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Advantages in Using Smaller, 600cc Silonite Canisters for Soil Gas Analysis
The US EPA Toxic Organic Methods TO1 through TO17 were developed for ambient air analysis over a period from 1980 to the mid-1990s. Methods TO14(A) and TO15 are canister methods initially developed around the utilization of a 6L canister to sample ambient air down to sub-PPB concentration levels for analysis. Over the past 20 years, laboratories have also applied Method TO15 to collection and analysis of soil gas samples. Surprisingly, between 50-80% of all “TO15” samples analyzed by today’s contract laboratories are related to soil gas analysis.
While very low level detection limits are required when measuring the impact of soil gas intrusion on Indoor Air Quality, progress over the past 20 years, in the form of both higher sensitivity mass spectrometers and sample preconcentrators with better system hygiene, has further enhanced the “signal to noise” ratio without requiring more signal (larger sample sizes).
These advancements are allowing for smaller sample volumes to be used to achieve required detection limits. In particular, since soil gas concentrations are expected to be 10-1000x higher than the indoor air in buildings directly above the contaminated soil, less sample is required when measuring sub-surface soil gas. Soil gas frequently needs to be diluted to bring it into the calibrated range of the method.
Prior to the use of canisters for soil gas analysis, most soil gas measurements were made using a 10cc gas tight syringe, with an analysis in a mobile lab, either using a Purge & Trap system, a loop injection inlet, or by a simple splitting of the 10cc injection. Most Purge & Traps do a 20:1 split upon injection from their 1/8” trap, so each of these techniques yields similar detection limits. When canisters became accepted for soil gas measurements, most of the canisters in existence were the 6L variety, so it made sense to start using those. However, compared to the 10cc syringe, the 6000cc volume of the canister was collecting far more sample than was needed for the analysis. It was also determined that such a large volume could more easily draw ambient air down from the surface, diluting the soil gas and creating a negative bias in the analytical results. Therefore, there was a need to reduce the size of the sampling canisters to avoid exceeding the volume of equilibrated downhole soil gas.
So how small can we make canisters while still being effective for soil gas monitoring? Some things that limit how small we can shrink canisters include.
- Minimum integration times
- Minimum volume to achieve detection limits
- Sufficient volume relative to soil gas sample train volumes
- Minimum volume to flush the analyzer to deliver “fresh” sample to the preconcentrator
- Enough volume for at least 2 analyses
"Collection rate is what is important..."
For sub-surface soil gas collection, a “collection rate” is what is important, rather than a “collection time”. Sampling rates of 200cc/min or less are normally specified for permanent wells, although slower flow rates may be needed for shallow probes (5, 10, 15 feet) and sub-slab measurements if soil permeability is not sufficient to maintain flow rates of 200cc/min while maintaining less than a 100” H2O downhole vacuum (approx. 7.3” Hg) during the sampling process. In the case of low permeability soil, flow rates of 50cc/min or even less may be needed for shallow, temporary probes. Sampling rates rather than sampling times take the canister volume out of the equation, and in fact, a smaller canister size allows shorter sampling times, which improves efficiency, and reduces the amount of tracer gas, such as helium, that is needed. When using a tracer gas, continuous attention is generally required during sample collection, so a quick sampling of 5-10 min rather than 30-60 min is highly desirable.
When collecting within 5 feet of the top of the vadose zone, a 10x attenuation is considered a conservative estimate of what may be introduced into the indoor air environment. To achieve the necessary levels of detection, an analytical volume of 25-100cc is typical when performing a splitless injection. Therefore, in order to allow at least 2 analyses per sampling event, a minimum sample volume collected should be 400cc. Since canisters are often not filled to atmospheric pressure, adding another 50% to that volume would provide a safety factor to ensure there is plenty of sample for a rerun. That means a total canister volume of 600cc is optimum to allow repeat analysis if needed.
