Sample Handling and Injection

Sample handling and injection techniques are greatly influenced by the source of the analysed samples such as liquid samples containing anaesthetics (e.g. blood, urine, sperm, tissue), low concentration gas samples (e.g. anaesthetics in pollution studies) and high concentration gas samples (e.g. inhaled and exhaled gas mixtures).

Direct injection of a liquid sample to the chromato-graphic column is very simple, but it is a rather crude approach and has serious disadvantages such as contamination of the sample port, column and detector, alterations in the baseline characteristics and interference by water vapour. The problems associated with the presence of the liquid in the chromatographic system are avoided by the technique of headspace analysis, whereby the vapour above the sample is injected under controlled conditions. Headspace sampling is rapid and is suitable for direct determination of the partial pressure of anaesthetics in blood.

Low concentration samples of liquid anaesthetics trapped in an adsorbent-filled cartridge (integrated sampling or passive dosimeter) in pollution studies are introduced into a gas chromatographic system via a gas sampling valve. Trapped anaesthetics are desor-bed from the adsorption cartridge and transferred by the carrier gas to the main chromatographic column by heating the adsorption cartridge rapidly.

Low concentration (spot sampling) and high concentration samples in the gaseous state may be introduced to a gas chromatographic system by a gas-tight syringe (0.1-5.0 mL) with the usual septum-type inlet system. However, this is not a reproducible sample introduction method and creates problems of reliability where quantification of the components is needed. In addition to this, polymeric material such as rubber (e.g. on the barrel of a disposable syringe), plastics, and even glass itself adsorb liquid anaesthetics (& 1-3%) on the contact surface. Adsorption on glass surfaces becomes more important when dealing with mixtures at lower concentrations (Figure 2). Therefore, syringe injection should be avoided in quantitative studies.

If gas samples are to be taken repeatedly to produce reproducible quantitative data, a gas sampling valve fitted with the desired size of sampling loop (0.25-10 mL) should be used at a constant temperature and filling pressure (usually ambient). It should be noted that, when using a concentration-sensitive detector such as TCD, the sample size and column diameter relationship must be taken into consideration to avoid column overloading. Several commercial gas sampling valves are available in various configurations. Some operate on the slider with the

Figure 2 Adsorption of halothane on glass surface at lower concentrations. Squares, cylinder preparation; circles, syringe dilution.

O-ring principle, while others operate by rotation of a TeflonĀ® (polytetrafluoroethylene) or polyimide rotor in various flow paths. The analyst should be aware that some polymeric materials (e.g. silicone rubber O-rings) adsorb anaesthetic vapours to some extent (halothane > isoflurane > enflurane). Gas switching valves made of a stainless-steel body and TeflonĀ® rotor or O-rings are the most suitable choice for anaesthetic purposes. It is important to note that gas sampling valves must not be used with flow control of the carrier gas, as this restricts the filling rate and hence the rate of flushing of the loop, resulting in tailing peaks, Pressure control is used instead.

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