Equilibrium and dynamic headspace methods
Sampling of headspace volatiles is a well-established technique used widely in the food industry. It is often employed alongside other techniques such as purge & trap, solvent extraction and SPME, in order to provide qualitative and quantitative analysis of VOCs in a sample.
Headspace techniques can be divided into two categories:
- Equilibrium (static) headspace is a convenient sampling technique for many samples, but in cases where there is a wide range of volatiles present in a sample it is difficult to optimise the equilibration temperature.
- Dynamic headspace sampling is a more versatile option, because it more efficiently extracts the less volatile components, leading to the generation of more comprehensive sample information in a single analysis.
What Markes can offer
Markes offers three approaches for carrying out headspace analysis, as described below.
Microchamber technologies for dynamic headspace analysis
Markes’ Micro-Chamber/Thermal Extractor uses the principles of dynamic headspace sampling to collect vapours released from a wide range of sample types.
The sample is placed in one of the compact chambers, which is held at a specified temperature as a flow of gas is applied. Analytes are collected onto a sorbent-packed TD tube for analysis by TD–GC.
A benefit of this approach is the minimal sample preparation and the rapid processing – up to six samples can be collected simultaneously.
Headspace sorptive extraction
Use of Markes' HiSorb probes allow materials in standard headspace vials to be sampled onto a section of PDMS sorbent. The probe is then inserted into an empty TD tube, and analysed by TD–GC–MS. This sampling approach is more sensitive than SPME methods, and less labour-intensive that solvent extraction, while the use of TD allows extraction of a wide range of volatiles in a single run.
Direct desorption, a variation on the principle of
dynamic headspace, is useful to understand the overall volatile content
of a sample, and in some cases can achieve exhaustive extraction of the
volatiles from a small sample.
Small samples of solids or semi-solids are weighed
directly into empty TD tubes or tube liners. They are then heated in a
stream of inert gas to sweep volatiles from the sample matrix onto the
focusing trap of the thermal desorber, for analysis by GC in the usual
Applications include the odour profiling and quality
control of a range of relatively homogeneous materials, such as dried
foods, powders or pastes.
|Dynamic headspace sampling || |
Dynamic headspace sampling, for example using micro-scale chambers such as Markes’ Micro-Chamber/Thermal Extractor, is an increasingly popular and versatile alternative to equilibrium (static) headspace, because of its ability to extract a wider range of analytes.
Continuous removal of the vapours from the sample chamber means that the sample–vapour equilibrium is constantly being re-established, leading to a greater proportion of the less volatile components being released into the headspace, and consequently the generation of more comprehensive sample information in a single run.
The size of the individual microchambers also makes this approach valuable for odour-profiling less homogeneous samples otherwise suitable for direct desorption – such as pieces of fruit, cheese or meat.
Like direct desorption, raising the temperature of the sample speeds up the release of volatiles and allows the extraction of a meaningful aroma profile in a short space of time.