Dynamic headspace DHS

The difference between SHS and DHS analysis mainly lies in the sensitivity that can be reached. In the static mode, part of ''one equilibrium'' is injected whereas in the dynamic mode the equilibrium concentration is continuously removed and trapped resulting in exhaustive extraction and thus high sensitivity for the analysis of volatiles. For liquid samples, DHS sampling is mostly carried out by purge-and-trap analysis. An inert gas is bubbled through the liquid sample and the purgeable organics are moved from the aqueous to the vapor phase. The volatile compounds are then trapped (solid phase extraction, SPE) on an adsorbent such as Tenax or active charcoal. The trap containing the adsorbent is built in a desorption chamber equipped with a powerful heating mechanism which when activated, permits desorption (gas phase extraction) of the trapped compounds. This technique has the distinct merit of providing a clean sample, free from its often complex matrix. A purge-and-trap device can easily be mounted on a GC equipped with a flame ionization detector (FID), flame photometric detector (FPD) or MS. This technique is most appropriate for mg/L (ppb) level analysis of low molecular weight, slightly water-soluble volatile organics with a boiling point below 200°C. A variation of purge-and-trap is closed-loop stripping (CLS) analysis [13] which is a combination of gas phase extraction with SPE in a closed system. CLS allows ng/L (ppt) analysis of contaminants in drinking water.

A recently introduced DHS sampling method is in-tube extraction (ITEX; CTC Analytics, Zwingen, Switzerland) [14]. The enrichment of volatiles can be fully automated as the ITEX device can be mounted on a CTC Combipal sampler. The principle of ITEX is illustrated in Figure 2.

Dynamic Headspace Restek

The sample is heated and agitated in a sealed vial until equilibrium is achieved. A special syringe equipped with a trap containing an adsorbent (Tenax, Carbosieve, etc.) is pierced through the vial septum and the syringe pumps the headspace through the adsorbent, e.g., 10 times 1 mL. The trap is then flash-heated and desorbed in the GC injector. Afterwards, the hot ITEX trap is cleaned with inert flush gas before the next sampling. The comparison SHS and ITEX will be presented in Section 2.5.1 for the determination of diacetyl, responsible for the cardboard flavour, in beer.

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