This involves sampling first onto a sorbent tube. The most widely used tubes are those following the pattern laid out by WG5 (see above). After sampling (for which a variety of accessories are available), the tube is desorbed to transfer the analytes to the focusing trap before the second desorption stage transfers them to the GC. The greater sensitivity of this method has made it increasingly popular for sampling dilute gas streams, or in exploratory work where the target atmosphere is unknown.
*Diffusive (or passive) sampling – A tube is packed with a single sorbent bed and allowed to adsorb analytes from the air diffusively. It is suitable for sampling known compounds over a period of hours (for analyte concentrations of 2–10 μg/m) to weeks (for analyte concentrations of 0.3–300 μg/m).
*Pumped (or active) sampling – A tube is packed with up to three sorbent beds and a flow of the sample gas passed through it. It is suitable for sampling high and low concentrations of known and unknown compounds over timescales of minutes to hours.
*Direct desorption – This is used for sampling emissions from small pieces of solid or semi-solid materials. The material is placed inside a tube and heated to release the vapours directly into the focusing trap.
*Headspace – The material is placed in a (micro-)chamber or other sampling container, and a flow of gas passed through it to transfer the headspace dynamically onto a sorbent tube.