Tenax, Carboxen and other sorbents used for thermal desorption sampling
Tuesday, 26 May 2020 at 4:49:PM
How correct sorbent choice is essential for tube-based TD
Sorbent choice is one of the most critical factors in tube-based thermal
desorption. If the sorbent(s) aren’t strong enough to retain the compounds of
interest during sampling, no amount of instrument manipulation will allow you
to see them in the chromatogram.
Choosing the right sorbent or sorbent combinations can sometimes seem
confusing, particularly when analysing unknown samples. People will often start
by asking for brand names such as Tenax® or Carboxen. These are sensible
choices for many applications – but they aren’t the best choice for everything
and there may be more hydrophobic, thermally stable, or cost-effective options
available which would make the sampling and analysis easier.
Sorbents selectively trap and retain compounds of interest while
excluding the bulk constituents of air such as permanent gases and (ideally)
water. Selectivity can also be used to eliminate some interferences and enhance
results for low-level compounds. Provided tubes are carefully sealed, the retained
compounds can usually be stored safely on the tube for several days or weeks
ready for thermal desorption and analysis using heat and a flow of carrier gas.
There are several key considerations when choosing sorbents for TD:
- Strength: A sorbent has to be strong enough to retain target analytes but able to release them efficiently during desorption. As a rough guide:
Use a weak
sorbent, such as Tenax TA, when working with compounds boiling above 100°C.
Use a medium-strength sorbent, such as a graphitised carbon black, for compounds
boiling between 30°C and 100°C.
Use a strong
sorbent, such as a carbon molecular sieve, for compounds boiling in the range
–48°C to 30°C.
- Compounds with
boiling points below –48°C are typically too volatile for ambient-temperature
- Thermal stability of sorbent and compounds of interest:
operating temperatures. Some sorbents cannot be heated above certain
temperatures without causing irreversible damage, and some compounds of
interest can’t be heated above certain temperatures without undergoing some
- Inertness: Reactive species are more
suited to inert sorbents, so try to avoid non-inert sorbents such as graphitised
carbon blacks and use an inert tube material, either glass or inert-coated
- Artefact levels: Some sorbents, such as the carbon blacks,
exhibit low artefact levels, making them ideal for ultra-trace level analysis.
Others can have higher artefact levels causing background noise or potential
interferences with certain analytes.
- Hydrophobicity: Needs to be considered to
avoid water retention on tubes sampled in a humid environment. Where possible,
hydrophilic sorbents, such as carbonised molecular sieves, should be avoided
when working in high humidity.
The types of available sorbents are grouped as follows:
- Porous polymers: Most porous polymer
sorbents are inert, making them suitable for the analysis of labile and
reactive compounds such as terpenes and CS gas.
- Graphitised carbon blacks: These are non-specific
carbon sorbents, widely used for trace-level applications due to their minimal
artefact levels. Examples are Carbograph, Carbopack and Carbotrap.
- Carbonised molecular sieves: These are the strongest
sorbents and are used to trap the most volatile compounds. Examples are SulfiCarb,
Carboxen 1000 and Carboxen 1003.
- Zeolite molecular sieves: These are very selective
hydrophilic sorbents used for specific applications.
Download the handy reference guide for sorbent selection
Can I use more than one sorbent in a tube?
Multi-bed tubes, able
to screen a wide range of compounds, are growing in popularity, especially for
indoor air applications and research. With active (pumped) sampling, a common
practice is to combine up to three sorbents, to increase the volatility range
that can be collected. In such cases, a good choice for reliably sampling a
wide analyte range (about C3 to n-C32) would be three
separate beds – a porous polymer, graphitised carbon black and carbonised
After sampling, the
tubes should be desorbed in the TD instrument in the opposite direction to
sampling. This sampling/desorption process is then replicated during transfer
to the focusing trap, where analytes pass onto the weakest sorbent bed first
and are desorbed in the opposite direction (known as backflush operation)––
this is a feature with all Markes systems.
Note that single sorbent tubes are
more commonly used for passive (diffusive) sampling. Multi-bed tubes can be
used diffusively, but only the first sorbent (nearest the sampling end) plays
any part in the passive sampling process.
Can I use sorbents for untargeted
There are many
sorbent combinations that can be used to sample analytes over a wide volatility
range. Markes offers a large choice of versatile, general purpose tubes for
broad-ranging samples, ideal for research and screening purposes where
untargeted analytes may be of interest. Our range of pre-packed sorbent tubes also
includes sorbent combinations tailored to the requirements of specific targeted
applications and methods.
Pre-packed sorbent tubes are available in the following tube types:
- Stainless steel, inert-coated stainless steel and glass industry-standard 3.5” thermal desorption tubes.
- DAAMS tubes (Depot Area Air Monitoring Systems) used for air sampling in chemical defence and counter-terrorism applications, available in 4.5” and 3.5”. These are packed to ensure optimum recovery of challenging and toxic analytes such as chemical agents.
- Glass TD Mini-tubes for use with Gerstel TD systems, packed with popular sorbents.
Do I need to match
the sorbent in my focusing trap to the sorbent in my tubes?
In most cases, a general-purpose
trap will be suitable for use with a variety of sorbent tube combinations,
enabling flexibility between applications in an analytical sequence. For very
specific applications it may require use of a different trap, and matching it
to the sorbent tube can be a good place to start. Look out for a future
blog on focusing traps and sorbents
Where can I get more advice?
Markes provides unrivalled expertise in sorbent selection and the
broadest range of sorbent packings for sampling known and unknown compounds of
interest. Our sorbent tubes and focusing traps have been designed by TD experts
and optimised for key applications and methods to simplify system and method set-up.
We also provide tubes and traps suited to wide-analyte-range applications for
the sampling of unknowns. We can offer advice on which trap is best suited to
your sorbent tubes or to your online sampling method. Alternatively, we can
custom pack according to your needs. Contact our sales team to discuss your requirements
Offer on top tubes
At Markes we have the widest range of industry-standard sorbent tubes
available. And for a limited time we’re offering 25% off some of our
most popular sorbent mixes. You can choose from the following tube
- Glass, stainless steel or inert-coated stainless steel
- Unconditioned or conditioned and capped
- Untagged or tagged with TubeTAG RFID tracking technology
- Custom etching of up to 10 alphanumeric characters and/or up to 5 bands
See our offer page
Nick van Noorden
Nick van Noorden joined Markes
International in 2018 as Marketing Manager – Supplies. He started his career in scientific
publishing and has a background in marketing communications & strategy for organisations
in the business, consumer and not-for-profit sectors.