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Reverse logic – Backflush operation for thermal desorption

Wednesday, 11 April 2012 at 3:28:PM

Desorbing analytes from sorbent beds in the opposite direction to which they were sampled greatly extends the analyte range of thermal desorption – but what's the logic behind this essential technique?

Public awareness of a healthy environment and consumer products free from harmful chemicals continues to increase. This, and the resulting legislation, means that analytical laboratories are being asked to identify and quantify a wider range of compounds than ever before, and at ever-lower levels.

As a result, manufacturers have put a lot of effort into making thermal desorbers compatible with as wide a range of compounds as possible. Modern instruments can now routinely trap and inject compounds ranging from acetylene all the way up to n-C40, as well as thermally labile compounds and ‘sticky’ compounds such as phthalates, all on one system.

Having taken the trouble to make the flow-paths of thermal desorbers compatible with this range of analytes, we need to ensure that the sorbent-packed tube and trap are also up to the job – especially when compounds differing greatly in volatility and thermal stability can be present in the same sample. 
 

Backflush diagram

 

Unfortunately, no one sorbent can quantitatively trap every relevant analyte. One way of getting around this is to use liquid cryogen to cool the sampling trap, so that volatile components have less chance of breaking through. However, this is not compatible with field sampling, and on-instrument use of liquid cryogen brings its own problems of logistics and cost, quite apart from the issue of ice formation.
A much better solution is to use a combination of different-strength sorbents in each tube, as this allows the widest possible range of compounds to be trapped and released in a single run. This is best done by packing the sorbents in series, separating the beds by short plugs of glass or quartz wool.

But this raises a question – as there’s now more than one sorbent in the tube, does it matter which end of the tube is used for sampling? The answer is yes it does. The overriding factor is to prevent the low-volatility analytes from coming into contact with the stronger sorbents, as if this happens, you won’t be able to quantitatively recover them (if at all). The sample therefore has to be drawn onto the weakest sorbent first, so that the high-volatility components pass through it and adsorb onto the stronger sorbents further in (see figure).

When the sample is desorbed, we still need to ensure that the low-volatility analytes don’t come into contact with the strong sorbent. Therefore the gas flow must be reversed to desorb the sample. Applying this backflush principle to both the field sampling stage and the internal refocusing trap is what allows modern thermal desorption systems to handle the wide range of analytes demanded. In addition, because the analytes leave the tube in a much narrower band of gas than when they entered, it delivers the narrow on-column peak shape we’ve all become accustomed to.

Matt Bates, Markes' Thermal Desorption Product Manager


Further reading

The use of multi-sorbent tubes and backflush desorption is covered in: E. Woolfenden, Sorbent-based sampling methods for volatile and semi-volatile organic compounds in air. Part 2: Sorbent selection and other aspects of optimizing air monitoring methods, Journal of Chromatography A, 2010, vol. 1217, pp. 2685–2694.

Backflush operation also gets a couple of pages in: E. Uhde, Application of solid sorbents for the sampling of volatile organic compounds in indoor air, in Organic Indoor Air Pollutants: Occurrence, Measurement, Evaluation, eds. T. Salthammer and E. Uhde, Wiley-VCH, 2009, chapter 1, pp. 9–10.

Technical aspects of backflush operation are covered in our Application Note 064 (Simultaneous TD–GC analysis of VOCs and SVOCs),

  

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