The purpose of this review is to highlight the developments in coupled-column gas chromatography methods for qualitative analysis of selected environmental toxicants such as dioxin, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), etc. In particular, the new technique of GC x GC will be introduced, and its role, and the promise it offers to this field is discussed. The benefits of enhanced separation to quantitative analysis will be considered. In order to perform an accurate risk assessment, both the dose and biological effects of environmental toxicants have to be determined with a high degree of certainty. This is most often achieved by using chromatographic methods. Given the complexity of most environmental sample extracts, single-column gas chromatography is unable to fully resolve all the components of interest frequently leading to a positive bias in the reported concentrations. Advanced separation tools, such as multidimensional gas chromatography (MDGC), were investigated quite early and demonstrated improvements in separation. However, limitations in the number of target analytes that could be analyzed in a single run as well as insufficient robustness lead to a continued interest in alternative solutions. The parallel development of mass spectrometric identification and quantification strategies proved useful in many cases, although it frequently failed to provide positive identification of chromatographically unresolved isomeric compounds. More recently, comprehensive two-dimensional gas chromatography (GC x GC) may offer a solution to that problem, especially because it offers enhanced resolution for complex mixtures containing trace level environmental toxicants.

Molecular separations are at the heart of our understanding of the chemical construction of many complex materials. The most powerful methods providing unparalleled capacity for separation of chemical compounds are based on chromatography procedures

Pressurised liquid extraction (PLE) was applied to the extraction of polycyclic aromatic hydrocarbons (PAHs) from contaminated soils from Husarviken in Stockholm, Sweden. The extraction step was followed by conventional gas chromatography (GC), comprehensive two-dimensional gas chromatography (GC x GC) (both with flame ionisation detection) and gas chromatography-quadrupole mass spectrometry (GC-MS) analysis. Qualitative and quantitative aspects of the results are considered. Qualitatively, results from all chromatographic analyses are in good agreement, and PLE provides a reliable extraction technique with all PAHs extracted in one extraction step; no carry over was observed. With respect to PAH quantification, some variability in results was noted, with better agreement in PAH concentrations for GC and GC x GC measurement, as compared to GC-MS. GC analysis compares favourably with GC x GC apart from the few exceptions where peaks are not fully resolved from other co-extracted analytes, which compromises GC-FID measurement. For example, acenaphthene shows a much higher concentration when measured by GC-FID, demonstrating the superior separating powers of GC x GC; the latter is the preferred technique if precise and accurate quantification of analytes are required. GC-MS results compare reasonably with GC x GC for low-molecular mass PAHs but not for high-molecular mass PAHs; results for GC-MS are consistently higher than those for GC x GC for high-molecular mass PAHs. Since PLE-GC x GC is proposed as a broad screening tool, the demand for precise quantification may be relaxed in the present situation. GC x GC has the added advantage of providing chemical structural information within the two-dimensional contour presentation. Reproducibilities for GC x GC results (peak area) and 2tR were acceptable with relative standard deviations (R.S.D.) of 8 and 1%, respectively (at the mg/kg level), and good repeatability within samples was achieved.

Comprehensive gas chromatography (GC×GC) is now established as a powerful technique, which offers unprecedented separation power. For complex samples, the distribution of peaks in the two-dimensional (2D) space still may need to be optimised. Since temperature (T) is a critical variable, and compounds can be shifted in relative positions on column 1 arising from temperature program rate (rT) changes, and since retention in the second dimension, D2 (2tR) is likewise affected by the prevailing T (elution temperature; Te), then any factors which alter Te will affect the extent of separation in D2. Since temperature program and carrier gas velocity rate will affect the Te of the solutes, these two factors are considered in this paper. Apart from these two parameters, results of different stationary phase choice for the second dimension column as well as the second dimension column length are reported. rT is found to have the most profound impact on the Te of solutes and will be more likely to cause an inversion of elution order if such behaviour can occur. Peak widths and 2tR increase with a decrease in Te. On the other hand, flow-rate has less impact on peak widths and 2tR although Te is affected by a change in flow-rate. Specific solute–stationary phase interactions will cause the elution order of certain solutes to be altered, and may be observed when a different stationary phase is employed as the second column, depending on the solute–stationary phase separation mechanism. Experiments conducted on different second dimension column length showed that although a longer column will lead to better separations, wrap-around may confound the separation process and may cause the solutes from sequential modulation events to co-elute. Thus a suitable second dimension column phase and length must be employed in order to obtain good separation. The factors investigated in this study will cause different extents of changes in the solute elution order and solute separations, and will affect the 2D contour presentation.

