This involves sampling direct onto the focusing trap of the thermal desorber. It is generally used for situations where the analytes are too volatile to be retained on sorbent tubes.
*Bags – Commonly known as Tedlar bags, these are made from poly(vinyl fluoride) film.
*Canisters – These are available in a range of sizes up to 1 L, and are popular especially in the US and Japan for monitoring of air for compounds lighter than about n-dodecane (n-CH). The canister is evacuated and allowed to refill with the target atmosphere via a flow regulator.
*Headspace – The material is placed in a headspace vial or other sampling container, and the headspace introduced directly into the focusing trap. Multiple samplings onto the same trap allow sensitivity to be increased, but it is increasingly common for two-stage thermal desorption to be used instead.
*On-line – The target atmosphere is simply pumped directly onto the focusing trap.
*Purge-and-trap – A flow of gas is bubbled through an aqueous sample (a beverage or aqueous extract), and the gas stream then introduced directly into the focusing trap.
*Solid-phase microextraction – This is based on adsorption of analytes onto a polymer-coated fibre or cartridge. The small sample size taken onto fibres means that analytes are usually desorbed directly into the GC, while the larger cartridges are usually placed in a TD tube and subjected to single-stage thermal desorption.