Multidimensional Chromatography

38	Multidimensional Chromatography

Figure 2.17 Schematic representation of the set-up used for on-line liquid–liquid extraction coupled with capillary GC when using a membrane phase separator. Reprinted from Journal of High Resdution Chromatography, 13, E. C. Goosens et al., ‘Determination of hexachlorocyclohexanes in ground water by coupled liquid–liquid extraction and capillary gas chromatography’, pp. 438 – 441, 1990, with permission from Wiley-VCH.

Figure 2.21 shows the on-line extraction gas chromatogram of 2.25 ml of water spiked at 5 ppb levels with 14 different organic pollutants (40). In this case, the authors concluded that wall-coated open tubular traps (thick-film polysiloxane phases) can be used for the on-line extraction of organic compounds from water. However, when using swelling agents such as pentane, non-polar analytes can be trapped quantitatively, while for more polar compounds chloroform is the most suitable solvent.

2.6CONCLUSIONS

Coupled liquid chromatography–gas chromatography is an excellent on-line method for sample enrichment and sample clean-up. Recently, many authors have reviewed in some detail the various LC–GC transfer methods that are now available (1, 43–52). For the analysis of normal phase eluents, the main transfer technique used is, without doubt, concurrent eluent evaporation employing a loop-type interface. The main disadvantage of this technique is co-evaporation of the solute with the solvent,

Coupled HPLC with HRGC

39

Figure 2.18 (a) Gas chromatogram of a standard solution of various hexachlorocyclohexanes (HCHs) in water, obtained after on-line isooctane extraction: 1, -HCH; 2, -HCH; 3, - HCH; 4, -HCH. (b) Gas chromatogram obtained for a reference blank (distilled water) after the same on-line extraction treatment. Reprinted from Journal of High Resolution Chromatography, 13, E. C. Goosens et al., ‘Determination of hexachlorocyclohexanes in ground water by coupled liquid – liquid extraction and capillary gas chromatography’, pp. 438 – 441, 1990, with permission from Wiley-VCH.

thus leading to the loss of the more volatile components. If more-volatile compounds have to be analysed, then transfer from the LC unit to the GC unit is best achieved by using retention gap techniques. Due to the solvent effects explained above, this technique allows the analysis of compounds which elute immediately after the solvent peak. The main drawback of this approach is that it is restricted to the transfer of small fractions, due to the limited capacity of the uncoated precolumns. Larger fractions can be transferred by partial concurrent evaporation, which still retains the advantages of the retention gap technique. Indirect injection of water-containing eluents seems to be the appropriate choice for the analysis of such samples (LLE, SPE and OTT). However, direct injection of water via a vaporizer chamber/precolumn solvent split/gas discharge interface seems to be a promising technique for transfering reversed-phase eluents.

40

Figure 2.19 Schematic representation of an on-line liquid–liquid extraction – GC/AED system. Reprinted from Journal of High Resolution Chromatography, 18, E. C. Goosens et al., ‘Continuous liquid–liquid extraction combined on-line with capillary gas chromatography – atomic emission detection for environmental analysis’, pp. 38 – 44, 1995, with permission from Wiley-VCH.

Chromatography Multidimensional

Coupled HPLC with HRGC

41

Figure 2.20 Schematic representation of the set-up used for on-line extraction – GC: V1 and V2, valves; P1 and P2, syringe pumps; L, sample loop; CC flow, countercurrent flow; CT, cold trap. Reprinted from Journal of High Resolution Chromatography, 16, H. G. J. Mol et al.,

‘Use of open-tubular trapping columns for on-line extraction – capillary gas chromatography of aqueous samples’, pp. 413 – 418, 1993, with permission from Wiley-VCH.

Figure 2.21 A gas chromatogram of a sample of river water (2.25 ml) spiked at 5 ppb levels with: 1, toluene; 2, ethylbenzene; 3, methoxybenzene; 4, p-dichlorobenzene; 5, dimethylphenol; 6, dimethylaniline; 7, chloroaniline; 8, indole; 9, dichlorobenzonitrile; 10, trichlorophenol; 11, dinitrobenzene; 12, trifluranil; 13, atrazine; 14, phenanthrene. Reprinted from Journal of High Resolution Chromatography, 16, H. G. J. Mol et al., ‘Use of open-tubular trapping columns for on-line extraction – capillary gas chromatography of aqueous samples’, pp. 413 – 418, 1993, with permission from Wiley-VCH.

