Canister based Soil Gas Monitoring-Still the Gold Standard
A collection of Soil Gas samples using 1L canisters has a lot of advantages over the use of thermal desorption tubes. The sampling of Soil Gas into evacuated canisters is a very straightforward and fast process that can be done accurately with very little training. There is no concern by the investigator as to what the contaminants are to be monitored, and what their concentrations are. Sampling is always done exactly the same way, except that the sampling line pre-purge time or volume will have to be adjusted based on the depth of the well. No problem, as this is a very simple formula (length x Vol/Length = Vol). After a short pre-purge, simply attach the canister using a small, inexpensive sampling train that controls the flow into the can, then just wait 3-10 minutes for the canister to fill as shown by the vacuum gauge on the sampling train. That’s about it. Disconnect the sample train, cap off the hole and you’re on your way.
For the laboratory, the advantage of using canisters is almost immeasurable. Soil Gas can vary in concentration by nearly 1 million fold, whereas most GCMS analyzers are only calibrated over 100 fold range. That can be like the proverbial needle in a haystack to be lucky enough with tubes to be in the right range for the analyzer. With canisters, no problem. You can screen the sample by taking a small amount out of the canister for testing, or you can just dilute a small aliquot into another canister (or inexpensive Bottle-Vac) to dilute the sample down 100-1000 fold as needed. What labs DON’T want to do is just try running a tube sample blindly, as this many not only inject far too much into the GCMS, but it can contaminate their inlet system so the next several “automated” tube injections are lost as well, not knowing how much carryover there was from the previous “hot”, overloaded tube sample. This is a huge issue for labs, as this not only creates downtime, but they are faced with telling their customer that they “lost” their samples. Many air labs are very concerned about losing customers who are not pleased with having to re-sample. The use of canisters virtually prevents this, as a general rule for canister monitoring is to collect enough sample to allow 2, 3, or even more analyses from each canister, allowing quantitative results to be reported for >99% of all samples sent to the laboratory. Ultimately, there is usually a governing SOP that suggests small changes to the canister filling procedures, so these need to be reviewed and implemented. However, lab analysis is typically done by “best TO15 procedures”, of which there is an extensive network of labs that are set up to do just that.