GC Detectors

The most common detector is the flame ionization detector (FID). The outlet from the column is directed into a carefully controlled hydrogen flame. The electrical potential across the flame is measured continu­ously, as this changes when an organic compound elutes from the column and is ionized in the flame. The analogue signal from the detector appears as a series of peaks over the duration of the analysis; these correspond to each material that elutes from the column. The amplitude of the signal is directly proportional to the amount of material passing through the detector and therefore the area under a peak on the recorded chromatogram is proportional to the amount of that material in the sample. The data are usually processed by an integrator or computer and the results are commonly expressed as a percentage of the total relative peak area (%RPA), the simplest quantitative measurement of the components in the mixture.

Coupling the column from the GC to a mass spectrometer provides a very powerful combination, GC-MS, which can identify and quantify almost all the compounds in a complex mixture, such as an essential oil or perfume, by reference to libraries of mass spectra of known compounds. Careful investigation of the mass spectrum can be used deductively to determine a possible structure for an unknown material using fragmentation theories to identify sub-structural components of the molecule. Recent developments in benchtop mass spectrometers have brought a range of specialized MS techniques into the realm of GC-MS machines; techniques such as chemical ionization and MS-MS are now available, which provide more information on individual sample components and allow better identification of unknown com­pounds.

GC-Fourier transform infra-red (GC-FTIR) spectroscopy is less frequently used than GC-MS, but involves a similar principle in which the outlet from the column is coupled to an infra-red spectro­photometer. The technique currently suffers from a lack of library spectra, as the IR spectra taken in the vapour phase can be subtly different from condensed-phase spectra or spectra collected using the well-established KBr disc method.

The use of nitrogen — or sulfur-specific detectors for GC enables small quantities of nitrogen — or sulfur-containing molecules to be detected.

These often have very powerful odours (Boelens and Gemert, 1994); for example, 2-isobutyl-3-methoxypyrazine from galbanum oil has a powerful green note and l-^ara-menthene-8-thiol, a very powerful natural material, has a strong grapefruit odour at concentrations in the p. p.b. range.

GC-sniffing is an adaptation of special importance in the fragrance industry. The effluent from the column is split between a conventional detector and a smelling port that allows the individual components to be smelled by the human nose which is more sensitive to certain materials than sophisticated detectors (Acree and Barnard, 1994). If the nose belongs to a perfumer, then the odours can be recognized and described immediately. This is particularly useful when trying to establish the odour of a single component in a complex mixture, as GC-sniffing provides information from a few micrograms of sample which would only otherwise be available if the individual material could be isolated and purified in quantities of several grams (a very time-consuming process requiring relatively large amounts of sample).

Some separations can only be achieved by GC and if it is necessary to isolate such a material, then preparative GC is required. The flow from the column is momentarily directed to a cold trap as the desired compound elutes, which then condenses in the trap. The amounts that can be collected in this way are minute, but a few hundred micrograms are sufficient for a NMR or IR analysis.

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