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Kathy Holley: Part-time Researcher, at the School of Environmental Science, Nene College of Higher Education, Northampton, UK
Mark Pennington: Lecturer, at the School of Environmental Science, Nene College of Higher Education, Northampton, UK
Paul Phillips: Senior Lecturer, at the School of Environmental Science, Nene College of Higher Education, Northampton, UK
Introduction
Gas chromatography (GC) is an analytical technique widely used in the food and drink industry. It is capable of rapidly separating and detecting the volatile organic components of a food or drink. A volatile substance is one that readily passes into the gas phase; normally these are liquids or solids with low boiling and melting points. A good example would be ethanol which is present in alcoholic drinks.
GC is particularly valuable when complex mixtures need to be analysed. During the quality control process in food production a processor may wish to check if toxic substances such as pesticide residues are present. Chemical analysis using traditional techniques would be slow and laborious, but by using GC it is possible to analyse to detect many such substances in a few minutes. Advanced GC systems can easily separate and detect 100 different compounds in about one to two hours. The analysis of the flavour components of herbs was a very time-consuming process before the advent of GC, but when GC was applied to the analysis of the flavour components from coriander leaves, it was possible to separate and detect the 30 major aromatic components in about 60 minutes.
Principles of GC
The basis of the separation is a retardation of the individual components as they are moved through a long column by a carrier gas, usually helium or nitrogen. The column consists of a steel or glass tube filled with an inert packing material such as glass or ceramic beads (see Figure 1). In gas-liquid chromatography (GLC), these are coated with an involatile liquid, so that the surface area of the liquid in contact with the gas is large. For some applications, the packing may be a solid without any liquid coating; it is then called gas-solid chromatography (GSC), but this is less widely used than GLC.
The sample is injected into the carrier gas stream. As it moves through the column with the carrier gas,...