In sorbent tube sampling (Figure 3.5), volatile and semi-volatile compounds are pumped from the air and trapped on the surface of the sorbent.By sampling a measured quantity of air (typical volumes of 10-500 m3),quantitative sampling is possible. The sorbent tube is then sealed and transported back to the laboratory for analysis. Desorption of volatile and semi-volatile compounds takes place either by the use of organic solvents (solvent extraction) or heat (thermal desorption), followed by analysis using gas chromatography (see later)