The enrichment stage is added to the setup before the CEPAS cell as shown in figure . Directly after the sample input, the sample gas flows through a glass sorption tube containing about 150 mg of Tenax TA (60/80 mesh, Alltech). Tenax TA does not adsorb benzene particularly strongly, but the relatively low breakthrough volume is not an issue here, because the adsorption time is short. Furthermore, this ensures that the desorption is very efficient even at a relatively low temperature, so that the duration of the total measurement cycle can be kept short. The flow direction through the adsorbent can be chosen with voltage controlled three-way valves. To control the sorbent temperature, a resistive heater is coiled around the sorbtion tube and two computer case fans are used for forced air cooling of the sorben tube. The temperature of the sorbent is monitored with a thermistor probe inserted inside the sorbent tube through a rubber seal.