Flow regime at ambient outlet pressure and its influence in comprehensive two-dimensional gas chromatography.

With method development in one-dimensional GC already being a tedious task, developing GC x GC methods is even more laborious. The majority of the present GC x GC applications are derived from previously optimised 1D-GC methods, from which especially the carrier gas flow settings are copied. However, in view of the high pressure inside the first-dimension column (high flow resistance of the narrow-bore second-dimension column), diffusion in the first column is much slower than in 1D-GC. Proper optimisation of the column combination and the carrier gas flow can considerably improve separations in GC x GC. To assist in the process of selecting column dimensions and flow rate optimization, we have developed a computer programme, based on Excel, that enables quick and simple calculation for all types of column combinations. The programme merely needs column dimensions and carrier gas type as input parameters and calculates all resolution and velocity parameters of the GC x GC separation by using flow rate and plate height equations. From the calculations a number of interesting conclusions can be drawn. As an example, the calculations clearly show that the majority of column combinations reported up till now have been operated at a far from optimal flow -- and, consequently, a far from optimal resolution. Probably even more important is the conclusion that the majority of column combinations used so far, i.e. those with 100 microm I.D. second-dimension columns, are not necessarily the best choice for GC x GC.

Twin comprehensive two-dimensional gas chromatographic system: concept and applications.

A twin GC x GC system has been designed which enables the analysis of a sample by means of two different and independent column combinations simultaneously. Both combinations are incorporated in the same oven, using the same temperature programme, and are fed using a 50:50 column-entrance-split. It is demonstrated that, employing combinations of a conventional non-polar x polar and a reversed-type polar x non-polar column set, the information content is as high, and the analytical performance is as good as when using two separate GC x GC systems. That is, there is an appreciable gain of time and a reduction of costs without any loss of quality. The general usefulness of performing, and comparing two mutually different GC x GC runs is further illustrated with FAMEs in olive oil, and pollutants in a sediment sample.

Comprehensive two-dimensional gas chromatography: a powerful and versatile analytical tool.

Comprehensive two-dimensional gas chromatography (GC x GC) is a novel technique which is rapidly gaining importance for the analysis of complex samples. In the present review, attention is devoted to the principle and advantages, and main characteristics such as modulation, column combinations, detector requirements and data processing, of the technique. Specifically, GC x GC of a variety of real-life samples is discussed to demonstrate the applicability of the technique, with emphasis on the usefulness of the ordered-structure principle and on the analyte-identification power provided by a combination with time-of-flight mass spectrometric detection.

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometric detection applied to the determination of pesticides in food extracts.

The separation provided by conventional gas chromatography (1D-GC) can be significantly enhanced by using comprehensive two-dimensional GC (GC X GC) instead. Combination with mass spectrometric detection is desirable for unambiguous confirmation of target compounds and the provisional identification of unknowns. A GC X GC system using a cryogenic modulator was coupled to a time-of-flight mass spectrometric (TOF MS) detector. With the determination of pesticides in vegetable extracts as an example, it was demonstrated that GC X GC improves the separation dramatically. All 58 pesticides of interest could be identified using their full-scan mass spectra, which was not possible when using ID-GC-TOF MS. In addition, the high scan speed of the TOF MS allowed the deconvolution of compounds partly co-eluting in GC X GC.

Comprehensive two-dimensional gas chromatography with atomic emission detection and correlation with mass spectrometric detection: principles and application in petrochemical analysis.

Comprehensive gas chromatography (GC x GC) has been combined with atomic emission detection (AED) to enable element-selective detection. Under optimised experimental conditions, the requirement of minimum five data points across a peak can be obtained even for analytes eluting early from the second-dimension column. Simple manipulation of the results allows the combined presentation of up to four sets of elemental data in one two-dimensional plot. GC x GC with AED and mass spectrometric (MS) detection in petrochemical analysis for fingerprinting as well as the identification of N- and S-containing unknowns is presented as an application.

Comprehensive two-dimensional gas chromatography of complex samples by using a 'reversed-type' column combination: application to food analysis.

The practicability and potential of a non-orthogonal approach in comprehensive two-dimensional gas chromatography (GC x GC) were studied and compared to those of the orthogonal approach for two different complex matrices, and using conventional flame ionisation (FID) and time-of-flight mass spectrometry (ToF MS) detection. The separation of a diesel oil showed that the non-orthogonal approach also provides interesting, but completely reversed, ordered structures. For the more extensively studied flavour analysis in food samples, improved peak shapes and, also, different types of ordered structures and retention behaviour, and improved detectability for polar compounds make the two approaches complementary to each other. As a consequence, identification and/or determination of targets and/or unknowns can be performed more reliably. Analytical performance (close to three-order linearity; LODs, 2-30 pg injected in most cases; R.S.D.s, 1-6% (n = 6)) was fully satisfactory.

