Sampling by adsorption tubes

Monitoring of the atmosphere and workplace for trace pollutants (substances) and compounds of environmental interest is now a common routine requirement to meet environmental legislation such as the Control of Substances Hazardous to Health (COSHH) regulations in the UK. Substances to be analysed are present at concentration levels down ppbv, pgm so that pre-concentration techniques are necessary. The most effective method is to collect the substances using an adsorbent tube, although trapping in an appropriate solvent may also be used. 

Desorption is by two procedures, thermal desorption being the preferred and more universal method. 

Solvent desorption. Samples are collected on glass tubes containing 100-200 mg of 20-40 granular charcoal adsorbents made from coconut shells. Solvents such as carbon disulphide, dichloromethane or hexane (2-5 ml) are used to desorb the analytes followed by capillary column analysis. Many applications using charcoal adsorbent tubes are reported in the literature [55,56]. 

Thermal desorption. Substances collected on to the adsorbent are analysed by thermal desorption at a temperature up to 300°C using dry helium carrier gas to transport the substances into a small volume (approximately 500 il) cold trap . The cold trap is then flash heated, the desorbed sample being transferred to a GC for analysis using a WCOT column. 

Tenax TA and Tenax GC are a recently introduced porous polymeric material originally developed as a GCS column packing. It is hydrophobic and is excellent for adsorbing volatiles from the atmosphere at room 

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