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Bulk and Trace Gas Sampling – Determining the Most Appropriate Method

A number of methods exist for the sampling and analysis of gases. Testing may be required to determine the quality of the fuel for landfill engines and other energy from waste applications. Sampling of trace components in landfill gases is often the requirement of an environmental permit, where operators have to produce an annual report on the emission of certain substances from the site.

Sampling methods may include collecting the gases onto solid, dual bed or reactive sorbents, while some gases can be measured on site using direct reading instruments. At SOCOTEC, we understand that the sampling process can prove a complex business, as not only do you need to select the most appropriate medium, but you must also take into consideration such factors as the sampling location and the sample’s ability to be transported safely.

Simon Shead, one of SOCOTEC’s gas analysts, discusses the various collection media for the ‘grab’ sampling of gases, highlighting the strengths and limitations of each medium in order to ease the decision-making process for your next sampling project. 

One of the most frequently asked questions from our clients is what medium they should use to take gas samples. Simply put, this is dependent on what it is they are hoping to have analysed.

Tedlar bags                       

Should you wish to have a gas analysed for the major components of air (nitrogen, oxygen, carbon dioxide and methane), a Tedlar bag – more commonly known as a ‘gas bag’ – will suffice. Tough and flexible, Tedlar film features tensile strength and toughness for many applications and can be continuously used from -72°C to 107°C when applied with the appropriately rated fitting. This wide range of operation means that the bag can be used in extremely hot or cold environments, including stacks, vents and flues.

Tedlars are versatile in that they can be used purely as a sample medium, or as an intermediary in-sample transfer to an absorbent thermal desorption (ATD) tube at a later stage. Moreover, if you are looking to analyse trace components – whether this be hydrocarbons (C1-C8), hydrogen, hydrogen sulphide, helium or carbon monoxide – Tedlars are generally the go-to sample medium.

While a Tedlar is a good option, it must be noted that they are not completely infallible. When inflated, they become bulky, meaning sample transport is difficult. They are not light reflective, so gases could be affected by UV radiation, and this could lead to sample degradation within the bag. On average, roughly one out of every hundred Tedlar bags tend to get damaged in some way. They have a tendency to leak after several uses, although this can be avoided by performing visual inspections on the bag before use. We would also encourage our clients and field technicians to send in duplicate samples to avoid this being an issue.

FlexFoils

If you are hoping to reduce sample degradation by UV radiation, a FlexFoil bag could prove the ideal solution. FlexFoil has good stability for low molecular weight compounds such as VOCs and CO, CO2, methane and SF6. SOCOTEC often encourages SynGas clients to use FlexFoil bags, as the manufacturer guidance states that hydrogen does not effuse as quickly from a FlexFoil as it does from a Tedlar. Hydrogen plays a large part in the calorific value of a particular gas sample – losing any hydrogen through natural effusion or degradation could potentially impact the outcome of a commissioning process, which is why we tend to recommend FlexFoil for those looking to take hydrogen samples. 

FlexFoils do have their own individual weaknesses. Much like Tedlar bags, they are weakened following repeated use and can get bulky when filled. However, unlike Tedlars, they are not suitable for any clients who wish to sample sulphurous compounds. As a general rule, should you require hydrogen analysis, FlexFoils are the way to go.

Gresham tubes

For clients that used to work in the coal mining industry, Gresham (or CRE) tubes will need no introduction. Ranging in size from 10 to 50cm long, Gresham tubes have a very simple design, consisting of a metal tube with a non-return valve to allow the gas to maintain pressure inside. They are often made from aluminium or stainless steel, with the latter being the preferred material as hydrocarbons can bind to aluminium tubes. When a sample is introduced, the pin within the tube is pushed back to allow the sample in, creating internal pressure to then close off the sampling port. This is repeated for each stroke of the Gresham pump until the sampling is complete.

Due to the materials used in construction, Gresham tubes are extremely sturdy and compact for transportation, so are be the preferred option for Diesel Engine Emissions Monitoring as they can withstand high temperatures. While not the most effective sample medium, they are perfectly adequate for analyses of such gases as ambient air or hydrogen, helium or carbon monoxide, both of which a gas lab can provide. Ideally, Gresham tubes will be used if samples need to be transferred overseas, or if a large number of samples are required to be transported back and forth without worrying about the possibility of one leaking or being damaged in transit. The biggest drawback of a Gresham tube is that it is simple to use but is labour intensive – in order to physically take the sample, you will be required to use a Gresham pump, which we can provide along with the tube upon request.

How can SOCOTEC help you select the correct gas sampling method?

While Tedlars, FlexFoils and Gresham tubes are the three most common sampling mediums we see in our lab, they are by no means the only methods. As aforementioned, the sampling technique that you choose ultimately depends on what you are looking to analyse. Each medium has its own individual strengths and weaknesses – some require specialist analysis, while others require a more passive approach that can be examined after a set time period. 

Gas sampling can prove a tricky business, so it is highly recommended that you contact the lab you are working with to ask their opinion on which medium will work the most effectively in terms of the analysis you are intending to carry out. At SOCOTEC, we are happy to advise and provide information and bespoke solutions that best suit your needs. We can also send a member of our experienced UKAS accredited field team to take the sample, as well as provide training if you should require it.

For further information about gas sampling, please feel free to call a member of our team on 0845 603 2112 or contact us here.

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