Rotary valve based autosamplers that were common with ambient air canister analysis from 1990 through 2010 using 6L canisters requires a flush volume of 20-50cc to ensure that the gas in the “common line out” of the stream select valve is well equilibrated to the next sample concentration. This in itself requires a larger canister size when considering that it may be necessary to run a sample 2-3 times. The introduction of robotic autosamplers that do not require any preflush volume is a big reason why smaller canisters can now be used. Even HDS Personal Monitoring canisters that contain only 50cc are analyzed successfully with the Entech 7650-M robotic autosampler, so 600cc canister analysis is obviously not an issue. When the heated sample line makes a connection to the top of the canister, the volume measured by the pre-concentrator during trapping is actually the volume that passes the canister valve into the heated transfer line, not the volume that flows through the trap at that particular moment. After trapping the sample, the robotic autosampler moves the heated inlet to a Nitrogen or Helium flush port to flush the rest of the sample to the traps, ensuring a complete and quantitative delivery of the sample. There is simply no way to do this with a rotary valve autosampler. The 7650-M Entech robotic autosampler makes the analysis of 600cc canisters not only a reality but improves the accuracy relative to 6L canisters analyzed through rotary valve autosamplers by eliminating the line to line variability in the background created by the 16 separate lines used in rotary valve based systems.
Is a 600cc canister size within State guidelines? A few labs have been promoting the even smaller, 400cc canister for soil gas and IAQ over the past 20 years, using SIM monitoring of 100cc out of 400cc canisters to achieve 10 part per trillion MDLs for most compounds. Many States specify volumes of 400 to 1000cc as acceptable canister sizes, so 600cc is definitely within this range.
Adopting 600cc canisters has several advantages over larger canisters:
- Much lower shipping costs
- Lower acquisition costs
- Reduced storage space requirements
- Much higher autosampler throughput (48 instead of 16)
- Robotic Autosampler Friendly to reduce carryover and positive bias potential
- Much higher cleaning system throughput (40 instead of 12 per oven)
- Twice the 100% certification throughput (combined aliquots in one analysis)
"Canisters can be screened without the complication and hassle of 100% desorption..."
Items 1-3 above are obvious. Some labs have considered trying to bring on US EPA Method TO17 for soil gas analysis simply due to the cost of shipping larger canisters. Instead, 600cc canisters are smaller than the combined volume and weight of tubes and sampling pumps needed to perform TO17. On top of that, trying to perform dilutions using TD tubes to address the million-fold concentration range found in soil gas can be a nightmare. Alternatively, a simple syringe transfer from one canister into another, or into a clean Bottle-Vac makes dilutions with canisters extremely easy and reliable. In addition, canisters can be screened without the complication and hassle of 100% desorption and retrapping on other tubes that may have unknown levels of contamination. You can’t take just part of the sample out of a tube, but you can out of a canister.
The Entech 7650-M robotic autosampler can handle 48 samples unattended, all making a direct connection to a single heated line. That is far better analytically than attaching 16 canisters each to separate transfer lines with individual background levels based on their exposure to previous high or low-level canister samples. Using the 7650-M robotic autosampler, a full 48 600cc canisters be either screened prior to analysis, analyzed through the 7200 with zero potential for cross contamination, or blank tested with faster throughput than rotary valve based solutions. All of this means a more accurate solution and greater productivity for the laboratory, in a smaller amount of space.
Indoor Air Quality testing during vapor intrusion investigations
For Indoor Air Quality testing during vapor intrusion investigations, a 200cc sample size can provide low part per trillion level detection, and single digit part per trillion MDLs when using SIM analysis. With the ability to sample for 24 hours using a CS1200E6 flow controller into a 600cc canister, these smaller canisters could even be used in place of 6L canisters for low-level work. Alternatively, a 1L Silonite canister can be used with a CS1200E5 flow controller to collect a 24-hour sample as well, giving, even more, sample for more repeat analyses if needed. Reducing the volume from 6L to 1L certainly, provides a huge reduction in shipping costs and required lab space, while boosting productivity. A full 24 1L canisters can be analyzed unattended on a 7650-M which is generally enough to keep the system operating 24 hours a day at least from Monday through Saturday morning. For 600cc canisters and 48 sample automation, that runtime extends through to Sunday morning with no overtime scheduled.
The future of TO15 lies in the improvement in the efficiency of how this method is performed. Taking advantage of today’s higher sensitivity GCMS systems and improved sample preparation technology means being able to use smaller sample sizes, from smaller canisters. As long as the technology is now available to take advantage of these smaller canisters (low flow samplers,no pre-flush autosamplers), labs can dramatically improve their efficiency and competitiveness by moving towards smaller canisters. Considering that more and more States are approving smaller canisters for soil gas monitoring, the transition should occur fairly rapidly.