42	Multidimensional Chromatography

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13.J. Staniewski, H.G. Janssen, C. A. Cramers and J. A. Rijks, ‘Programmed-temperature injector for large-volume sample introduction in capillary gas chromatography and for liquid chromatography–gas chromatography interfacing’, J. Microcolumn Sep. 4: 331 – 338 (1993).

14.H. G. J. Mole, J. Staniewski, J. A. Rijks, H. G. Janssen, C. A. Cramers and R. T. Ghijsen, ‘Use of an open tubular trapping column as interface in on-line coupled reversed phase LC – capillary GC’ in Proceedings of the 14th International Symposium on Capillary Chromatography, Baltimore, MD, USA, Sandra P. (Ed.), Hüthing, Heidelburg, Germany, pp. 645 – 653(1992).

15.J. Staniewski, H. G. Janssen, J. A. Rijks and C. A. Cramers, ‘Introduction of large volumes of methylene chloride in capillary GC with electron capture detection’, in

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25.T. Hyötyläinen, K. Grob, M. Biedermann and M-L. Riekkola, ‘Reversed phase HPLC coupled on-line to GC by the vaporizer/precolumn solvent split/gas discharge; analysis of phthalates in water’, J. High Resolut. Chromatogr. 20: 410 – 416 (1997).

26.T. Hyötyläinen, K. Jauho and M-L. Riekkola, ‘Analysis of pesticides in red wines by online coupled reversed phase liquid chromatography with a vaporizer/precolumn solvent split/gas discharge interface’, J. Chromatogr. 813: 113 – 119 (1997).

27.E. Noorozian, F. A. Maris, M. W. F. Nielen, R. W. Frei, G. J. de Jong and U. A. Th Brinkman, ‘Liquid chromatographic trace enrichment with on-line capillary gas chromatography for the determination of organic pollutants in aqueous samples’, J. High Resolut. Chromatogr. & Chromatogr. Commun. 10: 17 – 24 (1987).

28.J. J. Vreuls, W. J. G. M. Cuppen, G. J. de Jong and U. A. Th Brinkman, ‘Ethyl acetate for the desorption of a liquid chromatographic precolumn on-line into a gas chromatograph’,

J.High Resolut. Chromatogr. 13: 157 – 161 (1990).

29.J. J. Vreuls, R. T. Ghijsen, G. J. de Jong and U. A. Th Brinkman, ‘Drying step for introduction of water-free desorption solvent into a gas chromatograph after on-line liquid chromatographic trace enrichment of aqueous samples’, J. Chromatogr. 625: 237 – 245 (1992).

30.J. J. Vreuls, A. J. Bulterman, R. T. Ghijsen and U. A. Th Brinkman, ‘On-line preconcentration of aqueous samples for gas chromatographic – mass spectrometric analysis’, Analyst 117: 1701 – 1705 (1992).

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31A. J. Bulterman, J. J. Vreuls, R. T. Ghijsen and U. A. Th Brinkman, ‘Selective and sensitive detection of organic contaminants in water samples by on-line trace enrichment – gas chromatography – mass spectrometry’, J. High Resolut. Chromatogr. 16: 397 – 403 (1993).

32A. J. H. Louter, C. A. van Beekvelt, P Cid Montanes, J, Slobodnik, J. J. Vreuls and U. A. Th Brinkman, ‘Analysis of microcontaminants in aqueous samples by fully automated

on-line solid-phase extraction – gas chromatography – mass selective detection’,

J. Chromatogr 725: 67 – 83 (1996).

33T. Hankemeier, A. J. H. Louter, J. Dallüge, J. J. Vreuls and U. A. Th Brinkman, ‘Use of a drying cartridge in on-line solid-phase extraction – gas chromatography – mass spectrometry’, J. High Resolut. Chromatogr. 21: 450 – 456 (1998).

