Detectors

The thermal conductivity detector (TCD) was used extensively in early work and is still used in a much improved form which makes it compatible with wide-bore open-tubular columns. The relatively poor sensitivity of the early TCD meant that it was largely displaced by the flame ionization detector (FID). The combustion of carbon-containing compounds in a small hydrogen/air diffusion flame produces ions that can be detected by applying a voltage between the flame jet and a collector electrode situated around the flame. The detector has a low dead volume, a high sensitivity for nearly all carbon-containing compounds and an extremely wide linear range of response. As already pointed out, the discovery of the FID played a crucial role in the development of open tubular columns. The FID is rugged, reliable and relatively insensitive to operating variable so that it is now by far the most widely used of all detectors.

The TCD and the FID are universal detectors; that is, they give a response for all substances. This is not strictly true for the FID since it gives no response for the permanent gases and water (which makes it very suitable for the analysis of aqueous samples). Another universal detector, now becoming much more widely employed, is the mass spectrometer. Coupling to a mass spectrometer (GC-MS) dates almost from the beginning of gas chromatography, but in the early days the practical problems and high cost meant that the combination was confined to a few research laboratories. The advent of the silica open-tubular column, improved designs of mass spectrometers, the availability of computers to handle the large amount of data produced and a considerable reduction in cost have resulted in the GC-MS combination becoming very widely used. The great advantage of the mass spectrometer as a detector is its ability to identify the compounds being separated. It is not quite ideal in this respect since isomers sometimes give almost identical spectra, but techniques such as tandem mass spectrometry (MS-MS) and different methods of ionization can overcome some of these problems. Another advantage is that the mass spectrometer can also be used in a selective mode, often with greatly increased sensitivity.

Fourier transform infrared spectroscopy detectors for gas chromatography are also available; their range of application is not as wide as the GC-MS combination but to some extent they are complementary.

In addition to the universal detectors, a number of selective detectors are commercially available. The nitrogen and phosphorus detector (NPD) is similar in design to a conventional FID but has an electrically heated rubidium-glass bead situated between the flame jet and the collector electrode and (for nitrogen compounds) uses a very small hydrogen flow so that there is a heated plasma in the working zone rather than a flame. This detector is widely used for the analysis of drugs and pesticides in environmental and biological research. Other variants of the FID such as the hydrogen atmosphere FID, which gives a response to some gases, and the O-FID for the selective detection of oxygen-containing compounds, also exist.

The electron-capture detector has an outstandingly high response for polyhalogenated compounds and so has found extensive application in pesticide and environmental analysis. Indeed, the start of the concern for the distribution of compounds such as DDT in the environment can be attributed to the development of this detector in the early 1960s.

Other selective detectors include the microwave plasma emission detector, which can detect a number of elements simultaneously. The photoionization detector gives a high response for environmentally important compounds such as benzene and vinyl chloride and finds use as a portable field instrument. The flame photometric detector and the chemilumines-cence detector have a high response to sulfur and are used extensively in the petroleum industry. Chemi-luminescence detection has also been used for the selective determination of nitrosamines in foodstuffs. The Hall detector catalytically decomposes the compounds emerging from the gas chromatography column into simple inorganic gases such as hydrogen chloride (for chlorine) and ammonia (for nitrogen), which are absorbed in a circulating stream of aqueous organic solvent followed by monitoring the electrical conductivity of the solution obtained.

Although selective detectors find extensive application for particular problems they are all more demanding in the control of operating parameters than the FID.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment