Helium Diffusion Sampling Advantages
Advantages of Helium Diffusion Sampling for Air and Indoor Air Monitoring
HDS is a new approach for filling air sampling canisters and Bottle-Vac samplers that uses the principle of helium/air exchange rather than just the filling of air into an evacuated container. Monitoring of the amount sampled into the canister or Bottle-Vac is done using a weight measurement rather than measuring the container pressure, as the pressure using HDS is always at 1 atmosphere absolute, or more precisely is the atmospheric pressure where the sampling occurred. By filling the container with helium, then opening the valve or attaching a very low-cost diffusion zone at the sampling site, the helium will start to diffuse out of the canister at a constant rate, with replacement by the same molar volume of air, all of which time the container remains at atmospheric pressure. In the laboratory, the canister is weighed, analyzed, evacuated, and weighed a second time to determine what of the weight of the sample minus the container was. This provides the %air down to +-1%, allowing easy determination of the dilution ratio. Typically, 35-50% of the helium is exchanged with air, so the dilution is around 2-3x. With GCMS determinations in the laboratory, some of the biggest deterrents to ultimate detection limits is contamination and background, not the ultimate sensitivity of the GCMS. With HDS, there is virtually no chance for cross-contamination, say, due to a large contaminated sampling trained used in classical vacuum sampling, so the 2-3x dilution in practice has little effect on the ultimate detection limits using HDS when compared to time integrated vacuum sampling. Unlike passive tube collection where there is no direct confirmation of the actual amount of sample collected on the adsorbent in the tube, the change in canister weight is a huge advantage offered by HDS. Using HDS Bottle-Vacs, the cost of acquisition is roughly the same as that of a passive tube, so accuracy and definisibility do not necessarily come at a high price tag. See the line below to download an application note showing the comparison of HDS sampling to that of classical TO15 for monitoring chemicals well down into the sub-PPB range.