Laboratory testing of gas samples

Gas samples can be taken in sample containers and sent to a laboratory for analysis of the composition of the gas. The results can be compared to the in situ monitoring results. This is not routine practice, but is useful where anomalous 

A hand piunp is used to pump gas into a cylindrical stainless steel or aluminium container. The standard Gresham tube, when filled to its maximum of 14 bar, will contain about 0.751 of sample gas from the well. Care should be taken when using Gresham tubes to take gas samples from 19 mm diameter boreholes that gas is not drawn from the surroxmding ground because the volume of gas in the headspace is low. The volume of tire headspace for 19 mm and 50 mm diameter wells at various depths is given in Table 5.4. 

If gas samples are taken from 19 mm standpipes with a headspace less than 3 m the samples may be drawing gas from the surrounding ground if the tube is filled to its maximum volume, and this should be considered when assessing any difference in results. Smaller size sampling tubes can also be used. 

Adsorptive tubes are sometimes used to take samples for analysis for VOCs. A 4 mm diameter hard glass or stainless steel tube is filled with granular adsorbent (e.g. Tenax tubes). The air to be sampled is pumped through the tube using a small battery powered piunp and the VOCs in the air are adsorbed and condensed. The tube is then sent to a laboratory for analysis. 

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