutes 3 dimensions of identification. In addition to accTOFMS specificity, and first dimension retention indices ((1)I), a simple method involving direct multiple injections with stepwise isothermal temperature programming is described for construction of isovolatility curves for reference alkane series in GC × GC. This gives access to calculated second dimension retention indices ((2)I). Reliability of the calculated (2)I was evaluated by using a Grob test mixture, and saturated alkanes, revealing good correlation between previously reported I values from the literature, with R(2) correlation being 0.9997. This essentially recognizes the retention property of peaks in the GC × GC 2D space as being reducible to a retention index in each dimension, which should be a valuable tool supporting identification. The benefit of (2)I data, in supplementing (1)I and MS library matching, was clearly demonstrated by the progressive reduction of the number of possible compound matches for peaks observed in saffron. 114 analytes were assessed according to (1)I and (2)I values within ±20 index unit of reference values, and by MS spectrum matching above a match statistic of 750 (including mass accuracy of the molecular ion <20 ppm) and their possible identities derived. The described method provides a new avenue to utilize the full capability of the two-dimensional separation (GC × GC), in combination with MS library matching in complex sample analysis, to provide improved component identification.

Liquid-phase enantioselective chromatographic resolution using interpenetrated, homochiral framework materials.

Effective separation of mixtures of enantiomers is of continuing interest in analytical and preparative chromatography, with new materials frequently designed and tested. We report two new enantiomerically pure 2D→3D interpenetrated materials used as stationary liquid chromatographic (LC) phases that are shown to resolve selected racemic mixtures with enantiomeric and chemical selectivity. Dicarboxylate ligands derived from amino acids on naphthalene and perylene cores form 2D frameworks that interpenetrate to give 3D structures. Selectivity is initially tested by uptake from solution; subsequent LC methods show that the materials exhibit resolution of racemic analytes in 'micro-columns' and that the two closely related materials show markedly different selectivity for different analytes with much greater activity than the ligands alone. Comparison with a close-packed analogue suggests that the separation activity is largely due to surface effects.

Enantiomeric separation in comprehensive two-dimensional gas chromatography with accurate mass analysis.

Chiral comprehensive two-dimensional gas chromatography (eGC×GC) coupled to quadrupole-accurate mass time-of-flight mass spectrometry (QTOFMS) was evaluated for its capability to report the chiral composition of several monoterpenes, namely, α-pinene, ß-pinene, and limonene in cardamom oil. Enantiomers in a standard mixture were fully resolved by direct enantiomeric-GC analysis with a 2,3-di-O-methyl-6-t-butylsilyl derivatized ß-cyclodextrin phase; however, the (+)-(R)-limonene enantiomer in cardamom oil was overlapped with other background components including cymene and cineole. Verification of (+)-(R)-limonene components based on characteristic ions at m/z 136, 121, and 107 acquired by chiral single-dimension GC-QTOFMS in the alternate MS/MSMS mode of operation was unsuccessful due to similar parent/daughter ions generated by interfering or co-eluting cymene and cineole. Column phases SUPELCOWAX, SLB-IL111, HP-88, and SLB-IL59, were incorporated as the second dimension column ((2)D) in chiral GC×GC analysis; the SLB-IL59 offered the best resolution for the tested monoterpene enantiomers from the matrix background. Enantiomeric ratios for α-pinene, ß-pinene, and limonene were determined to be 1.325, 2.703, and 1.040, respectively, in the cardamom oil sample based on relative peak area data.

Miniaturized molecularly imprinted polymer extraction method for the gas chromatographic analysis of flavonoids.

In this study, the use of monolithic molecularly imprinted polymers in a micropipette tip format allowing the simple and fast extraction of flavonoids from standard solutions and a black tea sample is demonstrated. The imprinted polymer employed quercetin, methacrylic acid or 4-vinylpyridine, and ethylene glycol dimethacrylate as template, functional monomer, and cross-linker, respectively. Surface morphologies of the quercetin-imprinted polymers and the corresponding nonimprinted polymers were characterized by SEM. Extraction of flavonoid standards was performed to evaluate the selectivity and recovery with these imprinted and nonimprinted polymers. Flavonoid compositions in aliquots eluted from the tips were identified using fast GC with flame ionization detection. Maximum specific capacities of 0.2, 5.7, and 16.0 mg/g for catechin, morin, and quercetin, respectively, were obtained with the imprinted polymer prepared with methacrylic acid, with the corresponding recoveries of 99.8, 98.8, and 95.4%, respectively. Efficient extraction by the quercetin-imprinted polymer of epicatechin, catechin, and quercetin from an apple-flavored black tea sample was achieved, with GC-MS employed for compound identification for both the tea and extracted samples.

