Measuring Volatile Organic Air – Can it or Bag it?
Oft times, I wonder why a method will specify a certain type of container, preservation, or treatment prior to analysis for a specific target analyte. The collection of air for volatile organic compound (VOC) analysis is one of the methods for which I have come to understand why certain sample-collection media must be used.
The collection and analysis of VOCs in air can be a tricky thing. How does one effectively capture and quantitatively analyze a specific compound? Air samples are collected and analyzed for a variety of purposes, including community air monitoring, stripper effluent monitoring, or soil gas testing. These air samples are often analyzed for a long list of VOCs by US EPA Method TO-14 or TO‑15. These methods dictate the use of a Summa® canister as the sample container or media. A Summa canister is a specially prepared stainless-steel container that has an unreactive surface on the interior and is decontaminated and evacuated prior to use. The canisters are expensive and, fortunately, are re-usable after decontamination and confirmation testing.
Environmental Standards has observed that some air samples analyzed for VOCs by TO-14 or TO-15 have been collected in Tedlar® or Teflon® bags. These single-use bags can be obtained directly from a vendor, and do not necessarily need to be acquired from a laboratory. The bags are also cheaper than canisters. However, Methods TO-14 and TO-15 do not address their use. The most current version of TO‑15 (TO-15A, dated September 2019) indicates that non-rigid containers (e.g., Tedlar, Mylar, or foil bags) are not appropriate for sample collection.
In general, single-use, non-rigid containers are not tested for the absence of target analytes and are not cleaned prior to use. In addition, laboratories and manufacturers do not test the non-rigid containers to ensure that they are non-reactive with the target compounds.
Like all scientists, the question of Tedlar bag performance needed to be explored. Two laboratories agreed to assist with testing of Tedlar bags. The laboratories utilized the TO‑15 laboratory control sample (LCS) standard solution (yup, air standards are still solutions) and placed the LCS standard into Tedlar bags. The laboratories analyzed the contents immediately after placing the LCS standard in the Tedlar bag and then after 3 days. The laboratories utilized the Tedlar bags that they had on hand. Unsurprisingly, by day 3, some VOCs recovered poorly, and others recovered well.
While some regulatory agencies will allow the use of non-rigid bags for the collection of samples to be analyzed by Methods TO-14 or TO-15, the agency approval does not mean high‑quality data will result. Environmental practitioners should consider the analytes and importance of the sample collection when selecting a collection container. Due to the reactivity of the Tedlar bags tested, these bags do not appear to lend themselves to good behavior with all the targeted VOC compounds.
The sample container or media matters for the successful outcome of the project. If you intend to use non-rigid bags for sample collection, you should work with the laboratory to determine if the bag will perform appropriately for the compounds or concern. If you have questions about your air projects and/or would like to see the recovery data discussed earlier, please contact us.
