Two dimensional GC

Most developments in the past two decades, however, have involved coupled column systems which are much more amenable to automation and more readily permit quantitative measurements, and such systems form the subject of this present book. A review on two-dimensional GC was published (43) in 1978 (and recently updated (29)), and the development by Liu and Phillips in 1991 of comprehensive 2D GC marked a particular advance (33). The fundamentals of HPLC-GC coupling have been set out (37) with great thoroughness by Grob. Other work on a number of other aspects of multidimensional chromatography have also been extensively reviewed (44,45). 

In common with all multidimensional separations, two-dimensional GC has a requirement that target analytes are subjected to two or more mutually independent separation steps and that the components remain separated until completion of the overall procedure. Essentially, the effluent from a primary column is reanalysed by a second column of differing stationary phase selectivity. Since often enhancing the peak capacity of the analytical system is the main goal of the coupling, it is the relationship between the peak capacities of the individual dimensions that is crucial. Giddings (2) outlined the concepts of peak capacity product and it is this function that results in such powerful two-dimensional GC separations. 

The ultimate extension of two-dimensional GC was introduced in the early 1990s (4) and involves the reanalysis of all components from the primary dimension on a secondary column. To enable this, the peak capacity of the secondary column is often very much smaller than the primary thus allowing completion of the separation in a time that may be considered insignificant as a fraction of the time required to complete the primary stage. This is discussed in much further detail in Chapter 4. 

Pigure 3.1 shows several potential on-line modes of two-dimensional GC operation. These couplings demonstrate HRGC-HRGC performed by using a single heart-cut from the primary to the secondary column, multiple heart-cuts, transferred to multiple intermediate traps, and heart-cuts transferred to a multiple parallel secondary column configuration. 

Although the ability to generate separation systems with significantly enhanced peak capacities is the most obvious practical usage of two-dimensional GC, there are several ancillary benefits which are often also achieved when analysis is performed using this approach. 

The non-intrusive manipulation of carrier gas effluent between two columns clearly has significant advantages in two-dimensional GC. In addition, a pressure-driven switch between the columns introduces no extra band broadening to an eluting peak. 

What is common to all of these areas is that the relevant number of published GC-GC papers is very small when compared to those concerning single-column and GC-MS methods. While approximately 1000 papers per year are currently published on single-column GC methods and, in recent years, nearly 750 per year on GC-MS techniques, only around 50 per annum have been produced on two-dimensional GC. Of course, this may not be a true reflection of the extent to which two-dimensional GC is utilized, but it is certainly the case that research interest in its application is very much secondary to that of mass spectrometric couplings. A number of the subject areas where two-dimensional methods have been applied do highlight the limitations that exist in single-column and MS-separation analysis. 

The production of petrochemicals from feedstocks has also been an area of wide application of two-dimensional GC. A detailed knowledge of the composition of feedstocks and intermediates in the manufacturing process is central to obtaining... 

Figure 3.5 Two-dimensional GC analysis of tobacco essential oil using non-polar primary and polar secondary separ-ations. The top tr-ace indicates the primary separ-ation, with the four resulting heart-cut cliromatograms shown below being obtained on the transfer of approximately 1-2 min fractions of primary eluent. Reproduced from B.M. Gordon et al. J. Chwmatogr. Sci. 1988, 26, 174 (23).

The introduction of synthetic materials into natural products, often described as adulteration , is a common occurrence in food processing. The types of compounds introduced, however, are often chiral in nature, e.g. the addition of terpenes into fruit juices. The degree to which a synthetic terpene has been added to a natural product may be subsequently determined if chiral quantitation of the target species is enabled, since synthetic terpenes are manufactured as racemates. Two-dimensional GC has a long history as the methodology of choice for this particular aspect of organic analysis (38). 

Isolated problems of racemization, rearrangement or dehydration should not overshadow the fact, however, that the range of species amenable to enantiomeric two-dimensional GC is very wide indeed, including not only terpenes and lactones. 

