The tube of adsorbent is placed in an oven and connected to the column inlet and the column is either cooled to about 10°C or to room temperature. The trap can then be temperature programmed up to about 300°C. A flow of carrier gas (of a few ml/min.), then sweeps the desorbed benzene onto the column where it is focused onto the cool front of the column. After the benzene has been desorbed, the column temperature is increased (the precise temperature will depend on the type of column that is used) and the benzene peak monitored in the normal way. If solvent regeneration is to be used, a different type of trap is employed which is usually about 7 cm long and 6 mm I.D and packed with coconut charcoal or similar adsorbent (Supelco Charcoal trap 32 small is an example). The sample is taken using the same procedure as that described previously and stored and transported in the same way.
Figure 4. Sample Storage Vial
However, the benzene is displaced from the trap with about 2 ml of carbon disulfide into a vial with a septum cap. A diagram of a vial is shown in figure 4. The vial has a metal cap crimp-sealed to the glass tube with a hole in the top that exposes the silicone septum. The lower face of the silicone septum may be protected from the sample by a film of inert plastic (Teflon). The vials can have capacities of 1, 2 or 5 ml. The sample syringe pierces the septum and dips into the sample solution and an appropriate amount is then withdrawn into the syringe. The two methods provide similar accuracy, but temperature desorption is the simpler procedure. However, using temperature desorption, the sample must have adequate thermal stability. Thermal desorption is not usually suitable for essential oils as many of the terpene based substances are thermally labile.
