Headspace Analysis – A Summary of Possibilities
Classical Static Headspace analysis works by analysing the vapour above the sample at equilibrium. Doing this gives good quantitative data for volatile constituents, keeps your whole GC or GC-MS perfectly clean and operating reliably with limited sample preparation and enables samples to be analysed quickly.
The niggling issues have always been:
- A perceived lack of sensitivity.
- How to get larger volumes of headspace into the column without peak broadening.
- How to extend the volatility range beyond C12.
As far as sensitivity is concerned, in reality, careful optimisation of the whole system, coupled with better mass spectrometer performance yields a degree of sensitivity that is fine for most purposes. For volatiles in waste water down to the 100 microgram per litre range is achievable in routine analysis. For details of what is possible see this application note.
It is possible to inject larger volumes of headspace onto the column if you use cryofocusing to trap the volatiles at the head of the column. This is easy with the right hardware and gives detection limits in the part per trillion range. For more details, see this application note.
Extending the volatility range is more involved. Better sensitivity helps a bit, but the trick is to use a three-phase approach where analytes are removed from the headspace and concentrated elsewhere. This means that the equilibrium between the matrix and the gas phase is disturbed, driving the transfer of analytes into the headspace from which they are continuously removed. There are several ways of doing this:
- Headspace Solid Phase Micro Extraction (HS SPME) – A SPME fibre is introduced into the headspace and analytes partition into the coating on the fibre. After a fixed time the fibre is removed and desorbed in the inlet. This technique gives a slight improvement in the volatility range of analytes that can be analysed. While retaining the advantages of headspace sampling. The wide range of fibre coatings available enables the analyst to experiment with selective extraction of target compounds.
- Headspace Stir Bar Sorptive Extraction (HS SBSE) – A Twister stir bar is suspended in the sample headspace and allowed to equilibrate with the headspace. The outcome is similar to HS SPME, but with 100x the amount of coating compared to SPME, the recovery of analytes to the column is much greater giving better sensitivity. For more information see this application note.
- Dynamic Headspace (DHS) – the headspace is continuously swept with a flow of inert gas and analytes are concentrated onto a tube packed with adsorbent. The tube is then thermally desorbed onto the column. DHS gives much better recoveries than other headspace techniques and pushes the usable volatility range a bit further still. You can learn more here.
- Fully Evaporative Dynamic Headspace (FE DHS) – this is a brand new approach where a small amount of sample (10-100 microlitres) is placed in an empty headspace vial. The vial is heated so that all volatiles (including the liquid part of the matrix) volatilise fully and are swept onto the packed tube – leaving non-volatile matrix material behind.
FE DHS is the technique that, currently, works for analytes with the widest range of volatilities and gives rise to a chromatogram that most closely matches the composition of the original sample.
It is not always clear, without experimentation, which of these approaches is appropriate to any given task. Most of the volatiles work carried out in our application laboratory involves comparisons between these different ways of working on behalf of prospective customers.
In almost all cases, when customers finally settle on what to buy, they end-up with systems that have the flexibility to support some or all of these approaches. This gives the best chance of meeting current and future needs.
If you are interested in making similar comparisons with your samples – please feel free to contact me.