Sorbent tube sampling
There are two types of sampling with sorbent tubes – pumped and passive. Using pumped sampling, air is pulled through a sorbent tube using a pump. Single-bed or multi-bed tubes can be used. A constant flow of gas into the vessel aids release of analytes onto a sorbent tube. Typical analytes monitored by this method are in the range C3 to C44. Sampling is quick (minutes to hours) and a wide range can be monitored in one run. The technique is ideal for all types of ambient air monitoring. With passive sampling, analytes migrate across an air gap onto a bed of sorbent. Here, only single-bed tubes can be used. Typical analytes monitored are also in the range C3 to C44. Passive sampling is easy to deploy but is relatively slow. The technique is commonly used for factory fenceline monitoring and personal monitoring.
During on-line sampling, air is pulled directly into a sorbent-packed focusing trap. Typical analytes monitored by this method are freons and C2 hydrocarbons up to C14. On-line sampling is ideal for ultra-volatiles and is commonly used for continuous monitoring for source apportionment, rapid detection of hazardous compounds and atmospheric research.
Canister & bag sampling
With canister and bag sampling, air flows at a controlled rate into an evacuated canister or bag. The air sample is then transferred to a sorbent-packed focusing trap. Typical analytes monitored are freons and C2 hydrocarbons up to C10. Canister and bag sampling are ideal for ultra-volatiles and are commonly used for ambient air monitoring for volatile ‘air toxics’ and atmospheric research.
Accessories to enhance your thermal desorption instruments and sampling.
Overview of thermal desorption
Thermal desorption (TD) arose out of the need to improve upon conventional sample preparation techniques such as solvent extraction, solid-phase microextraction, purge-and-trap and static headspace. It gives greater sensitivity than these techniques and can be used for a wider range of compound classes (from C3–C44). TD is applicable to a wide range of sample types – solids (using dynamic headspace, headspace sorptive extraction or direct desorption), liquids (using immersive sorptive extraction) and gases (using pumped sampling, passive sampling, on-line sampling or canisters). It is safer and more environmentally friendly than solvent extraction, is easily automated, easy to validate and complies with key standard methods.
What is thermal desorption?
TD is a versatile preconcentration technique for gas chromatography, which is used to analyse volatile and semi-volatile organic compounds. The majority of TD applications use sorbent tubes and a two-stage desorption process to focus the analytes into a narrow band of gas, and so achieve the maximum sensitivity enhancement.
The process consists of three steps:
- Sample collection: Up to several hundred litres of vapour is sampled off-line onto a sorbent tube.
- Tube desorption: The sorbent tube is heated and the analytes are swept onto a focusing trap in 100–200 mL of carrier gas.
- Trap desorption: The focusing trap is heated rapidly and the analytes are transferred to the GC column in 100–200 µL of carrier gas.