Time-resolved cryogenic modulation for targeted multidimensional capillary gas chromatography analysis

Multidimensional gas chromatography (MDGC) is performed in a new manner, described in this paper. The method incorporates two directly coupled columns and employs a longitudinally modulated cryogenic trap located between the columns. No heartcutting process is used, but rather a method better termed selected zone compression pulsing is used. Compared with normal MDGC, where primary column effluent has to be temporarily diverted either to a monitor detector or to the second dimension column, the new procedure in its simplest mode passes all of the first column effluent to the second column. It is simply the times at which the modulation of the trap is performed that determines which target solutes will be selected for enhanced separation. This approach allows almost instantaneous separation of selected zones on the second column, and has the potential to significantly simplify the MDGC method. Since data are presented in a time-response format, and do not require transformation as previously described for comprehensive GC when using the longitudinal modulator, quantitation and report generation are essentially the same as in any GC method and data system. Advantages also include significant sensitivity improvement. By using cryofocussing, and benefiting from the zone compression effects along with fast GC conditions on the second dimension, new possibilities for MDGC can be realised. The method is demonstrated by using a mixture of semi-volatile aromatic hydrocarbons.     

Targeted multidimensional gas chromatography for the quantitative analysis of suspected allergens in fragrance products

Two approaches are described and compared for the analysis of suspected allergens (SAs) in fragrance products, which are defined by the Scientific Committee of Cosmetics and Non-Food Products (SCCNFP). The first consists of a comprehensive two-dimensional gas chromatography (GCxGC) experiment using both a "conventional" non-polar/polar column combination and an "inverse" polar/non-polar column set. The second approach uses a targeted multidimensional gas chromatography (MDGC) system employing a Deans type pneumatic switch and a longitudinally modulated cryogenic system (LMCS). It was found that the conventional and inverse column sets complement each other well, providing identification of SAs present. Compounds well retained on the second dimension of one column set were the first to be eluted from the other. In some instances SAs co-eluting with matrix components on the second dimension for a given column set were clearly resolved on the other, although this has the disadvantage of requiring two analytical runs. Targeted MDGC with a non-polar/polar column set, successfully separated all SAs identified within a fragrance product. The instrument is set up in a similar fashion to a GCxGC system though with longer second dimension ((2)D) column, a cryogenic trap at the beginning of the second column, and a pneumatic switch coupling both columns. The data are easier to process than for a GCxGC experiment. The targeted MDGC method has the capacity to deliver far greater efficiency to targeted regions of a primary separation than a GCxGC experiment, whilst still maintaining overall run times similar to those of a conventional one-dimensional (1D) GC experiment. Cryogenic focussing at the beginning of the (2)D column delivers enhanced sensitivity, accurate (2)D retention times and narrow peak widths; these are responsible for an increased resolution obtained from the fast, relatively short ( approximately 5m) (2)D column. The two column set GCxGC analysis provided a quick and effective means to qualitatively determine the presence of six SAs in a commercially available air freshener, however all were not adequately resolved from matrix components. In contrast, quantitation was straightforward using the targeted MDGC method.     

Development of a switchable multidimensional/comprehensive two-dimensional gas chromatographic analytical system

In this study, a new system for analysis using a dual comprehensive two-dimensional gas chromatography/targeted multidimensional gas chromatography (switchable GC × GC/targeted MDGC) analysis was developed. The configuration of this system not only permits the independent operation of GC, GC × GC and targeted MDGC analyses in separate analyses, but also allows the mode to be switched from GC × GC to targeted MDGC any number of times through a single analysis. By incorporating a Deans switch microfluidics transfer module prior to a cryotrapping device, the flow stream from the first dimension column can be directed to either one of two second dimension columns in a classical heart-cutting operation. Both second columns pass through the cryotrap to allow solute bands to be focused and then rapidly remobilized to the respective second columns. A short second column enables GC × GC operation, whilst a longer column is used for targeted MDGC. Validation of the system was performed using a standard mixture of compounds relevant to essential oil analysis, and then using compounds present at different abundances in lavender essential oil. Reproducibility of retention times and peak area responses demonstrated that there was negligible variation in the system over the course of multiple heart-cuts, and proved the reliable operation of the system. An application of the system to lavender oil, as a more complex sample, was carried out to affirm system feasibility, and demonstrate the ability of the system to target multiple components in the oil. The system was proposed to be useful for study of aroma-impact compounds where GC × GC can be incorporated with MDGC to permit precise identification of aroma-active compounds, where heart-cut multidimensional GC-olfactometry detection (MDGC-O) is a more appropriate technology for odour assessment.     