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Tricks for Getting Canisters Clean the First Time
Canister sampling and analysis remains the most effective means of monitoring C1 to C12 Volatile Organic Compounds in air. Today’s analytical systems are capable of achieving reproducibility in the range of 2-3%, which is as much as 5-10 times better than typically achieved when using thermal desorption tubes, where the tube (the preconcentration device) is changing on a per-sample basis. The only issues remaining in achieving the true potential of the canister technique is eliminating positive and negative bias resulting from inconsistencies in the canisters themselves. Negative bias is caused by using canisters that are no longer capable of maintaining compounds in the gas phase, causing them to be under-reported in the analysis. Positive bias is the result of using canisters which were not cleaned properly after the prior sampling event, leading to results that are artificially high. The condition that causes positive and negative bias are actually linked, as the same absorptive and adsorptive issues that causes negative bias losses, make canisters harder to clean up contributing to a greater potential for a positive bias, especially after collecting and storing samples with elevated levels of target compounds. Improving the inertness of canisters generally contributes to the reduction of both kinds of bias.
Getting canisters clean the first time is always the goal of every air lab, as the need to re-clean failed canisters adds significantly to overhead costs, while delaying the return of canisters to the field to collect the next sample. To improve the cleaning of canisters so they pass the first time, it’s important to know what is actually going on inside of the canister when samples are collected, stored, and then cleaned. EPA Method TO15 discusses the need to verify the sample train to show that it is recovering 80% of the target compounds, and the canister itself is certainly a part of that sample train. TO15 doesn’t specifically address the recovery from the canister after a reasonable holding time of, say, 2 weeks. In general, shooting for an 80% or better recovery is well within reason for this technique, again when using canisters that are inert. So, what happened to the 10-20% that are no longer in the gas phase, even with canisters that are considered to be acceptable (>80% recovery)? In most cases, the lost chemicals have not reacted, but rather have just partitioned into the walls of the canister. What’s actually happening is that chemicals will create an equilibrium concentration between the walls and the gas phase within the canister. This equilibrium will mostly set up over the first 24 hours after sampling but will continue to reach a final equilibrium over even a longer period of time. The trick to cleaning canisters is reversing this equilibrium and allowing some time for a “new equilibrium” to become established. What does this mean?
How long will it take for Compound X to come out of the walls?
Equilibrium is achieved when the rates between two states are equal. In the case of canisters, a given target compound X is going onto the walls of the canister as fast as it’s coming out. The moment canister cleaning commences and the first evacuation occurs, the equilibrium between the gas phase and the adsorbed phase in the walls has been disrupted, because the rate of transfer back into the walls drops to zero as the canister is evacuated. So how long will it take for Compound X to come out of the walls? That depends on the compound. It also depends on how much needs to come out to achieve required blank levels. Remember that if 10% of Compound X is in the canister walls, but that compound was at 1000x over the reporting limits, then there exists 100x too much of Compound X within the walls. This needs to be eliminated from the canister.
Time, temperature, and vacuum are all important to removing Compound X, and all compounds from the canister walls. With this understanding, we can see that getting canisters under vacuum ASAP after they are analyzed is critical for getting the cleanest canister. By using a separate dual stage, oil-less vacuum pump exhausted to a hood, canisters can be evacuated to just 1% of atmospheric pressure within 3-4 minutes. This will cause chemicals in the walls to start “re-equilibrating” back into the gas phase. Then, removing these canisters from the vacuum source and just allowing them to sit is actually causing them to be cleaned, as the extraction of Compound X and others is easy once they are in the gas phase. If there is an equilibrium such that 90% will be in the gas phase and only 10% in the walls, then by just sitting for a while these canisters will have 10x less in their walls compared to those canisters that were not pre-evacuated. Then, when the canisters are officially cleaned on the canister cleaner, they will be able to achieve a final cleanliness that may approach 5-10x lower than if this initial evacuation had not occurred the day before, or to a lesser yet still significant extent just several hours before. As an added benefit, the canister cleaning system will be subjected to 10x less contamination, making it easier to maintain very low background levels.
Don’t let canisters sit around with their original samples in them while they are waiting their turn to be cleaned!
Once the test results are in and the canisters can be cleaned, perform the quick, rough evacuation. Then let them sit and wait their turn. The result will be canisters that pass the first time, and a lab manager with a bigger smile on his face. And of course, when very hot samples are encountered, use this same concept but beef it up a bit. Try evacuation for a day, refill the canister, and then another quick evacuation for another day. Each time should give an attenuation of 10-50x, depending on how much has partitioned into the walls. Multiplied together, those hot cans are not going to be a problem.