Unravelling the composition of very complex samples by comprehensive gas chromatography coupled to time-of-flight mass spectrometry. Cigarette smoke.

The potential and current limitations of comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC x GC-TOF-MS) for the analysis of very complex samples were studied with the separation of cigarette smoke as an example. Because of the large number of peaks in such a GC x GC chromatogram it was not possible to perform manual data processing. Instead, the GC-TOF-MS software was used to perform peak finding, deconvolution and library search in an automated fashion; this resulted in a peak table containing some 30000 peaks. Mass spectral match factors were used to evaluate the library search results. The additional use of retention indices and information from second-dimension retention times can substantially improve the identification. The combined separation power of the GC x GC-TOF-MS system and the deconvolution algorithm provide a system with a most impressive separation power.

Evaluation of modulators and electron-capture detectors for comprehensive two-dimensional GC of halogenated organic compounds.

Different cryogenic and a heated GC x GC modulator(s) were evaluated and compared for the analysis of high-boiling halogenated compounds. The cryogenic modulators investigated were: (i) the longitudinally modulated cryogenic system; (ii) the liquid-nitrogen-cooled jet modulator (KT2001); (iii) a dual-jet CO2 modulator (made in-house); (iv) a semi-rotating cryogenic modulator (made in-house) and (v) a CO2 loop modulator (KT2003); the heated modulator was the slotted heater system (sweeper). Each modulator was optimised with respect to analyte peak widths at half height in the second-dimension. n-Alkanes, chlorinated alkanes, polychlorinated biphenyls (PCBs) and fluorinated polycyclic aromatic hydrocarbons (F-PAHs) were used as test analytes. The flow rate of the coolant was found to be an important parameter, i.e. the flow rate of the gaseous nitrogen in the KT2001, and of the liquid CO2 in the other cryogenic modulators. For the slotted heater the stroke velocity and pause time were important parameters. This modulator had a limited application range in terms of temperature due to a necessary 100 degrees C difference between sweeper and oven temperature. All cryogenic modulators were found to be suitable for the GC x GC analysis of high-boiling compounds, but the CO2 modulators are to be preferred to the KT2001 due to a wider application range and slightly narrower peaks. As regards the performance of three commercially available electron-capture detectors (ECDs), the aim was to obtain narrow peak widths in GC x GC, i.e. to avoid band broadening caused by the cell volume. The most important parameters were the flow rate of the make-up gas and the detector temperature which both should be as high as possible. Comparison of analyte peak widths obtained with ECD mode and flame ionisation detection (FID) showed that all ECDs exhibited band broadening compared to the FID. The narrowest peaks were obtained with the Agilent micro-ECD, which has a cell volume of only 150 microl.

Comprehensive two-dimensional gas chromatography with time-of-flight mass spectrometric detection for the trace analysis of flavour compounds in food.

The practicability and potential of comprehensive two-dimensional gas chromatography coupled to time-of-flight mass spectrometry (GC x GC-TOF-MS) for the analysis of complex flavour mixtures in food were studied. With the determination of key flavour targets in dairy samples as an example, it was demonstrated that GC x GC dramatically improves the separation. As a consequence, identification and, more importantly, quantification down to the ng/g level can be performed more reliably: background interferences largely disappear. Next to the peak table generated from the GC-TOF-MS software after data processing, the additional use of well-ordered patterns in the 2D-plane and information from second-dimension retention times can substantially help the identification of unknowns. The technique was successfully used for an evaluation of extraction techniques and the characterisation of different types of samples.

A comparison of ten different methods for the analysis of saturates, olefins, benzene, total aromatics, and oxygenates in finished gasolines.

The Technical Committee 19 of the European Committee for Standardization (CEN/TC19) in the "Lisbon resolution" requested to evaluate replacement methods to the fluorescent indicator absorption (FIA) (American Standardization for Testing and Material D 1319) method for the determination of aromatics and olefins in gasolines. In the same resolution it was requested to review the two existing methods for the determination of benzene content of gasolines, anticipating lower limit values in future European gasoline specifications. As a result of this request, a round robin (RR) was organized in which 8 gasoline samples are analyzed using 10 different methods in 33 laboratories. The methods used in the RR include, apart from the FIA method, one-dimensional gas chromatography (GC) and multidimensional GC, with and without specific detectors and spectroscopic analysis methods. This study describes these methods in short, gives an evaluation of the results of the RR, and draws a conclusion on the outcome.

Rapeseed oil methyl ester pyrolysis: on-line product analysis using comprehensive two-dimensional gas chromatography.

