Last time, we left you with a teaser, how to take your canister blanks from the following red trace down to the blue trace:
Before we get to that, let us back up to 2015, where both Wayne Whipple (retired US EPA) and I coincidentally presented on canister cleaning and canister blank levels at the National Environmental Monitoring Conference (NEMC). Neither of us knew of the other’s presentation, but apparently we both saw the same thing… No, it was not the new canister cleanliness requirements of the pending Method TO-15A on the horizon. Rather, we both saw that canister cleanliness was the rate-limiting step in achieving lower detection limits with evacuated stainless-steel canisters. It was not the instrumentation then, nor is it the instrumentation now. With reasonably up-to-date equipment, I know of several laboratories (including ours), which routinely achieve 5 to 10 pptv detection limits for most of the VOCs targeted by TO-15A. For the record, Wayne Whipple routinely had single-digit pptv detection limits with his Leco Pegasus V TOF-MS. Obviously, most of us are not as lucky as Wayne; regardless, it does not matter how sensitive your instrumentation is if your canister blanks remain higher.
Because I like a challenge and I am a geek who enjoys researching the minutia of a subject like canister cleaning, we proceeded to experiment with canister cleaning variabilities and presented these results at Air and Waste Management (A&WMA), NEMC, and other global conferences from 2015 to 2018. We are going to take the next several blogs to break out some of these experiments and results to shine a light on what will move the needle for your canister cleanliness, as you strive to achieve the new Method To-15A guidelines. Although obvious for most, I feel compelled to point out just how many different variables come into play when looking at canister cleanliness. The following is a screen shot from one of our canister cleaning presentations:
As you may see above, we have a list (that may not be exhaustive) of a dozen or so variables, which offer the opportunity to increase the canister blank concentrations via anyone or combination of the following: a leak, contamination (intrinsic to the material), carry over, etc. For example, use contaminated water to humidify your canisters and it simply does not matter how clean the canister was prior to that point. Long story short, there will be a prevailing theme in the following blogs: “garbage in = garbage out.” With that said, the first thing I want to start off with is fill gas. As you will notice in section 9.4.2 of TO-15A:
“Canister zero-air challenges are performed by pressurizing clean evacuated canisters with humidified (40% to 50% RH) HCF zero air. Note that performing this qualification with ultrapure nitrogen does not adequately test the canister as the inert nitrogen atmosphere does not permit reactions within the canister that may occur when ambient air is sampled.”
So why does the EPA make a point to call out air over nitrogen? Well, that question was rhetorical, as the EPA tells us point blank that nitrogen is an inert gas, which does not permit reactions to take place, which may otherwise take place during and/or after field sampling of AIR. Okay, so I added the last part, but it is true. You know why… because we sample air, which has oxygen and that means there is an oxidative potential, which is otherwise absent in the inert nitrogen. This is just sound logic folks! So, then the question becomes why is anyone using nitrogen in the first place? The answer to that questions is steeped in history. Most laboratories were setup back in the day with preconcentrators, which required liquid nitrogen to cryogenically cool the traps. The resultant presence of a liquid nitrogen dewars throughout air laboratories meant everybody had access to a very clean source of fill gas, which was literally just going to be wasted anyway. And since nitrogen represent 80% of air, somewhere along the line there seemed to have been this leap of faith to the suitability of nitrogen as a fill gas. I know the vendor that installed my first preconcentrator successfully convinced me of this and I ran nitrogen as a fill gas for several years, until I generated the data to show this was not the best practice. So yes, the need for air over nitrogen as a fill gas is not speculation or theory on the behalf of the EPA. We presented the results to support this back in 2017 to the U.S. EPA and scientific community.
In the following table, we evaluated canister cleanliness with helium and air as the fill gas. For the following results, all canisters were humidified to 50% RH (more on the importance of this in future blogs) and aged for the canisters for 7 days (more on this as well in future blogs); and everything else was equivalent for an apples-to-apples comparison:
|Compound||Sample Size (n)||Average Helium Conc. (pptv)||Average Air Conc. (pptv)||P-Value|
As you may see in the table above, acrolein (which was a hot topic for quite some time) grows more in canisters filled with air when compared to canisters filled with an inert gas like helium. Even for a more mundane compound like benzene we make the same observation. In both cases, this was a statistically significant trend. Now I know you are probably thinking one or more of the following:
- This is the product of the gases coming from different sources and/or lines and thereby contributing to the blank levels.
- We used helium and not nitrogen.
- These results are not orders of magnitude different.
- These results would not meet the new Method TO-15A cleanliness requirement of 20 pptv.
My responses to those thoughts:
- Both gases were run through the same lines and the above blank results were background corrected for each gas (i.e., each gas was plumbed directly to the preconcentrator 1st and analyzed independent of any canisters).
- Yes, but an inert gas is an inert gas in this scenario. So, long as it does not contain the oxygen, which air contains. I encourage you try it for yourself with nitrogen and air. I have yet to see any results to contradict what we are saying. Oh… and we recently had a customer with ethylene oxide growth (the next acrolein in my opinion) in their canister blanks. They never saw the EtO in their blanks (oddly enough, filled with nitrogen), but it grew in their field samples (filled with air). I do not want to give too much away on this, as I know my colleague Jason Hoisington will be blogging on this in the very near future.
- You are correct, at best we see a 2x difference for acrolein. However, we are not talking about orders of magnitude in improvement anyways. We are talking about moving the needle in the correct direction with incremental improvements to the above dozen or so variables originally identified as blank contributors. All in the name of trying to consistently achieve the 20 pptv cleanliness levels.
- Remember that teaser from the beginning, well it looks like I have rambled on long enough, so that will have to wait until next time. I know, I said that last time, but I promise this time.
Final thought: if you fill your blank canisters with an inert gas like nitrogen, you risk getting artificially biased low blank concentrations for some of your target analytes, which may look fantastic for meeting cleanliness requirements. However, the problem is that these blank results are not consistent with what those canisters will experience when filled with humid air in the field. So, now you have some background information on why TO-15A calls out the use of air as the fill gas for the blank challenge. Stay tuned to see how we make sure that air is clean…