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Comparing SiloniteTM Canisters vs SUMMA & other TO-15 Canisters for Air Monitoring - What's Best?
It was about 30 years ago that the first release of EPA Method TO-14 appeared for monitoring of VOCs in ambient air that specified the use of 6L stainless steel canisters having a special internal “SUMMA” passivation to improve chemical inertness. Although “SUMMA” really refers to the process used to apply a Nickel/Chromium oxide layer to the inside of the canister, the canisters themselves became known as SUMMA canisters. The whole purpose of applying the Ni/Cr Ox layer was to prevent exposure to iron, as iron is known to catalyze the reaction and subsequent loss of several of the VOCs in the TO-14 method list, thereby reducing the effectiveness of the canister as a sampling and storage device. With canister whole air sampling, it’s all about how long the sample can remain in the canister without any of the concentrations changing. The Ni/Cr Ox layer greatly improved the stability of TO-14 compounds in 304 stainless steel canisters relative to uncoated canisters. Considering that 304 stainless is 70% iron, applying something to the internal surface to cover up the iron is really not an option.
The SUMMA passivation process has some drawbacks. First, although the Ni/Cr Ox layer prevents exposure to iron, it leaves an ionically bound surface that will adsorb polar VOCs and aromatic compounds unless there is enough water vapor in the canister to out-compete these compounds for this somewhat active surface. This works for the compounds on the TO-14 list, but more reactive compounds including reduced sulfurs and amines still exhibit poor stability. Also, it is important to realize that extra water must be added to SUMMA canisters to keep VOCs in the gas phase when performing sampling when relative humidities dip below about 20%. Remember that “percent relative humidity” is temperature dependent. When sampling on a Winter day where the ambient temperature is below 0 deg C, collected canisters will almost certainly be below 20% RH when brought back into a laboratory that is at a more comfortable 25 deg C. In these cases, water should be added to SUMMA canisters “prior” to sampling, by injecting about 50ul into the evacuated 6L canister before deployment to the field. Also, during canister cleaning, it is recommended to fill and evacuate SUMMA canisters a few times at room temperature before heating the canisters, as raising the temperature to 80 deg C will drop the Relative Humidity to just 1-2%, and polar compounds will stick quite effectively to the SUMMA passivated surface, preventing their removal from the canister.
SUMMA Canister manufacturing is also expensive, as it creates extremely toxic chromium waste that requires disposal into specialized landfills. For the past 15 years, deactivation layers consisting of vapor deposited silica have been replacing the SUMMA passivation process, as the silica layer is more inert, requires no water vapor to complete the canister passivation, and produces no toxic waste. The gas phase deposition process also makes for a smoother deposition, relative to the diffusion limited liquid phase SUMMA deposition process. Silonite Canisters have this internal silica layer and have been demonstrated by independent laboratories to be the most inert canister available, based specifically on the recovery of hydrogen sulfide. The silica layer is similar to the inside of a GC column, and Silonite canisters are helping to increase the range of compounds recoverable to approach nearly that of “all GC compatible compounds”. For SVOC range compounds (Semi-Volatiles), Silonite canisters must be heated during analysis to ensure that these heavier compounds are driven quantitatively into the gas phase, just like a GC column needs to be heated to elute heavier injected compounds. Entech has developed laboratory analyzers that automate the heating process, making it possible to extend the range of recoverable compounds from C1-C12, to C1 to over C25.
So the next time someone asks you about SUMMA canisters...
You can tell them that almost all canisters made today are not SUMMA Canisters, but are either Silonite Canisters or some other brand. Please see our complete discussion of canisters in our “Understanding Canisters Technology” document in our Technical Library. Beware of other canisters on the market with “no internal coating at all”. These manufacturers have simply electropolished and nitric acid passivated the 304 stainless, which only removes the iron that is right on the surface. When brand new, these canisters are shown to be inert because initially there is little iron on the very surface after nitric acid stripping during manufacturing. However, as these canisters age, oxidation and corrosion will expose the iron just under the surface, drastically reducing the stability and recovery of extremely carcinogenic compounds such as carbon tetrachloride. Many of these canisters are marketed as “SUMMA-Like”, but they don’t have the actual SUMMA applied NiCrOx surface that is hundreds of atoms thick, preventing the iron well below the surface to ever be exposed, even after 20 years. Finally, “all ceramic” deactivated glass canisters, called Bottle-Vacs, do not have reactive iron to worry about, and have shown recoveries equivalent or superior to those of SUMMA canisters for TO-14A and TO-15 compounds, and these are slowly gaining popularity as a low cost solutions for canister sampling. Bottle-Vacs are also becoming popular for diluting very high concentration canister samples in the laboratory prior to analysis.