Practical aspects of Pt/Ir effluent splitters for multidetector GC and pneumatic solute switching

Practical details are given to produce low dead volume Pt/Ir to glass connections and a variety of components for effluent splitting and pneumatic solute switching. It is shown that the column effluents from a glass capillary column can be split in any desired ratio and maintained constant, regardless of column flow rate. Bandbroadening in the splitter remains negligible even for very low flow rates. A complex pneumatic system to be used for heartcutting with two glass capillary columns is shown.     

Enantiomeric separation of chiral polychlorinated biphenyls on beta-cyclodextrin capillary columns by means of heart-cut multidimensional gas chromatography and comprehensive two-dimensional gas chromatography. Application to food samples

Three commercially available chiral capillary columns, Chirasil-Dex, BGB-176SE, and BGB-172, have been evaluated for the separation into enantiomers of the 19 chiral polychlorinated biphenyls (PCB) congeners stable at room temperature. The enantiomers of 15 chiral PCBs were, at least to some extent, separated using these beta-cyclodextrin based columns. Multidimensional techniques, such as heart-cut multi-dimensional gas chromatography (heart-cut MDGC) and comprehensive two-dimensional gas chromatography (GC x GC), were investigated for their ability to solve coelution problems with other PCBs present in commercial mixtures and real-life samples. Heart-cut MDGC improved the separation as compared to one-dimensional GC, and enantiomeric fractions of the investigated chiral PCBs could be determined free from interferences. However, limitations on the number of target compounds that can be transferred to the second column in a single run and, therefore, the time consumption, have led to the evaluation of GC x GC as an alternative for this type of analysis. With GC x GC, two column set-ups were tested, both having a chiral column as first-dimension column, and two different polar stationary phase columns in the second dimension. On using both column combinations, congeners 84, 91, 95, 132, 135, 136, 149, 174, and 176 could be determined free from coelutions with other PCBs. Results on the application of heart-cut MDGC to food samples such as milk and cheese are given, as well as the first results on the application of GC x GC to this type of samples.     

Multidimensional gas chromatography (MDC) in capillary columns using double oven instruments and a newly designed coupling piece for monitoring detection after pre-separation

Summary Eluate transfer between coupled columns by flow switching, backflushing of the pre-column, and other procedures of multidimensional chromatography (MDC) are invaluable techniques to save analysis time to improve resolution, to gain more and better chromatographic information and to prevent contamination of main (high resolution) column and detectors such as ECD and TID. It is of advantage to maintain the coupled columns at different temperatures. Double oven instruments should therefore be used with advantage in multidimensional gas chromatography (MDGC). Flow switching can be done in between and after the system of coupled columns. Also, reactors can be coupled to high resolution columns with advantage [5].Presented at the 14th International Symposium on Chromatography London, September, 1982     

Enantiodifferentiation of α-ketols in sherry by one- and two-dimensional HRGC techniques

The enantiomeric distrubution of solerone (5-oxo-4-hexanolide) 1, ethyl 4-hydroxy- 5-oxohexanoate 2, ethyl 5-hydroxy-4-oxohexanoate 3, 4-oxo-5-hexanolide 4, and solerol (5-hydroxy-4-hexanolide) 5 in sherry wines was determined by several HRGC techniques. While gas chromatography-mass spectrometry on chiral cyclodextrin phases (chiral GC-MS) prevented any racemization of α-ketols 2 and 3 caused by keto-enol tautomerization during analysis, multidimensional gas chromatography MDGC (coupled either by “live-T” switching or by “moving column stream switching” MCSS) led to rearranged constitutional-and stereoisomers. The stereochemical results are discussed regarding the biogenesis of sherry constituents 1–5.     

