The New U.S. EPA Method TO-15A Blog Series - Part 4: Clean Lines for Clean Air16 Jul 2020
In previous blogs (Part 1, Part 2, Part 3) we’ve covered the need to use air instead of nitrogen as a fill gas, the new lower blank requirements, and how these increase the importance for very clean lab air for canister filling. We’ve shown that using high quality gas filters or generators will get you the clean air required, but getting it to the canister ends up being more complicated than one might expect.
When we first started testing the new TO-15A blank requirements, we wanted to get the best baseline possible for our air quality. To do this we hooked our fill gas line up directly to our preconcentrator, allowing us to test the dry air without worrying about canister cleanliness. Note that TO-15A recommends testing your gas directly if possible in section 9.3. We found that while we were able to meet the 20 pptv requirements for most compounds, some of them were giving us trouble. Note that while we did test for the full 75 compound list for TO-15A and NJ mix I’m only showing the problem compounds for simplicity’s sake.
Table 1: Selected TO-15A blank results. Average of 3 tests.
We were confident in the quality of our incoming air, so where was the contamination coming from? We were using a setup that let us blend dry and humid air to a desired relative humidity, and while it’s very handy it also complicates the flow path with extra tubing, fittings, valves, and gauges. It had been in use for quite a while, and even low levels of contamination in the air and water could have potentially built up over time. So to test that we bypassed the system, which gave us much improved results.
Table 2: Selected TO-15A blank results after bypassing mixing system. Average of 3 tests
Acetonitrile is still slightly high, and toluene has increased as well. The air line used was next to our SPME prep area which had some small bottles of solvents, including acetonitrile. To better isolate the air from this we installed a new air line across the bench, about 15 feet away.
Table 3: Selected TO-15A blank results after moving air line. Average of 3 tests.
The new line was a bit higher in methylene chloride and tetrahydrofuran, but finally got us to 20pptv or less for the other compounds. From all of this I can give a few key pieces of advice. • Test your air with the shortest, simplest flow path to get baseline cleanliness • Compare any extra systems, such as mixing and humidification chambers, to this baseline to determine their contribution to your blanks. Over time these systems may need to be cleaned or replaced to meet the 20pptv limits. • Isolate your systems from solvents as much as possible. Having your air lab (and volatiles lab if you run 8260 or similar methods) completely isolated from any semivolatile GC, GC prep, or LC instruments is ideal, but if they can’t be in separate rooms at least keep them as distant as possible.
Once you are confident in the cleanliness of your air, the next complication is humidification. The next blog in this series will address that, so stay tuned.