34 J. Roeraade, ‘Automated monitoring of organic	trace components in water.
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35E. Fogelqvist, M. Krysell and L-G. Danielsson, ‘On-line liquid – liquid extraction in a segmented flow directly coupled to on-column injection into a gas chromatograph’, Anal. Chem. 58: 1516 – 1520 (1986).

36E. C. Goosens, R. G. Bunschoten, V. Engelen, D. de Jong and J. H. M. van den Berg, ‘Determination of hexachlorocyclohexanes in ground water by coupled liquid – liquid extraction and capillary gas chromatography’, J. High Resolut. Chromatogr. 13: 438 – 441 (1990).

37C. de Ruiter, J. H. Wolf, U. A. Th Brinkman and R. W. Frei, ‘Design and evaluation of a sandwich phase separator for on-line liquid/liquid extraction’, Anal. Chim. Acta 192: 267 – 275 (1987).

38E. C. Goosens, M. H. Broekman, M. H. Wolters, R. E. Strrijker, D. de Jong and G. J. de Jong, ‘A continuous two-phase reaction system coupled on-line with capillary chromatography for the determination of polar solutes in water’, J. High Resolut. Chromatogr. 15: 242 – 248 (1992).

39E. C. Goosens, D. de Jong, G. J. de Jong, F. D. Rinkema and U A Th Brinkman, ‘Continuous liquid – liquid extraction combined on-line with capillary gas chromatography – atomic emission detection for environmental analysis’, J. High Resolut. Chromatogr. 18: 38 – 44 (1995).

40H. G. J. Mol, H.-G. Janssen and C. A. Cramers, ‘Use of open-tubular trapping columns for on-line extraction – capillary gas chromatography of aqueous samples’, J. High Resolut. Chromatogr. 16: 413 – 418 (1993).

41H. G. J. Mol, J. Staniewski, H.-G. Janssen and C. A. Cramers, ‘Use of an open-tubular trapping column as phase-switching interface in on-line coupled reversed-phase liquid chromatography – capillary gas chromatography’, J. Chromatogr. 630: 201 – 212 (1993).

42H. G. J. Mol, H.-G. Janssen, C. A. Cramers and U. A. Th Brinkman, ‘On-line sample enrichment – capillary gas chromatography of aqueous samples using geometrically deformed open-tubular extraction columns’, J. Microcolumn Sep. 7: 247 – 257 (1995).

43A. J. H. Louter, J. J. Vreuls and U. A. Th Brinkman, ‘On-line combination of aqueoussample preparation and capillary gas chromatography’, J. Chromatogr. 842: 391 – 426 (1999).

44L. Mondello, P. Dugo, G. Dugo, A. C. Lewis and K. D. Bartle, ‘High performance liquid chromatography coupled on-line with high resolution gas chromatography. State of the art’, J. Chromatogr. 842: 373 – 390 (1999).

45L. Mondello, P. Dugo, G Dugo, K. D. Bartle and A. C. Lewis, ‘Liquid chromatography – gas chromatography’, in Encyclopedia Separation Science, Academic Press, London, pp. 3261 –3268 (2000).

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46E. C. Goosens, D. de Jong, G. J. de Jong and U. A. Th Brinkman, ‘On-line sample treatment – capillary gas chromatography’, Chromatographia 47: 313 – 345 (1998).

47T. Hyötyläinen and M-L. Riekkola, ‘Direct coupling of reversed-phase liquid chromatography to gas chromatography’, J. Chromatogr. 819: 13 – 24 (1998).

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49H. G. J. Mol, H.-G. M. Janssen, C. A. Cramers, J. J. Vreuls and U. A. Th Brinkman, ‘Trace level analysis of micropollutants in aqueous samples using gas chromatography with on-line sample enrichment and large volume injection’, J. Chromatogr. 703: 277 – 307 (1995).

50K. Grob, ‘Developments of the transfer techniques for on-line high performance liquid chromatography – capillary gas chromatography’, J. Chromatogr. 703: 265 – 276 (1995).

51M.-L. Riekkola, ‘Applications of on-line coupled liquid chromatography –gas chromatography’, J. Chromatogr. 473: 315 – 323 (1989).