Data independent acquisition for gas chromatographic MS/MS analysis of volatile compounds.

This study presents a novel tandem mass spectrometry (MS/MS) approach utilizing a data independent acquisition (DIA) concept specifically designed with gas chromatography-electron ionization-triple quadrupole mass spectrometry (GC-EI-QqQMS). This allows compound identification based on comparison between all the experimental MS/MS product ion spectra and the simulated library data of >1,000 MS/MS transitions of 71 compounds. The simulation data were generated by using the Competitive Fragmentation Modeling (CFM-ID) 3.0 program. The approach for calculation of the DIA MS/MS library match scores was then established and applied for identification of a range of terpenoids and oxygenated compounds in perfume. The identity of each peak was confirmed using 4-241 MS/MS transitions. The established data collection and analysis methods are expected to be useful for increased confidence in volatile compound analysis.

GC for flavonoids analysis: Past, current, and prospective trends.

Flavonoids represent one of the more abundant classes of phytochemicals. They are renowned for their health benefits against age-related ailments and diseases. Several chromatography techniques have underpinned many chemical analysis methods, developed for superior flavonoid separation and identification. Among these, GC is one of the most powerful tools in separation science, providing precise measurement of a wide range of flavonoids. Combined with various detectors - most commonly MS-GC offers a sensitive and accurate tool for quantitative and qualitative flavonoids analysis. This review features developments in the application of GC and MS for flavonoids determination during past decades, progressing to recent developments and considering future trends. The review will highlight the state-of-the-art of GC, with opportunities for multidimensional GC analysis also briefly discussed.

Exploring thermal isomerisation in gas chromatography analyses using natural pyrethrins: Comparison of comprehensive two-dimensional and one-dimensional gas chromatography.

This study aims to assess and qualitatively compare the visual presentation of chromatographic data from the isomerisation of natural pyrethrins - a group of pesticides derived from Chrysanthemum flowers - using one-dimensional gas chromatography (1DGC) and comprehensive two-dimensional gas chromatography (GC×GC). Molecular structural changes, such as thermal isomerisation in this case, occur during gas chromatography injection and separation, to provide characteristic patterns which may not be routinely recognised on the 1D chromatogram. To demonstrate the influence of analytical method parameters on isomerisation processes, variations in oven temperature (isothermal vs. temperature programmed analysis), inlet mode (split vs. splitless), inlet temperature, and carrier gas flow rate were investigated. Increasing oven temperature was the most significant factor affecting isomerisation. Splitless injection mode and increasing inlet temperature promoted isopyrethrin formation, while the effect of inlet temperature appeared minimal with a split injection technique, most likely due to the short residence time in the inlet. Increased carrier gas flow rates in a temperature programmed analysis reduced retention time and minimised isomerisation. The unique presentation of isopyrethrin peaks on a GC×GC contour plot allows for facile recognition of isomerisation especially at low concentrations, simplifies chromatogram interpretation, and aids in analyte identification. It also confirms that the isomerisation process is irreversible since the pyrethrin I and II compounds are absent throughout the bridge formation. These benefits support the use of GC×GC over 1DGC to study isomerisation. Additionally, due to limited data in the literature, Kováts retention indices and linear retention indices of the natural pyrethrins, including isopyrethrins, were experimentally determined on four columns: DB-5 ms UI, Rxi-17Sil MS, SLB-IL60i, and SLB-IL111i.

Use and abuse of retention indices in gas chromatography.

The value of the concept of retention indices (RI) to the practice of gas chromatography (GC) is highlighted, where the RI of a compound is one component of the strategy to identify the compound. The widespread reliance on GC and then on mass spectrometry for 'identification', may result in inadequate confirmation of molecular identity. However, RI do provide a useful tentative indication of the possible molecule(s). Thus, the RI value is a useful first measure of the molecule identity, and shown here to be valuable provided limitations are recognised. An author has a responsibility to correctly calculate the index and then use the values for (tentative) identification. Tables of reference RI values are useful in this respect, but finding an 'exact match' RI value does not confirm the identity. Hence, it is necessary to understand how the RI value may be incorrectly used in this respect. The reviewer of written research is charged with ensuring the index values are applied in a rigorous manner. Selected case studies from our own work, support the care that must be exercised when reporting RI values. In terms of advanced GC operations, mention is made of multidimensional gas chromatography and comprehensive two-dimensional gas chromatography to acquire RI values on both the first and second columns in the two-column separation experiment.