High Temperature Gas Chromatography

Gas chromatography (GC) is generally believed to be restricted to the analysis of ‘volatiles’ and is less applicable to the analysis of so-called ‘heavy’ compounds. The introduction of persilylated glass and fused silicea columns, of thermostable stationary phases and of non-discriminative injection devices in capillary GC (CGC) have made the deRnitions of ‘volatile’ and ‘heavy’ very Sexible. High temperature CGC (HTCGC) was developed in the 1980s as a result of work carried out by Grob, Geeraert and Sandra, Trestianu et al., Lipsky and Duffy, and Blum and Aichholz (see Further Reading). Although HTCGCwas initially not accepted as a ‘robust’ analytical technique for the analysis of high molecular weight compounds, in recent years several research groups have demonstrated the capabilities of HTCGC for the analysis of hydrocarbons with carbon numbers in excess of 130 (simulated distillation), of lipid compounds, of emulsiRers, of detergents, of polymer additives, of oligosaccharides, of porphyrins and of many more solutes. DeRning a temperature in HTCGC is not straightforward but it is now generally accepted that 4203C is the maximum allowable column temperature limit in practice. Applications at higher column temperatures have been carried out but, with the exception of hydrocarbons, most organic compounds do not withstand temperatures higher than 4203C. Moreover, the maximum allowable operating temperatures (MAOT) of the stationary phases applied in HTCGC are all lower than 4503C. A prerequisite in HTCGC is that the solutes to be analysed are thermally stable and can be volatilized. The thermal stability of organic compounds depends not only on their nature, but also on the activity of the environment to which they are subjected and on the thermal stress given to the solutes. HTCGC is nowadays performed in a completely inert system, i.e. high purity carrier gas, dedicated and puriRed stationary phases, fused silica columns with less than 0.1 ppm trace metals and specially deactivated, etc. Moreover, thermal stress is reduced by applying cool on-column (COC) or programmed temperature vaporizing (PTV) injection. Lipids may serve as a good illustration. When oils or fats are used in food preparation, they decompose (formation of volatiles) or polymerize (formation of dimers, trimers, etc.) as a function of time, which makes them no longer useful for cooking. These alterations are caused by the presence of water and oxygen. When heated under inert conditions, however, fats and oils are stable and evaporate. Figure 1 shows the thermogravimetric proRles for triolein with a molecular mass of 886 Da under a stream of pure nitrogen (A) and of air (B). In present state-of-the-art HTCGC, the systems are operated under the circumstances shown in curve A. Volatility, on the other hand, is related to the vapour pressure (boiling point) of the compounds. Polydimethylsiloxanes with molecular masses as high as 5000 Da are volatile enough to be analysed by HTCGC, whereas low molecular weight (oligo)saccharides, for example, are not volatile at all. This is because of the polarity of the functional groups. Derivatization is often employed to impart volatility and to yield a thermostable product, thereby also improving chromatographic performance and peak shapes. Silylation, alkylation and acylation are used to modify the active hydrogen in compounds containing }OH, }COOH, and }NH2 functionalities. HTCGC is a reliable analytical method if a number of prerequisites are fulRlled. The different aspects of the technique are discussed and the potential illustrated with a number of relevant applications.