Stainless steel canisters used to collect air samples for analysis of PPM to sub-PPB level VOCs are generally cleaned using several cycles of filling with Ultra High Purity air or nitrogen, followed by evacuation to a fairly high vacuum, with a final evacuation down to or below 50 mtorr. During the cleaning process, the ability to reach 50mtorr while several canisters are attached generally means that there are no leaks into the system, either through the cleaning system manifold or the canisters themselves. However, it does not mean that the canister valves themselves are leak tight when closed. Below are several tips that can be used with any canister cleaning system to achieve the cleanest possible cans to avoid false positives during the next cleaning event. These are just some of the issues Entech has discovered in supplying canister cleaning systems over the past 20 years.
There are several types of canisters being used for EPA Methods TO-14A and TO-15. In general, these can be summarized as “Coated” and “Uncoated”. Both Silica lined canisters (Silonite and SilcoCan) and SUMMA canisters are coated to prevent exposure to the iron which represents 70% of the composition of 304 stainless. Entech has been warning for years that “SUMMA -Like” canisters which have no internal coating can easily oxidize and/or corrode to increase the exposure to surface iron, causing may important compounds such as Carbon Tetrachloride to be completely lost in as little as 1 week, especially at lower humidity levels where a thin film of water isn’t available on the surface of the canister to reduce the catalytic nature of surface iron. Although the NiCrOx coating found in SUMMA cans to contribute significantly to their inertness over uncoated canisters, both SUMMA and uncoated canisters expose the sample to a metal oxide surface, which has been shown to adsorb polar and aromatic VOCs if relative humidity levels are not above 10-20% RH. When heating canisters during cleaning, relative humidity levels in canister can easily drop down below 3%, causing these polar and aromatic compounds to adsorb to the metal oxide surface and resist removal during cleaning. When cleaning these canisters, it is important to run a couple of cycles at room temperature for better removal of these compounds prior to heating the canisters. With Silonite Canisters, water is not needed to keep VOCs off of the surface, so heating can commence at the start of cleaning. In general, heat canisters to no more than 80-100 deg C, as higher temperatures are not necessary, and can dry out the lubrication in canister valves, affecting their operation long-term. Also, just like heating GC columns to higher temperatures in the presence of oxygen isn’t a good idea, don’t use zero air to clean Entech Silonite canisters at temperatures above 80 deg C.
Cleaning canisters using a cleaner capable of creating a strong vacuum, such as the Entech 3100A based systems (3112 and 3108) provides the perfect opportunity to leakcheck canister valves every time canisters are cleaned. When attaching canisters with packless valves (Entech’s Toxic Organics Valve – TOV, or Nupro Valves), simply keep the valve closed after attaching the canisters to the cleaning manifold, and attempt to pull a strong vacuum against the closed valves. Since the canister will still be at or near atmospheric pressure, this will create about 13-15 psid (psi differential) across the valve. If the vacuum system cannot reach it’s potential vacuum, and also if the pressure increases rapidly when the manifold is isolated from the vacuum pump, there is either a leak in the control valve or in the valve connection to the manifold. To determine where the leak is, leave the manifold connected to the vacuum system and go from can to can applying just “a little” extra sealing force to the valve’s control knob. Do not over-tighten Nupro valves, as this will permanently damage them. Entech TO Valves cannot be damaged by hand, so these can be tightened more aggressively. When you find the leaking canister, you’ll see the pressure drop at least a little when the slight increase in sealing force was applied. Make sure to check the vacuum between each canister valve tightening to avoid missing the affected canister valve. If the vacuum can drop down to an acceptable level when tightening the affected valve without having to apply an unreasonable amount of torque to the valve, then that valve may still be useable, but should be flagged for repair or replacement in the near future. If the vacuum does not drop at all when snugging all valves, try slightly tightening the valve/manifold compressing fitting, but again do not over-tighten. Some canisters use thin-walled tubing for connection of the valve, and this can cause damage to the valve fitting itself by causing the fitting to “flair out” as the thin walled tube collapses under the pressure of the ferrule. This has been found to be a problem even with softer Vespel/Graphite ferrules. Finally, if a strong vacuum cannot be achieved, consider that water may have been introduced into the cleaner, either during the canister sampling process (a big issue with soil gas sampling, so shake these canister before attaching to the cleaner to listen and feel for the presence of water), or by over-filling the system humidifier. If water is suspected, the most effective way to remove it is to cycle clean with the oven heated to 80-100 deg C, with evacuation to only about 1 psia during each cycle. With the Entech 3100A, just cycle with the diaphragm pump, not with the molecular drag pump to remove the excess water.
Sometimes it is believe that a vacuum pump creates a “force” that pulls gas molecules in the direction of the vacuum source. This, of course, is not the case. Vacuum pumps just remove whatever gas molecules find their way into the pump, creating an ultimate mass transfer across the pump. When a pump achieves its ultimate pressure (vacuum) differential across it, there will be no more “net” flow. Under these conditions, chemicals can freely migrate in “all” directions in this vacuum system, and their diffusion rates can be extremely high at higher vacuums due to the lack of collisions with other molecules. When using vacuum pumps that can introduce oil vapor into the vacuum system, there is a good possibility of oil vapor getting into the manifold and ultimately into the canisters themselves. Some suppliers will suggest the use of a cryo-trap to prevent this from happening, and indeed a cryo-trap will temporarily prevent these vapors from backing up into the cleaning manifold, but what happens when the LN2 in the dewar all evaporates, leaving this oil film to again experience the full vacuum of the system at room temperature? Yes, it can continue on it’s way to the canisters. So what happens if it reaches the canisters? These oils are heavy, and are not on the TO-14A or TO-15 lists, and canisters at room temperature will not be recovering them, nor will most GCMS preconcentration systems. Instead, this oily film is just going to change the characteristics of the canister, probably causing much lower recoveries as VOC absorption into this layer increases. The second issue to be concerned about with oil-based pumps is back-streaming. If power goes out prior to venting the system “upstream” of the pump, the existing vacuum can pull oil back into the lines, creating quite a mess. Entech eliminated the use of oil based pumps back in 1994 due to these issues, replacing them with a rough pump (diaphragm) and high vacuum pump (molecular drag – 10^-5 torr) that prevents oil from ever getting into the cleaning system. The Entech 3100A Cleaning System incorporates these dual stage oiless pumps into its vacuum system, and used by more laboratories than any other canister cleaning system.
As required detection limits continue to decrease to the low part-per-trillion levels, it is becoming increasingly important to maintain very good canister cleaning system hygiene. In particular, with so much soil gas work being accomplished using whole air sampling into canisters, cleaning systems today can be bombarded with concentration levels at thousands of PPM, while requiring to re-establish blank levels millions of times lower. There are several things that labs can do to accomplish this, no matter which cleaner they are using;
Please contact Entech if you have any questions concerning canister cleaning and what works best. We’d be happy to help you obtain the best possible use of the investment you have made in you canisters, samplers, and supporting laboratory equipment.