Multidimensional Chromatography

Figure 12.21 SFC – GC heart-cut analysis of chrysene from a complex hydrocarbon mixture: (a) SFC trace (UV detection); (b) GC trace without heart-cut (100% transfer); (c) GC trace of heart-cut fraction (flame-ionization detection used for GC experiments). Reprinted from Journal of High Resolution Chromatography, 10, J. M. Levy et al., ‘On-line multidimensional supercritical fluid chromatography capillary gas chromatography’, pp. 337 – 341, 1987, with permission from Wiley-VCH.

SFC – GC has also been used for group-type separations of high-olefin gasoline fuels, including saturates, olefins, and aromatics (25). The SFC – GC characterization of the aromatic fraction of gasoline fuel was carried out by using CO2 on four packed columns in series, i.e. silica, Ag -loaded silica, cation-exchange silica and NH2 silica. The heart-cut fractions were transferred into a capillary column coated with a methyl polysiloxane stationary phase, with cryofocusing at 50 °C being used for focusing during the transfer into the GC system. The SFC –FID and GC –FID chromatographs are shown in Figure 12.22.

12.11 SFC – SFC APPLICATIONS

An on-line SFC – SFC coupled technique involving a rotary valve interface was used to provide an efficient separation of coal tar (see in Figure 12.23) (26). A schematic diagram of the multidimensional packed capillary to open tubular column SFC – SFC system is shown in Figure 12.24. The rotary valve interface was used to provide the flexibility of using two independently controlled pumps, which gave an increased performance of the system when compared to the traditional one-pump system. In addition, an on-column cryogenic trap was used to suppress efficiency losses due to the first packed column. This cryogenic unit efficiently traps the selected fractions and focuses the sample, then allowing the transfer of the fractions into a narrow band

Figure 12.22 SFC – GC analysis of aromatic fraction of a gasoline fuel. (a) SFC trace;

(b) GC trace of the aromatic cut. SFC conditions: four columns (4.6 mm i.d.) in series (silica, silver-loaded silica, cation-exchange silica, amino-silica); 50 °C; 2850 psi; CO2 mobile phase at 2.5 mL min; FID detection. GC conditions: methyl silicone column (50 m 0.2 mm i.d.); injector split ratio, 80:1; injector temperature, 250 °C; carrier gas helium; temperature programmed, 50 °C (8 min) to 320 °C at a rate of 5 °C/min; FID detection. Reprinted from Journal of Liquid Chromatography, 5, P. A. Peaden and M. L. Lee, ‘Supercritical fluid chromatography: methods and principles’, pp. 179 – 221, 1987, by courtesy of Marcel Dekker Inc.

onto the second column. The first column was an aminosilane stationary phase packed column and gave good resolution of the coal tar extract by chemical class separation (i.e. of aromatic rings) due to high selectivity. The second column was an open tubular fused-silica column (10.5 m 50 m i.d.) coated with a liquid crystalline polysiloxane stationary phase. The compounds were separated on the second column according to shape-selective separation of the isomeric compounds, thus leading to increased efficiency. By using a packed first column, this increased the loading capacity and increased sample concentration, which may not be obtained with an open tubular column.

Figure 12.23 SFC–SFC analysis, involving a rotary valve interface, of a standard coal tar sample (SRM 1597). Two fractions were collected from the first SFC separation (a) and then analyzed simultaneously in the second SFC system (b); cuts ‘a’ and ‘b’ are taken between 20.2 and 21.2 min, and 38.7 and 40.2 min, respectively. Peak identification is as follows: 1, triphenylene 2, chrysene 3, benzo[ghi]perylene; 4, anthracene. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al., ‘Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switching interface’, pp. 1384 – 1388, copyright 1990, with permission from the American Chemical Society.