Multidimensional GC with macrobore WCOT and PLOT Columns

Multidimensional GC (MDGC) with macrobore WCOT and PLOT columns has shown several benefits: (1) Higher flow rates, which are used, makes possible the use of common valves as the switching device; (2) very simple system configurations are possible; (3) because the columns have relatively large capacity, the TCD can be used to measure both organic and inorganic components; (4) micropacked or open tubular columns are precolumns which provide a wide range of selectivity; (5) both precolumn and analytical columns can be operated at the same flow rate without splitting of the sample. Four rapid sample analyses: (1) Oxygenates in gasohol; (2) natural gas; (3) refinery gas; (4) hydrocarbon types in naphtha, have been developed and performed on a less complex and economical gas chromatograph.     

Application of a thermal desorption cold trap injector to multidimensional GC and GC-MS

The Thermal desorption Cold Trap injector (TCT) was used as a part of modified multidimensional GC (MDGC) or MDGC mass spectroscopy (MS) systems. These systems were based on a preparative GC (GC1), an analytical GC (GC2), or GC-MS and the TCT. The TCT was mounted on the GC2 or GC-MS. Analysis was carried out as follows: first, the volatile compounds heart-cut after separation on the GC1 column were adsorbed onto the Porapak Q column out of the GC1 oven. This Porapak Q column was then coupled to the TCT, and the volatile compounds adsorbed on the Porapak Q were thermally desorbed, cold trapped, and injected onto an analytical column in the GC2 or GC-MS. Repeatability of the retention time (RT) and area % of model samples consisting of citronellol, decanol, and geranyl acetate was examined. Also, the volatile compounds present at very low concentrations in ethanol solution were concentrated on the Porapak Q column. These were injected onto the analytical column by the same method as described above, and the repeatability of the RT and area % on the chromatogram was examined. In the two experiments, the standard deviation of the RT and area % for each compound was about 0.02 and less than 2.85, respectively. A commercial geranium oil was successfully analyzed by this technique. The results indicate that this modified MDGC and MDGC-MS system are very useful for detection and determination of compounds in complex mixtures.     

Fast enantiomeric analysis of a complex essential oil with an innovative multidimensional gas chromatographic system

The present research is focussed on the evaluation of a recently developed high performance multidimensional gas chromatographic (MDGC) system employed in the fast analysis of a series of chiral compounds contained in rosemary essential oil. The heart of the MDGC system consists in a simple transfer device for the rapid sequential re-injection of analyte "heart-cuts" from the first to the second dimension. The transfer system has no temperature restrictions, presents very low dead volumes and achieves multidimensional analysis through a pressure-balance mechanism. The MDGC set-up is characterized by two GC ovens (enabling independent temperature programming) and the possibility of mass spectrometric (MS) and/or flame ionization detection (FID). Multiple-cut conventional and fast MDGC-FID methods were developed and the results obtained compared, in order to evaluate the effectiveness of the system. In this respect, the rapid method provided the same analytical result in a greatly reduced time (approximately five times less). Furthermore, quali/quantitative data reproducibilty was very good. Fast MDGC was achieved by using micro-bore (0.1mm I.D.) columns in both dimensions.     

Comprehensive two-dimensional gas chromatography applied to illicit drug analysis

Multidimensional gas chromatography (MDGC), and especially its latest incarnation—comprehensive two-dimensional gas chromatography (GC × GC)—have proved advantageous over and above classic one-dimensional gas chromatography (1D GC) in many areas of analysis by offering improved peak capacity, often enhanced sensitivity and, especially in the case of GC × GC, the unique feature of ‘structured’ chromatograms. This article reviews recent advances in MDGC and GC × GC in drug analysis with special focus on ecstasy, heroin and cocaine profiling. Although 1D GC is still the method of choice for drug profiling in most laboratories because of its simplicity and instrument availability, GC × GC is a tempting proposition for this purpose because of its ability to generate a higher net information content. Effluent refocusing due to the modulation (compression) process, combined with the separation on two ‘orthogonal’ columns, results in more components being well resolved and therefore being analytically and statistically useful to the profile. The spread of the components in the two-dimensional plots is strongly dependent on the extent of retention ‘orthogonality’ (i.e. the extent to which the two phases possess different or independent retention mechanisms towards sample constituents) between the two columns. The benefits of ‘information-driven’ drug profiling, where more points of reference are usually required for sample differentiation, are discussed. In addition, several limitations in application of MDGC in drug profiling, including data acquisition rate, column temperature limit, column phase orthogonality and chiral separation, are considered and discussed. Although the review focuses on the articles published in the last decade, a brief chronological preview of the profiling methods used throughout the last three decades is given.     