A number of toxic planar PCBs, that is, PCB 77, 105, 118, 126, 156, and 169, were successfully separated from other PCB congeners present in technical PCB formulations using comprehensive GC X GC. The planar PCBs were selectively retained using a liquid crystal column, which was used as the first-dimension column. A short and narrow bore (0.25 In. X 0.1 mm) nonpolar 5%-phenyl-methylpoly-siloxane column was used as the second-dimension column. A longitudinally modulating cryogenic system (LMCS) was used to modulate the first-dimension signal. Since the planar PCBs elute from the first column at relatively high oven temperatures they were rapidly eluted from the second column. Altogether, this resulted in a significant peak sharpening, and a 20-fold increase in the signal-to-noise ratio, as compared to standard one-dimensional GC. This column set also seems to separate seven frequently measured congeners, that is, PCBs 28, 52, 101, 118, 138, 153, and 180, from other major PCB congeners. (C) 2001 John Wiley & Sons, Inc.

Comprehensive two-dimensional gas chromatography (GC x GC) often incorporates a conventional capillary gas chromatography column in combination with a fast elution column. If this latter column has a thin stationary phase film, it may be supposed that the analyst risks loss of peak resolution due to the onset of non-linear conditions. This will normally be evidenced by overloading - or fronting - peak shapes. GC x GC methods which result from zone compression will be even more likely to exhibit overloading on the second column since the instantaneous peak concentration (CM and Cs), will be significantly greater than that of the band which enters the modulation region. Two cases of overloading may be considered - that on the first column, and that on the second column. Each of these cases will lead to different two-dimensional peak shapes when the GC x GC result is converted to a two-dimensional separation space. In each instance, however, resolution of components will be degraded, although the GC x GC experiment will still not be as badly affected as a single dimensional GC analysis. When overloading conditions are obtained, the GC x GC peak pulsing phenomenon will result in the extremities of the chromatographic band showing linear conditions even though the centre of the band is non-linear.

The two current technologies for achieving comprehensive gas chromatography (GC x GC) - the thermal sweeper and the cryogenic modulator - are compared in an interlaboratory study using a multicomponent semi-volatile aromatic compound sample, The same column set (phases, film thickness, dimensions of columns) and conditions of oven temperature program were used, Carrier gas flow settings however were different for the data reported here. The thermal sweeper has a longer overall length due to the extra ca. 30 cm length of narrow bore tubing used for the modulator/accumulator section. Data reveal that the two methods behave in an analogous manner in respect of delivering GC x GC results, with key peak parameters of peak widths and symmetry measures showing good correlation Retention time dissimilarity on the first dimension columns in the two systems arises from different flow rates used, however the second column retention is similar, and this is due to the resulting different elution temperatures that peaks elute on the first dimension in each system. Overall, the two approaches to GC x GC appear to produce equivalent results within the scope of the application studied. Each system does have its experimental limitations; the thermal sweeper has what may be called a 'thick film effect', where at high temperature it can be difficult to sufficiently trap the migrating bands in the accumulator column, and the pulsing of solutes in the cryogenic system may suffer from a 'thick wall effect' if a column with too thick a wall dimension is used at low oven temperature.

The new technique of comprehensive gas chromatography (GC x GC) is applied to the analysis of Vetiver essential oil, and the complex nature of the oil components is readily displayed in a two-dimension separation space. The GC x GC method involves the use of two directly coupled columns, with a cryogenic modulation system at their confluence; this allows coeluting peaks from the first column to be zone compressed and pulsed rapidly into the second column and separated. Greater sensitivity of detection is also obtained. Results show that the earlier eluting components from the gas chromatography analysis have similar chemical nature in respect of component polarity, being of low-to-moderate polarity. Later eluting compounds appear to be more polar, and are presumably of an oxygenated nature. Many of these show extensive co-elution, with up to 6-8 overlapping compounds on the first column. These results clearly demonstrate that the complexity of essential oils is much greater than that which might be suspected from single column GC analysis, and also suggest that GC-MS analysis with peak deconvolution capabilities will still probably not be able to adequately identify the interfering components. With GC x GC, minor constituents in the presence of larger components may still be well characterized and quantitated, and from the chromatographic retention patterns their chemical nature might be indicated; this would be aided by establishing the retention behaviour of known compounds. Copyright (C) 2000 John Wiley & Sons, Ltd.