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A stainless steel canister based method has been developed to measure both VOCs and light carbonyls in indoor air using a single GCMS analysis. MiniCan stainless steel samplers collect the sample using either a quick five second fill technique (grab sample) or by maintaining a slow constant flow into the canister which gives the average concentration of contaminants in air over time. The vacuum in the MiniCan samplers is used to draw in the sample, eliminating the need for sampling pumps or a source of power (AC or batteries). A new coating called SiloniteTM on the inside of the MiniCan samplers provides an ultra inert surface that prevents the adsorption of analytes. This makes the MiniCan an ideal choice for sampling reactive compounds, and performing site investigations where a more universal technique is needed to detect anything that may be present.
The analysis of MiniCans is performed by GCMS using a preliminary preconcentration step to improve method sensitivity. Up to 100cc of sample is concentrated down to a few microliters and injected rapidly onto a GC capillary column for analyte separation and detection by a mass spectrometer.
SiloniteTM coating throughout the preconcentrator’s internal tubing allows a wide range of compounds to be recovered and quantified down to 1-2 PPB(v) in a single analysis.
This application note details the use of MiniCans for sampling and analysis of formaldehyde, light carbonyls, and common VOCs in the concentration range commonly found in indoor air (1-200 PPBv). Included in the study is storage testing in MiniCans, analytical reproducibility, response linearity, and analyte detection limits.
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This is the complete Entech 2018 Catalog. You can download the PDF or use our Flipbook viewer (below) for a smoother experience. Please note that not all sampling products are included. The Entech store is the best place to find photos and descriptions of every product we carry.
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Air Sampling Canisters – Why Coating is not Optional
The Lessons Learned from SUMMA® (Nickel Chromium Oxide) and SiloniteTM (Ceramic) Canister Passivation Coating Technologies.
SUMMA® NiCrOx Coated Canisters – Early EPA Method Development
Sampling into stainless steel canisters was first conducted to measure freons in the atmosphere to help scientists determine which compounds were responsible for the depletion of the ozone layer. In 1982, the EPA Compendium of Methods for the Determination of VOCs in Ambient Air first appeared which specified the use of SUMMA® passivated canisters for collecting air samples to measure levels of 40 solvents and aromatics.
SUMMA® canisters were coated with a Nickel Chromium Oxide (NiCrOx) layer using a liquid deposition bath process that effectively covered the corrosion susceptible 304 stainless steel. Stainless steel features a chemical composition of 70% iron – a very reactive and catalytic metal.
The NiCrOx coating of 500-1000 angstroms prevented exposure to iron and was thick enough to prevent future exposure after years of corrosion / oxidation through sampling of moisture, ozone, NOx, oxygen, and other oxidizing compounds found in ambient air.
SUMMA® NiCrOx coating is generally effective for TO-15 compound recovery at relative humidities of > 40%. SUMMA® manufacturing challenges: The NiCrOx canister coating quality is subject to bath degradation from increased levels of iron released from previous canisters. In addition, glycols used in the bath are difficult to completely remove from the surface, creating aldehyde and alkene breakdown products to appear over time.
SiloniteTM Ceramic Coated Canisters – EPA TO-14 and TO-15
Later in the 1990’s, both the EPA and various agencies/investigators worked to increase the range of compounds that could be recovered from canisters. During this time, Entech Instruments, Inc. developed and introduced SiloniteTM – using a different kind of ceramic to prevent exposure to the iron comprising the majority of 304SS. After electropolishing, the additional SiloniteTM chemical vapor deposition process yielded an even more consistent and inert coating layer than was possible with the SUMMA® approach. Similar to SUMMA® passivation layer, this coating was typically over 500 angstroms thick to prevent any contact or corrosion of the underlying iron.