The study of biochemical natural products has also been aided through the application of two-dimensional GC. In many studies, it has been observed that volatile organic compounds from plants (for example, in fruits) show species-specific distributions in chiral abundances. Observations have shown that related species produce similar compounds, but at differing ratios, and the study of such distributions yields information on speciation and plant genetics. In particular, the determination of hydroxyl fatty acid adducts produced from bacterial processes has been a successful application. In the reported applications, enantiomeric determination of polyhydroxyl alkanoic acids extracted from intracellular regions has been enabled (45). 

It is in the study of this phenomenon where two-dimensional GC offers by far the most superior method of analysis. The use of chiral selector stationary phases, in particular modified cyclodextrin types, allows apolar primary and atropisomer selective secondary separation. Reported two-dimensional methods have been successful... 

Figure 13.4 Two-dimensional GC separation of isoprene, interference hydrocarbons, and DMS, with the inset showing the one-dimensional separation of isoprene and interference hydrocarbons for comparison. Reprinted from Environmental Science and Technology, 31, A. C. Lewis et ai, High-speed isothermal analysis of atmospheric isoprene and DMS using online two-dimensional gas cliromatography , pp. 3209-3217, copyright 1997, with permission from the American Chemical Society.

Although comprehensive two-dimensional gas chromatography has not been applied to any great extent in forensic analysis, the technique shows great promise when samples or sample matrices are complex. For example, when oil is spilled into waterways, assigning responsibility for the economic and environmental damage is often difficult. Gaines et al. employed comprehensive two-dimensional GC in the forensic analysis of samples collected at oil-spill sites and were able to obtain results which were comparable to those obtained by classical methods (39). This article also... 

Comprehensive two-dimensional GC has also been employed for the analysis of pesticides from serum, which, although not strictly a forensic analytical problem , provides an example of the promise of this technique to forensic applications, such as the analysis of drugs of abuse (40). Two-dimensional gas chromatograms of a 17-pesticide standard and an extract from human serum are shown in Figure 15.13. The total analysis time of about 5 min, high peak capacity and the separation of all... 

Figure 15.13 Comprehensive two-dimensional GC chromatogram of a supercritical fluid exti act of spiked human semm. Peak identification is as follows 1, dicamha 2, tiifluralin 3, dicliloran 4, phorate 5, pentachlorophenol 6, atrazine 7, fonofos 8, diazinon 9, cWorothalonil 10, terhufos 11, alachlor 12, matalaxyl 13, malathion 14, metalochlor 15, DCPA 16, captan 17, folpet 18, heptadecanoic acid. Adapted imm Analytical Chemistry, 66, Z. Liu et al., Comprehensive two-dimensional gas chromatography for the fast separation and determination of pesticides exuacted from human senim , pp. 3086-3092, copyright 1994, with pemiission from the American Chemical Society.

The starting configuration for a two-dimensional GCS. Each unit stores a vector of weights. 

Figure 1 Separation of the standard Grob mix using two-dimensional GC/MS. (See Color Plate Section at the end of this book.)...

M. Forrest, S. Holding and D. Howells, The use of two-dimensional GC-MS for the identification and quantification of low molecular weight compounds from high performance elastomers, Polym. Testing, 25 (2006) 63-74. 

Two-dimensional GC can be used to separate complex mixtures of polyaromatic compounds, and MS used to subsequently identify the compounds. In this method, the original sample is injected into a gas chromatograph with one type of column. As the components exit the first GC, they are fed into a second GC, with a different column, for further separation and finally into a mass spectrometer. In this way, compounds that coeluted from the first column are separated on the second. Focant et al. [19] were able to separate polychlorinated dibenzo-p-dioxin (PCDD), polychlorinated dibenzofuran (PCDF), and coplanar polychlorinated biphenyl (cPCB) using this type of analytical procedure, including isotope dilution TOF-MS. These compounds are frequently found as contaminants in soils surrounding industrial settings thus, the ability to separate and identify them is extremely important [6,12,19],... 

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