Despite its basic simplicity, this technique has been automated by a number of manufacturers and as a result the instrumentation has become quite complex. The sample can be transferred to the flask but the head-space of the container in which the material is stored, if suitable, can also be used as the sample source. To increase the volatility of the substances, the flask may be warmed to about 40°C before sampling is commenced. A defined volume of the air is withdrawn from the flask through the adsorption tube by a gas–syringe. Graphitized carbon can be used as the adsorbent although other substances (e.g. porous polymers) can also be employed. Carbon adsorbents with large surface areas (ca 100m2/g) are used for adsorbing small molecular weight materials: adsorbents with low surface areas (ca 5m2/g) are suitable for adsorbing large molecules. Adsorbing larger molecules on low surface area adsorbents allow them to be thermally desorbed more easily, in addition, it reduces the risk of thermal degradation.

To desorb the sample, the trap is placed in an oven and connected to the column. that is maintained at a low temperature (50°C or less) this causes the desorbed solutes to be concentrated on the front of the column. The trap is heated rapidly to 300°C and a stream of carrier gas then sweeps the desorbed solutes onto the column. When desorption is complete, the temperature of the column is programmed up to an appropriate temperature, and the components of the head-space sample separated and quantitatively assayed.

Solid Phase Micro Extraction

The solid phase micro extraction technique operates is similar to the head space sampler but is much smaller and, thus, can handle very small samples. The apparatus used is shown in figure 7. The device consists of a length of fused silica fiber, coated with a polymeric adsorbent, which is attached to the steel plunger contained in a protective holder. The steps in the procedure are included in figure 7. The sample is placed in a small vial (which may be heated if necessary) and allowed to equilibrate with the air (1). The syringe needle, containing the fiber, is the made to pierce the cap, and the plunger depressed to expose the fiber to the head-space vapor.

Figure 7 The Solid Phase Micro Extraction Apparatus