Analysis of cigarette smoke by an online thermal desorption system and multidimensional GC-MS

Single puffs of cigarette smoke with a wide continuous range of volatility are directly analyzed using a new system. The system consists of a smoking machine, an online thermal desorption system (TDS), and a multidimensional gas chromatograph-mass spectrometer (MDGC-MS) system. The online TDS with the smoking machine collects the single-puff cigarette smoke with glass beads as the cryogenic adsorbent. The MDGC is composed of three capillary columns, Poraplot Q, and DB-WAX for separation and a deactivated capillary column for pressure balance, which enables simultaneous separation of the two different phases. The smoke desorbed from the TDS is divided into vapor and semivolatile phases and analyzed individually with each column by the MDGC. Thus, the system enables the overall analysis of the two phases simultaneously, including acetaldehyde and 1,4-benzenediol. This system also provides more appropriate analysis for compounds crossing the two phases such as toluene and pyridine. For the approach of introducing internal standards, a gas mixture of toluene-d(8) and o-xylene-d(10) is applied and the compounds are detected in the vapor and semivolatile phases, respectively.     

Analysis of carbamazepine in serum by liquid chromatography with direct sample injection and surfactant-containing eluents

Surfactant-containing eluents are evaluated for the analysis of carbamazepine in serum with conventional reversed-phase columns. Bovine serum was quantitatively eluted at the column void volume using surfactant concentrations in conventional reversed-phase eluents. The effect of pH, guard columns and column switching was evaluated with respect to separating and detecting clinical levels of the drug and its primary metabolite. Column lifetime was also investigated.     

Radio gas chromatography on capillary columns using a multi-column system (column-switching)

Summary A gas chromatographic system with capillary columns (fused silica) for the analysis of radiolabelled compounds is described. The system presented is based on a dual column gas chromatograph equipped with column switching facllity and a variable splitter at the column outlet combined with a dead-volume free adapter for the radioactivity monitor for continous measurement of radioactivity in the column effluent. The first column works as a separation column and the second is roughly shortened and used as a feed to the mass detector. The adjustment of the split ratio is regulated by the inlet pressures for the carrier gas supplying both columns. For mass detection all conventional systems can be used. Detection of radioactivity by a gas proportional counter (system based on a combustion technique). Three flow modes can be adjusted: a) total column effluent to the mass detector or b) to the radioactivity monitor, and c) simultaneous flow (dependent on the chosen split ratio) to mass-and radioactivity detectors. The system was developed for use in clinical chemistry and tested with labelled and unlabelled steroids. The method for peak identification by means of relative retention times and methylene units was possible also for radioactive peaks when a heart cutting technique was used. The radio gas chromatographic system presented allows the development of radiochromatograms with the same peak characteristics as in conventional capillary gas chromatography.Presented at the 14th International Symposium on Chromatography London, September, 1982     

High-speed gas chromatography: an overview of various concepts.

An overview is given of existing methods to minimise the analysis time in gas chromatography (GC) being the subject of many publications in the scientific literature. Packed and (multi-) capillary columns are compared with respect to their deployment in fast GC. It is assumed that the contribution of the stationary phase to peak broadening can be neglected (low liquid phase loading and thin film columns, respectively). The treatment is based on the minimisation of the analysis time required on both column types for the resolution of a critical pair of solutes (resolution normalised conditions). Theoretical relationships are given, describing analysis time and the related pressure drop. The equations are expressed in reduced parameters, making a comparison of column types considerably simpler than with the conventional equations. Reduction of the characteristic diameter, being the inside column diameter for open tubular columns and the particle size for packed columns, is the best approach to increase the separation speed in gas chromatography. Extremely fast analysis is only possible when the required number of plates to separate a critical pair of solutes is relatively low. Reducing the analysis time by reduction of the characteristic diameter is accompanied by a proportionally higher required inlet pressure. Due to the high resistance of flow of packed columns this seriously limits the use of packed columns for fast GC. For fast GC hydrogen has to be used as carrier gas and in some situations vacuum-outlet operation of capillary columns allows a further minimisation of the analysis time. For fast GC the columns should be operated near the conditions for minimum plate height. Linear temperature programmed fast GC requires high column temperature programming rates. Reduction of the characteristic diameter affects the sample capacity of the "fast columns". This effect is very pronounced for narrow-bore columns and in principle non-existing in packed columns. Multi-capillary columns (a parallel configuration of some 900 narrow-bore capillaries) take an intermediate position.     

