Choice of Detectors

The most commonly used detectors in anaesthetic gas analysis are TCD, FID and electron capture detector (ECD).

TCD is concentration-sensitive and has been the most widely used in chromatographic analysis for the determination of gases, and for any applications in explosion hazard areas. If inorganic gases, besides

Figure 5 (A) Gas chromatograms for the single-column separation of anaesthetics by temperature programming (linear or nonlinear). A, Air; B, carbon dioxide; C, nitrous oxide; D, halothane; E, isoflurane; F, enflurane. (B) Simple set-up of a temperature-programmed (linear or nonlinear) dual-column chromatograph.

Figure 5 (A) Gas chromatograms for the single-column separation of anaesthetics by temperature programming (linear or nonlinear). A, Air; B, carbon dioxide; C, nitrous oxide; D, halothane; E, isoflurane; F, enflurane. (B) Simple set-up of a temperature-programmed (linear or nonlinear) dual-column chromatograph.

Figure 6 Chromatograms for dual detector chromatography (A) halothane (left) and isoflurane (right) in atmospheric air. (B) Simple set-up of a dual detector chromatograph.

liquid anaesthetics, need to be analysed, TCD is the detector of choice due to its universal response to almost all substances and its very large linear dynamic range. Because of its relatively poor sensitivity, it is unsuitable for the determination of low concentrations (<40p.p.m.) without employing extreme detector conditions and large sample volumes. The nondestructive character of the TCD enables it to be used in dual-column chromatography by utilizing two channels simultaneously or in series with another detector such as the FID. Sensitivity of the hot-wire TCD depends on the temperature difference between filament and cell wall temperature (Figure 4), and higher chromatographic responses are obtained at higher filament temperatures.

The ECD is very sensitive to electrophilic species such as polyhalogenated anaesthetics and also to nitrous oxide, but its linear dynamic range is limited to a range of about 104 and it can easily be saturated. For this reason, it is generally employed for the low concentration determination of liquid anaesthetics and nitrous oxide. Since oxygen and water influence the detector sensitivity, these compounds must be rigor ously removed from the carrier and make-up gases. Contamination also causes serious interference. The detector must be held at an elevated temperature, always with a steady flow of carrier gas, and must be regularly baked out to ensure cleanliness. All these factors make ECD a difficult detector in anaesthetic gas analysis.

The very widely used FID is a mass-sensitive detector, with the disadvantage compared to the TCD that it is destructive. It responds to virtually all organic components but does not respond to the permanent gases. In the great majority of studies where only the determination of the volatile liquid anaesthetics is needed (e.g. blood and body fluid analysis), FID is used. If the analysis includes nitrous oxide in addition to liquid anaesthetics, the ECD alone may be chosen. For low concentration analysis, TCD and FID may be connected in series to determine the permanent gases and the liquid anaesthetics.

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