12.12 CONCLUSIONS

Multidimensional chromatography has proven to be useful for the analysis of complex samples such as polymer or industrial mixtures. All of the separation techniques available today have definite limitations in terms of their selectivity and separation range, which leads to the necessity of multi-stage separation procedures for samples which contain a wide variety of different components. Current technological processes require fast and rugged analytical methods which can provide comprehensive information about the process stages and products. This dictates the necessity of development of automated complex separation procedures with minimal sample pretreatment, and the use of on-line multidimensional chromatographic techniques is a logical solution to these requirements.

Figure 12.24 Schematic diagram of the multidimensional packed capillary to open tubular column SFC – SFC system. Reprinted from Analytical Chemistry, 62, Z. Juvancz et al., ‘Multidimensional packed capillary coupled to open tubular column supercritical fluid chromatography using a valve-switching interface’, pp. 1384 – 1388, copyright 1990, with permission from the American Chemical Society.

One of the significant drawbacks of multidimensional analytical methods is the specificity of the conditions of each separation mode for a particular sample type, together with restrictive requirements for the type and operational conditions of the interface between them. Therefore, extensive work in the method development stage, along with the availability of highly skilled personnel for operating such systems, are required.

Methods developed for on-line technological control have to be tested for the variation of the product composition due to process variations. However, if rugged analytical procedures are developed these multidimensional methods may only require minimal attention during on-line operation. Multidimensional chromatography for the analysis of complex polymer and industrial samples offers chromatographers high productivity and efficiency and is an excellent alternative to off-line methods.

REFERENCES

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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)

13Multidimensional Chromatography in Environmental Analysis

R. M. MARCÉ

Universitat Rovira i Virgili, Tarragona, Spain

13.1INTRODUCTION

Multidimensional chromatography has important applications in environmental analysis. Environmental samples may be very complex, and the fact that the range of polarity of the components is very wide, and that there are a good many isomers or congeners with similar or identical retention characteristics, does not allow their separation by using just one chromatographic method.

The main aims in environmental analysis are sensitivity (due to the low concentration of microcontaminants to be determined), selectivity (due to the complexity of the sample) and automation of analysis (to increase the throughput in control analysis). These three aims are achieved by multidimensional chromatography: sensitivity is enhanced by large-volume injection techniques combined with peak compression, selectivity is obviously enhanced if one uses two separations with different selectivities instead of one, while on-line techniques reduce the number of manual operations in the analytical procedure.

For analytical purposes, environmental analysis can be divided into the control of pollution and the analysis of target compounds. For the control of pollution, it is important to monitor both well-known priority pollutants and all of the other nonpriority pollutants. The selectivity of the analytical column may therefore not be sufficient. In most cases, however, mass spectrometry (MS) detection can solve the problem and this is why gas chromatography GC–MS is widely used in routine analysis. Sometimes, however, MS, and even MS/MS (which requires complex instrumentation) may not solve the problem and multidimensional chromatography is then a suitable technique. The low levels at which the micropollutants are to be determined is another drawback and multidimensional techniques are a good solution to this problem. In pollution control, high throughput is required and this may be obtained by automating the analysis via multidimensional chromatography, which also reduces the possible sources of error.

In target-compound analysis, a particular compound, usually present in a complex matrix and at trace levels, needs to be quantified. Here, selectivity and

sensitivity are the most important requirements, and, as well as the use of a suitable selective detector, multidimensional chromatography has also played an important role.

Soil extracts are usually very complex. In water samples, humic and fulvic acids make analysis difficult, especially when polar substances are to be determined. Multidimensional chromatography can also make a significant contribution here to this type of analysis.

The use of multidimensional chromatography in environmental analysis has been reviewed in the literature (1 – 6). Of the multidimensional systems described in previous chapters, GC–GC liquid chromatography LC–LC and LC–GC, whose applications to environmental analysis will be detailed in this chapter, are the ones most often used in environmental analysis.

Other multidimensional systems, such as supercritical fluid chromatography (SFC–GC or LC –SFC), will not be described here because, although some applications to environmental analysis have been described (4, 7 – 9), they have not been very widely used in this field.