Some applications of multidimensional gas chromatography to the enrichment of minor components in essential oil analysis

The use of multidimensional gas chromatography (MDGC) for the analysis of essential oils is gaining in importance. A rarely used application consists in the enrichment of minor components through a MDGC system provided with a cold trap between trap column and analytical column. Under suitable conditions, in fact, the cold-trap can store a trapped compound (or a fraction) for a long time. Consequently, the same fraction can be heart-cut from several successive chromatographic runs on the first column and stored together in order to accumulate trace compounds; afterwards the accumulated fraction can be injected in the analytical column. The possibilities of this technique will illustrated through some examples of analysis of complex essential oils.     

Rapid determination of complex mixtures by dual-column gas chromatography with a novel stationary phase combination and spectrometric detection

Fast GC separations of a broad range of analytes are demonstrated using a capillary column coated with a novel immobilized ionic liquid (IIL) stationary phase. Both completely cross-linked and partially cross-linked columns were evaluated, yielding approximately 1600 and approximately 2000 theoretical plates per meter, respectively. Enhanced separation is demonstrated using a dual-column ensemble comprised of an IIL column, a commercially coated Rtx-1 column, and a pneumatic valve connecting the inlet to the junction point between the two columns. Enhanced separation of 20 components, with two sets of co-eluting peaks is shown in approximately 150 s, while sacrificing only a length of time equivalent to the sum of the stop flow pulses, or about 15.5 s. A novel application of a band trajectory model that shows band position as a function of analysis time as analytes move through the column ensemble is employed to determine pulse application times. The model predicts component retention times within a few seconds. Another method of selectivity enhancement of the IIL stationary phase-coated columns is demonstrated using a differential mobility spectrometer (DMS) that provides a second dimension separation based on ion mobility in a high-frequency electrical field. The DMS is able to separate all but one set of co-eluting components from the IIL column. The separation of 13 components found in the headspace above U.S. currency is demonstrated using the IIL column in a dual-column ensemble as well as with the DMS.     

Repeatability in column preparation of a reversed-phase C18 monolith and its application to separation of tocopherol homologues

This work investigated the repeatability of column preparation for a reversed-phase C18 monolith, namely stearyl methacrylate-co-ethylene glycol dimethacrylate (SMA-EDMA). The columns were thermally polymerised using three commonly available heating devices (GC oven, hot air oven and water bath) and their chromatographic performance evaluated using micro-liquid chromatography for separation of five test compounds. Precision in terms of %RSD of retention times were 9.0, 6.5, and 12.5 using GC oven, hot air oven and water bath, respectively. Between-batch precision for the hot air oven (n = 3 days) was less than 10.4% for retention time. The SMA-EDMA monolith was applied to the separation of tocopherol homologues by capillary electrochromatography. Usually tocopherol homologues cannot be completely separated by conventional reversed-phase C8- or C18-packed bed or C18-silica based monolithic columns. Polymer monolith has been shown to give remarkable selectivity towards the tocopherols compared to the conventional microparticulate phase and silica based monolith. Successful separation of the tocopherol isomers was achieved on the SMA-EDMA monolith without any column modification.     

气相色谱法高效快速分析多种燃气组分

采用热导检测器,通过几种柱的组合,采用多维柱切换系统并自动完成阀的驱动和信号切换,实现性质不同的人工煤气和液化石油气主要成分一次进样自动分析,方法操作简便,分离效果好,灵敏度高,结果准确可靠。