52I. L. Davies, K. E. Markides, M. L. Lee, M. W. Raynor and K. D. Bartle, ‘Applications of coupled LC–GC: a review’, J. High Resolut. Chromatogr. 12: 193 – 207 (1989).

Multidimensional Chromatography

Edited by Luigi Mondello, Alastair C. Lewis and Keith D. Bartle

Copyright © 2002 John Wiley & Sons Ltd

ISBNs: 0-471-98869-3 (Hardback); 0-470-84577-5 (Electronic)

3Multidimensional High Resolution Gas Chromatography

A. C. LEWIS

University of Leeds, Leeds, UK

3.1INTRODUCTION

The coupling of gas chromatography columns to enable multidimensional separations has been widely reported in many areas of industrial and environmental analysis. The application of multidimensional GC has been focused in essentially two areas: (i) increasing peak capacity of the separation system, and (ii) increasing the speed of analysis of the separation system. It was perhaps the former of these two that drove the early interest in two-dimensional GC couplings, and this still remains important today. Despite GC still being very much a developing technique, twodimensional systems, were being applied to the analysis of crude oil and refinery products as early as the late 1960s (1). These early applications focused on achieving a higher degree of deconvolution with a two-column system for the characterization of feedstock and refinery fuels, and this over the past three decades has become a recurring application of two-dimensional gas chromatography.

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.

This present chapter will not focus on the statistical theory of overlapping peaks and the deconvolution of complex mixtures, as this is treated in more detail in Chapter 1. It is worth remembering, however, that of all the separation techniques, it is gas chromatography which is generally applied to the analysis of the most complex mixtures that are encountered. Individual columns in gas chromatography can, of course, have extremely high individual peak capacities, for example, over 1000 with a 106 theoretical plates column (3), but even when columns such as these are

48	Multidimensional Chromatography

applied to moderately complex samples such as gasoline, they still suffer from incomplete resolution of the mixture. The analysts1 most common approach to improve resolution is generally a modification of single-column physical parameters, increase the length, decrease the column internal diameter, or a combination of the two. For many complex sample analyses, however, changes such as these offer only slight improvements in resolving typical target compounds, which may well be isomeric or enantiomeric in nature. It is a well-known theory that a doubling in column length results in only a √2N increase in the number of theoretical plates. What is more often required is not simply greater numbers of theoretical plates on the same column, but complementary selectivity, achieved by using a serially coupled secondary separation. The degree to which a multidimensional GC separation produces enhancement in peak capacity can be related to the degree of orthogonality between the stationary phase selectivities in each dimension. For any given application, therefore, single-column methods may be described as being reliant on column efficiency, whereas-two dimensional system depend on stationary phase selectivities.

Application of two-dimensional GC to increase the speed of analysis was pioneered initially by industrial and process applications, which required on-line highspeed analysis of only a single or very limited number of target analytes. In this mode, the primary GC column has taken on a role similar to the primary column in LC – GC, in that it is used more for sample pre-fractionation than high resolution separation. Through the use of pre-columns and backflushing, many applications have taken advantage of the power of two-dimensional GC, to allow the rapid analysis of relatively volatile components in a matrix of higher-molecular-weight species.

3.2PRACTICAL TWO-DIMENSIONAL GAS CHROMATOGRAPHY

In many respects, the coupling of GC columns is well suited since experimentally there are few limitations and all analytes may be considered miscible. There are, however, a very wide variety of modes in which columns may be utilized in what may be described as a two-dimensional manner. What is common to all processes is that segments or bands of eluent from a first separation are directed into a secondary column of differing stationary phase selectivity. The key differences of the method lie in the mechanisms by which the outflow from the primary column is interfaced to the secondary column or columns.

In most two-dimensional GC applications reported from the late 1960s until the early 1990s, coupling was via the transfer of limited numbers of discrete fractions of eluent from one standard capillary column to a secondary one. In early work, this focused on using packed columns for at least one of the dimensions, although currently most new techniques report the coupling of two or more capillary columns. This mode is often described in literature as ‘heart-cut’, or linear in nature. Since both columns have roughly equivalent peak capacities, the time taken for the analysis by each dimension is significant with respect to the other. That is to say that only