13.2MULTIDIMENSIONAL GAS CHROMATOGRAPHY

13.2.1 INTRODUCTION

Gas chromatography, because of its high resolution, is widely used in environmental analysis to determine a wide range of pollutants. This technique is applied to both volatile and, after a derivatization step, to nonvolatile analytes. Environmental samples are usually quite complex because of the different pollutants which may be present. Multidimensional chromatography (MDGC) or GC–GC coupling would therefore be expected to be widely applied in environmental analysis. Despite its many advantages, however, a major drawback with MDGC is that, in principle, many heart-cuts from the first column should be subjected to a second separation. In other words, the method can become extremely time consuming. One remedy is to combine several heart-cuts and analyse these in one second run. However, the risk of co-elution then markedly increases and this is particularly dangerous when detection with no identification power is used. This is what happens, for example, with chlorinated analytes, for which electron-capture detection is the most widely used, due to its high sensitivity (5).

In general, capillary gas chromatography provides enough resolution for most determinations in environmental analysis. Multidimensional gas chromatography has been applied to environmental analysis mainly to solve separation problems for complex groups of compounds. Important applications of GC–GC can therefore be found in the analysis of organic micropollutants, where compounds such as polychlorinated dibenzodioxins (PCDDs) (10), polychlorinated dibenzofurans (PCDFs) (10) and polychlorinated biphenyls (PCBs) (11 – 15), on account of their similar properties, present serious separation problems. MDGC has also been used to analyse other pollutants in environmental samples (10, 16, 17).

MDGC has also been used in the air analysis field. For instance, it has been applied to the analysis of volatile organic compounds (VOCs) in air, thus enabling a wider range of these compounds to be analysed (18).

Today, however, GC–GC coupling is seldom used to determine pesticides in environmental samples (2), although comprehensive MDGC has been applied to determine pesticides in more complex samples, such as human serum (19). On the other hand, new trends in the pesticide market, which is now moving towards the production of optically active enantiomers and away from racemic mixtures, may make this area suitable for GC–GC application. The coupling of non-chiral columns to chiral columns appears to be a suitable solution to the separation problems that such a trend might cause.

Multidimensional gas chromatography has also been used in the qualitative analysis of contaminated environmental extracts by using spectral detection techniques such as infrared (IR) spectroscopy and mass spectrometry (MS) (20). These techniques produce the most reliable identification only when they are dealing with pure substances; this means that the chromatographic process should avoid overlapping of the peaks.

Most applications in environmental analysis involve heart-cut GC–GC, while comprehensive multidimensional gas chromatography is the most widely used technique for analysing extremely complex mixtures such as those found in the petroleum industry (21).

13.2.2 EXAMPLES OF MULTIDIMENSIONAL GAS CHROMATOGRAPHY APPLIED TO ENVIRONMENTAL ANALYSIS

A typical example of MDGC in environmental analysis is the determination of PCBs. These are ubiquitous contaminants of the environment in which they occur as complex mixtures of many of the 209 theoretically possible congeners. The compositions of environmental mixtures vary according to sample type.

Attempts to optimize the capillary GC separation conditions of 209 PCBs on a single column of either single or mixed phases have had only limited success. MDGC has therefore been very important. In some cases, mass spectrometry and, in particular high-resolution mass spectrometry, may be enough to determine different isomers which co-elute in a single column, and sensitivity may be enhanced by selected ion monitoring (SIM) or negative chemical ionization (NCI).

In MDGC, the usual configuration normally has a non-polar phase (such as SE 54 or CPSil 8) on the first column to make the initial, well-characterized separation. The sample is chromatographed on this column to a point just before the elution of the unresolved peaks. The column flow is then switched into a second column of a different, usually more polar, phase such as CPSil 19 or CPSil88, for the duration of the elution of these resolved peaks only. The column is again isolated and the small group of unresolved peaks is separated on the second column (15). Other columns which have been used include BPX5 (22), OV1(23) or Ultra 2 (11) as the first column, and HT8 (23), OV-210 (12, 24) or FFAP (14) as the second column.