3-Stage Preconcentration has been the standard for performing canister analysis over the past 20 years. Two 3-stage techniques for trapping, water management, and on-column focusing have been Pioneered by Entech Instruments, and include Microscale Purge & Trap (MP&T) and Extended Cold Trap Dehydration (ECTD). An overview of the challenges confronting the quantitative preconcentration of air samples is presented below, followed by a description of the different techniques currently being used to proconcentrate VOCs and manage water vapor, and why MP&T and ECTD typically provide superior results to that of multi-bed adsorbent trapping and dry purge water management.
Today’s GCMS analyzers only have enough sensitivity to quantitatively measure gas phase chemical at levels above about 0.1PPM without first preconcentrating the sample prior to GCMS injection. Since VOCs exist in ambient air at levels ranging from 0.01 – 10 PPB, virtually nothing would be detected without initial preconcentration. One of the challenges that exists during sample preconcentration is the elimination of all bulk air compounds, including Nitrogen, Oxygen, Argon, Carbon Dioxide, and Water, without loss of any of the VOCs of interest. Most of the compounds in this list are easily separated from the VOCs during trapping due to their low boiling points, but water boils at 100 deg C and its removal without loss of important compounds becomes more difficult. Proper water eliminate is then typically a key factor in proper preconcentration of air samples. But what’s so important about eliminating water? GCMS system that are optimized to measure trace levels of chemicals cannot handle the co-injection of other chemicals at much higher concentrations, otherwise both the sensitivity of the mass spectrometer will be negatively effected, and chromatographic distortion of injected chemicals will also occur on the narrow bore capillary GC column due to over-loading. At 1-3% concentrations in air, water is at tens of millions of times higher in concentration than compounds to be measured. Most of this water must be removed, but care must be taken not to remove many water soluble classes of VOCs during the process. Water is very polar, having very strong hydrogen bonding tendencies, and many of the VOCs of interest are also polar and water soluble. Removal of water without loss of these polar VOCs can be tricky, and any surfaces in the trapping system that are not completely inert can attract water, reducing the ability to remove it efficiently from the sample.
A second issue is trap reactivity, which in turn is affected by the composition of the packing, the addition of any foreign material on the front of the trap over time, and the necessary temperature needed to desorb the VOCs after initial trapping. Trapping systems that are designed with stronger adsorbents will require higher temperatures during the desorption process. As a general rule, organic chemical reactions double with every 10 deg C rise in temperature, so with all things being equal, traps requiring higher desorption temperatures will have problems recovering more reactive chemicals, such as reduced sulfur compounds, amines, and formaldehyde. Cold trapping allows the use of very week adsorbents, that allow recovery of the trapped VOCs at lower desorption temperatures. The reason why most commercial systems are not using cold trapping is that this creates a greater challenge in the removal of the 1-3% water vapor. Multiple stages are typically required for cold trapping to both the sample and remove water, and two examples are given in the MP&T and ECTD explanations below.
Most systems attempt to trap VOCs using a single or multi-bed hydrophobic adsorbent trap which should allow water vapor to pass through the trap faster than VOCs due to water’s highly polar nature. However, this somewhat oversimplified approach has some major limitations. First, water concentrations in air are anything but predictable. Concentrations range from bone dry to 100% relative humidity, creating a challenge for the trapping system to be able to handle this wide humidity range. As water vapor approaches nearly 100% relative humidity, it begins to interact more strongly with surfaces, creating a more polar character to even hydrophobic surfaces. This causes water to travel through the adsorbent trap at a slower rate and has the adverse effect of causing most VOCs to travel through faster due to the increased polar character of the surface of the adsorbent. Moving the chemicals further into the trap has several disadvantages. For multi-bed traps that use second and third stages containing stronger adsorbents, allowing chemicals to reach these stages only at higher humidity levels causes their recoveries to drop off due to the higher surface areas and greater distance to traverse back through the adsorbent during desorption. Compounds that show even “some” reactivity at the high temperatures used to desorb these traps (230-400 deg C) will show reduced recoveries the longer they are exposed to these conditions. Therefore, what appears on the surface to be a straightforward process of trapping the sample, flushing off the water with a forward nitrogen or helium purge, and then back desorbing to recover the VOCs can actually show reduced recoveries based on humidity levels. For the lightest VOCs, humidity levels can even affect whether these compounds are completely trapped or not.
Strong adsorbents have another negative attribute. Due to the high surface area, it can be difficult to achieve low blank levels after accidental exposure to a high concentration sample (1000 to 1,000,000 PPB). These high concentration samples are becoming more commonplace in air laboratories due to the popularity of soil gas monitoring using canisters. As explained below, cold trapping allows the use of weaker adsorbents which can be more easily cleaned after exposure to high-level samples.
MP&T is similar to ECTD, except stage one is cooled to about -150 deg C for trapping of the entire sample, including the CO2 and water vapor. The first stage trap is then warmed to roughly 10 deg C, followed by purging of nitrogen or helium through the first trap to a second cold adsorbent trap. Just 40-50cc of helium can effectively purge all of the VOCs to the second trap, leaving most of the water behind. A 500cc air sample at 100% RH contains about 10ul of water, of which about 0.1-0.2ul will be swept through to the second trap. Today’s GCMS systems can handle this amount of water without a noticeable performance reduction.
Labs have lately been favoring ECTD over MP&T due to the lower amounts of LN2 required. New, more efficient LN2 delivery systems are being developed to drop the effective cost of LN2 per sample to about $0.25 per analysis, making LN2 extremely efficient and practical for all but remote, real time applications.
Extended Cold Trap Dehydration (ECTD) has become the most popular technique for analyzing canister samples. ECTD uses two separate stages for water management and cold trapping, followed by a third stage for final on-column cryofocusing to create a very fast injection onto the GC column to maximize sensitivity and compound resolution and separation from each other. The sample passes through a first trap at -40 to -50 deg C, freezing out almost all of the water found in the original sample while allowing VOCs to pass through unretained. Water management with ECTD takes advantage of the fact that water is usually within a factor of 10 of its saturation point in the air, while VOCs are thousands or millions of times below their saturation points. Passing the sample through an inert trap at -50C causes most of the water to drop out, but virtually none of the VOCs. Instead, the VOCs pass through to a second cold trap containing week adsorbents and trapping typically occurs on the very front end of this trap by lower the temperature of the weak adsorbent.
This combines to allow nearly complete recovery with a very non-reactive environment, so even compounds as reactive as methyl amine, hydrogen sulfide, and formaldehyde are recovered for GCMS analysis without derivatization. Occasionally, some of the very heavy VOCs or light SVOCs will drop out in the first cold trap, so the “extended” part of the ECTD name involves s second step whereby the first trap is warmed to just over the melting point of water, followed by the purging of a small amount of nitrogen or helium to transfer any temporarily condensed VOCs to the second cold trap with very little additional water vapor transferred. This provides consistent recoveries of all VOCs whether the initial samples were very dry or literally at 100% Relative Humidity. A final benefit to ECTD over Multi-bed Adsorbent trapping and Dry Purging is that any ozone in the air sample will pass through the cold traps at -50C without reacting. However, reactions will occur on multi-bed traps operating at 20 deg C or higher and have been shown to cause double bond containing terpenes to be converted to C6-C10 aldehydes. Chemicals created during the trapping process that were not actually found in the original air samples are called artifacts and can misrepresent the true composition of the original air sample. Unfortunately, a single trap design does not allow cold trapping, as then water will condense and cannot be dry purged through the trap effectively.