Thermal desorption fundamentally involves collecting volatile organic compounds onto a sorbent, and then heating this sorbent in a flow of gas to release the compounds and concentrate them into a smaller volume.
Early thermal desorbers used just single-stage operation, whereby the volatiles collected on a sorbent tube were released by heating the tube in a flow of gas, from where they passed directly into the GC.
Modern thermal desorbers can also accommodate two-stage operation, whereby the gas stream from the sorbent tube (typically 100–200 mL) is collected on a narrower tube integral to the thermal desorber, called the focusing trap or cold trap. Heating this trap releases the analytes once again, but this time in an even smaller volume of gas (typically 100–200 μL), resulting in improved sensitivity and better GC peak shape.
Modern thermal desorbers can accommodate both single-stage and two-stage operation, although single-stage operation is now usually carried out using the focusing trap to collect the analytes, rather than a sorbent tube.
It is normal for the focusing trap to be held at or below room temperature, although a temperature no lower than 0 °C is sufficient for all but the most volatile analytes. Higher trap temperatures also reduce the amount of water condensing inside the trap (when transferred to the GC column, water can reduce the quality of the chromatography).