Thermochemical conversion processes play a crucial role in all routes from fossil and renewable resources to base chemicals, fuels and energy. Hence, a fundamental understanding of these chemical processes can help to resolve the upcoming challenges of our society. A bench scale pyrolysis set-up has been used to study the thermochemical conversion of rapeseed oil methyl ester (RME), i.e. a mixture of fatty acid methyl esters. A GC×GC, equipped with both a flame ionization detector (FID) and a time-of-flight mass spectrometer (TOF-MS), allows quantitative and qualitative characterization of the reactor feed and product. Analysis of the latter is accomplished using a dedicated high temperature on-line sampling system. Temperature programmed analysis, starting at -40°C, permits effluent characterization from methane up to lignoceric acid methyl ester (C(25)H(50)O(2)), in a single run of the GC×GC. The latter combines a 100% dimethylpolysiloxane primary column with a 50% phenyl polysilphenylene-siloxane secondary column. Modulation is started when the oven temperature reaches 40°C, thus dividing the chromatogram in a conventional 1D and a comprehensive 2D part. The proposed quantification approach allows to combine the quantitative GC×GC analysis with 2 other on-line 1D GC analyses, resulting in a complete and detailed product composition including the measurement of CO, CO(2), formaldehyde and water. The GC×GC reveals that the product stream contains a huge variety of valuable products, such as linear alpha olefins, unsaturated esters and aromatics, that could not have been identified and quantified accurately with conventional 1D GC because of peak overlap.

Comprehensive two-dimensional gas chromatography for the analysis of synthetic and crude-derived jet fuels.

Fully synthetic jet fuel (FSJF) produced via Fischer-Tropsch (FT) technology was recently approved by the international aviation fuel authorities. To receive approval, comparison of FSJF and crude-derived fuel and blends on their qualitative and quantitative hydrocarbon composition was of utmost importance. This was performed by comprehensive two-dimensional gas chromatography (GC×GC) in the reversed phase mode. The hydrocarbon composition of synthetic and crude-derived jet fuels is very similar and all compounds detected in the synthetic product are also present in crude-derived fuels. Quantitatively, the synthetic fuel consists of a higher degree of aliphatic branching with less than half the aromatic content of the crude-derived fuel. GC×GC analyses also indicated the presence of trace levels of hetero-atomic impurities in the crude-derived product that were absent in the synthetic product. While clay-treatment removed some of the impurities and improved the fuel stability, the crude-derived product still contained traces of cyclic and aromatic S-containing compounds afterwards. Lower level of aromatics and the absence of sulphur are some of the factors that contribute to the better fuel stability and environmental properties of the synthetic fuel. GC×GC was further applied for the analysis of products during Jet Fuel Thermal Oxidation Testing (JFTOT), which measures deposit formation of a fuel under simulated engine conditions. JFTOT showed the synthetic fuel to be much more stable than the crude-derived fuel.

On-line analysis of complex hydrocarbon mixtures using comprehensive two-dimensional gas chromatography.

This paper discusses the first setup for on-line qualitative and quantitative comprehensive two-dimensional gas chromatography (GC × GC) of complex hydrocarbon mixtures. A built-in 4-port 2-way valve allows switching between flame ionization detection (FID) and time-of-flight mass spectrometry (TOF-MS) between runs, without the need to cool down and vent the MS. Proper selection of GC carrier gas flow rates enables maximal agreement between the obtained chromatograms in both configurations. For on-line analysis of reactor effluents, a dedicated sampling system allows automatic sampling of the hot reactor effluent gases and immediate injection of the sample on the GC × GC. To determine a complete effluent composition in a single run of the GC × GC, a subzero oven starting temperature was employed. Modulation is started when the oven temperature reaches 40°C, thus dividing the chromatogram in a conventional 1D and a comprehensive 2D part. This work illustrates the mature and robust character of GC × GC, extending its capabilities from mere laboratory use to on-line routine analysis for industrial processes in the (petro-)chemical industry.

Comprehensive two-dimensional gas chromatography--a powerful and versatile technique.

Comprehensive two-dimensional gas chromatography (GC x GC) was introduced about a decade ago and is, today, rapidly become a powerful and widely applicable technique for the characterisation and analysis of a variety of complex samples. GC x GC provides structurally ordered chromatograms in the 2D plane, and features an impressive peak capacity. Combining GC x GC and time-of-flight mass spectrometry (ToFMS), with its fast acquisition rates and excellent deconvolution potential, has been an important step forward which enables identification and identity confirmation of large numbers of target analytes and unknowns in one run. Up until now, some 150 papers have been published, fish and food, environmental and air analysis. Polyhalogenated aromatics and other priority pollutants, flavours and fragrances, fatty acid methyl esters and essential oils, are among the classes of compounds which frequently receive attention. In summary, GC x GC is increasingly being recognised as the separation technique of choice in problem areas.