Why You Must Know about Fully Evaporative Dynamic Headspace

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Over the last few months, Fully Evaporative Dynamic Headspace Sampling (DHS (FET)) has been gaining the attention of many people.

DHS (FET) is developing a bit of a buzz about it.

Compared to conventional static headspace, DHS (FET) offers:

• Significantly lower detection
  limits.
• To a large degree, the elimination of effects due to matrix variability.
• Uniform high recoveries for analytes of different polarities, without the need to modify the matrix.
• Good recoveries for analytes of a much wider volatility range.

If you have experience of using static headspace, then you will understand that
these four items represent fixes for four of the most significant limitations
that users of headspace sampling encounter. As a consequence DHS (FET) greatly extends the number of tasks that headspace sampling can perform. This is big news for some.

What can it be used for?

Important potential applications for the technique include:

• Trace impurities in pharmaceutical products (GTIs and PGTIs).
• Residual fragrances in consumer products.
• Profiling of low level flavour compounds in foods and beverages.

For certain, there will be others.

Like static headspace, you only transfer gas phase analytes from the sample to
the GC-MS, so you keep the inlet, column and source squeaky clean – however
grim the sample may be.

How does it work?

DHS (FET) can be performed using the GERSTEL Multi-Purpose Sampler. FET stands for: “Full Evaporation Technique”, a concept, was originally devised by
Michael Markelov and co-workers⁽¹⁾ in the 1990’s as a way of performing matrix
independent headspace sampling. DHS stands for Dynamic Headspace Sampling, which is also an established sampling technique, that gives better detection limits than static headspace. What is new, is this combination of the two techniques, that ends-up delivering the best attributes of DHS and FET techniques.

This is the sequence of events:

1. Around 100 microlitres of liquid phase sample (this can be aqueous) is placed into a 10ml headspace vial.
2. The vial containing the sample is moved to a sampling station where it can be shaken, and if appropriate, warmed.
3. The whole liquid portion of the sample is allowed to evaporate in the vial, moving all of the volatile components in the vapour phase and leaving any solids in the bottom of the vial.
4. The whole of the vapour phase contained in the vial is swept out and onto an
   adsorbent packed trap that is closely coupled to the vial.
5. The trap is then moved to a thermal desorber mounted on top of a cooled, PTV injector.
6. The trap is heated and thermally desorbed into the cold injector, where the
   analytes are focused into a tight band and then injected, splitless, onto the
   column.

You can see the effect by comparing the two chromatograms (below), a greater than 100 fold increase in sensitivity can be observed by switching from Static Headspace to DHS (FET) for this sample of a herbal based liquor.

For more details follow this link to download a PDF copy of the application note.

We are thinking of organising a training day on DHS (FET) in our Cambridge
laboratory. Are you are interested in the idea? Perhaps you can let me know.

(1) Matrix independent headspace gas chromatographic analysis. The Full Evaporation Technique M. Markelov, J. P. Guzowski, Analytica Chimica Acta, 276
(1993) 235.