SiloniteTM ceramic coating is inert and durable, enabling the complete recovery and storage of an extended range of compounds. SiloniteTM also greatly improves canister reliability and lifespan in the field.
Uncoated Air Sampling Canisters
The dangers of corrosion and limited canister lifetime
In order to lower the cost of production, many canister manufacturers started to promote uncoated canisters that were ONLY electropolished and acid passivated to remove the surface iron. Unfortunately, this process did not include a coating layer to prevent future exposure to iron and subsequent corrosion / oxidation – after these canisters were placed into field service.
Sometimes referred to as “SUMMA-Like” canisters, these uncoated metal canisters can show relatively good performance until they are exposed to ozone, moisture, and oxidants in ambient air, which results in a breakdown of this barrier via a few different mechanisms. As the internal polished surface degrades, the holding time data reveals that uncoated canisters can begin to show significant compound losses and reactions in as little as 6-12 months of field sampling use. This process ultimately renders these canisters inadequate for the analytical demands of EPA TO-14A and TO-15.
Surface breakdown mechanisms: Considering the small size of a water molecule, an oxygen molecule, and singlet oxygen as ozone breaks down, a thin 5-10 angstrom zone is simply not thick enough to keep these corrosion and oxidation promoting species from reaching the reduced metal surface of the 304SS canister. As oxidation and corrosion occurs, the canister surface begins to expand with the incorporation of both oxygen and hydroxyl group (–OH), causing surface bulging and cracking, leading to enhanced exposure to the iron below. This unrelenting process can cause rapid conversion of uncoated canisters that initially work reasonably well for TO-14A and TO-15 compounds when new and later renders them ineffective for even a 1 week storage of many important TO Method compounds.
Moisture and the Rapid Corrosion Paradox
Water is at a lower energy state when there are dissolved ions present. In fact, what is referred to as 18-megohm water (ultra pure de-ionized water) is rather corrosive when placed in contact with metal surfaces – at least until a small amount of the metal surface transfers into the water. This same kind of corrosion and stripping of the surface effect can certainly occur in air sampling canisters.
Unfortunately, a condition exists when collecting high humidity samples – especially during the Summer months – which causes excess water to be collected. This additional water condenses onto the sides and eventually drips down into the bottom of the canister. In uncoated canisters, this process strips a small amount of the metal/metal oxide surface away.
Subject to this mechanism occurring only once per sampling, even an uncoated canister may give a few years of effective use. However, most laboratories store canisters in non-temperature regulated space, and the temperature during shipping is anything but regulated. The potential daily cycling of temperatures from high to low to high again causes repeated evaporation, condensation, stripping, and re-evaporation – even after just a single sampling event. This accelerates the aging process, causing irreversible damage to an uncoated canister in months rather than years as illustrated in Figure 1.
Alas, once exposure to the underlying iron surface has occurred, there are no effective ways to return an uncoated canister to useful operation that would meet the expectations of EPA TO-14A or TO-15 Methods.
The Bottom Line
Insist on a coated canister to ensure the best possible canister inertness, corrosion resistance, and reliable sample storage for EPA TO-14a and TO-15 Compounds.
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Stainless steel canisters offer a tremendous advantage over other air sampling media, namely Tedlar® bags and adsorbent traps. When canisters were first introduced, the “SUMMA” passivated canister was the only option available. Today, with the availability of electropolished, silica lined (Silonite™ and others), and glass (Bottle-Vac) canisters, as well as multiple valves and gauges to choose from, air monitoring personnel must be further educated on how to properly choose, clean, evaluate and certify a canister for TO14 and TO15 analysis. This document gives a background overview of different types of canisters as well as the procedure for properly certifying them for use.
For chemicals to remain in the gas phase, the internal canister surface must be inert to minimize adsorption.
Research efforts to create this level of inertness have led to SUMMA, electropolished, silica ceramic coated, and deactivated glass canisters. For more detail into the difference of each type of canister and properties, please refer to Entech Application Note Document 501, Understanding Sampling Canister Technology.
Due to the range of canister types and the wide variability of samples and conditions that each canister sees, it is a long established fact that not every canister will perform up to the original manufacturer’s specifications. For this reason, canisters must be evaluated individually to ensure that all target compounds can be recovered after a reasonable holding time. Entech recommends that canisters go through the evaluation and certification process described in this document once every two years or whenever a canister is suspected of having a problem.