15 Headspace Techniques Figure 2: Number of SBSE publications by year (up to Oct 2015).


As most aroma compounds are volatile, gas chromatography is the instrument of choice – either with a fl ame ionisation detector (FID) or more selective detectors for quantitation of target compound or mass spectrometric detectors for identifi cation of unknowns. Each of the automated sample preparation techniques outlined in this article can be integrated online with a range of GC instrumentation, or can be used as standalone sample preparation solutions. As can be seen from Figure 1, solutions can range from a basic liquid extraction system to a much more complex dual rail approach enabling automation of several complex sample preparation protocols. These include liquid-liquid extraction with an automated vortex mixer (Gerstel mVorx), evaporation of solvent using vacuum with the Gerstel automated solvent evaporator (mVap), as well as different liquid handling and agitator options to enable reconstitution, derivatisation and direct injection into the GC (if required).


Sample Preparation Options Direct (liquid) Extraction Approaches


Liquid extractions are thought to be more exhaustive, but may require several extractions and invariably require some enrichment, either through evaporation or solid phase extraction clean up. Traditionally this approach is labour intensive and often the bottleneck in analysis. However, the use of automated systems not only increases effi ciency, and aids in method development, but also leads to an improvement in robustness and in some cases can have an additional benefi t of the analyst not having to handle potentially harmful chemicals.


One option that provides some selectivity and enrichment is the use of stir bar sorptive extraction (SBSE). This is effectively a miniaturised direct liquid-liquid extraction and when thermal desorption is used for injection, can provide extremely good enrichment factors. SBSE is now an established technique, as the number of publications (Figure 2) illustrates. It is particularly suited to low level target analytes in aqueous matrices, and has been demonstrated for determination of food taints [1], malodour compounds in water [2] and for analysis of volatiles for characterisation and differentiation of high quality vinegars [3], as well as analysis of wine, ham, and a range of other food aroma applications.


Previously this technique was limited to non-polar analytes, but now there is a second phase available that is designed to extract more polar analytes and is particularly suitable for compounds such as phenols, which can be very aroma active. For a broader screen, both extraction phases can be used and if desired thermally desorbed simultaneously to obtain a single chromatogram. To enable multi-stir bar extraction Gerstel developed the ‘Twicester’ (Figure 3) which can also be used to sample a liquid and headspace concurrently.


As most aroma compounds are volatile, headspace sampling methods are often employed. Static headspace approaches rely on the equilibrium between the sample and the headspace, following heating and agitation for a defi ned period of time. By measuring the concentration in the headspace, the levels in the samples can be compared. Direct static headspace can be employed for most aroma compounds, although it has limited sensitivity as it provides no enrichment and typically only 1ml of sample headspace is taken. Multiple headspace injections can be taken from one sample, to provide a more exhaustive extraction.


Alternatives that provide enrichment include headspace solid phase microextraction (HS- SPME), where the analytes in the headspace are extracted onto a fi bre coated with an extraction phase. This provides some selectivity (depending on the choice of fi bre) and as the fi bre is then thermally desorbed also provides enrichment, resulting in improved limits of detection. This approach can also be used for sampling liquids directly, although this is much less common, due to matrix interferences and issues with cleaning the fi bres between samples.


Figure 3: Gerstel Twicester enables 2 or more Twister stir bars to be extracted simultaneously


Another way of enriching sample headspace is to sample dynamically, by purging the headspace with a fl ush gas and trapping analytes onto a sorbent. Several automated purge and trap systems are available, including the Gerstel dynamic headspace system (DHS), which uses the thermal desorption unit as the injector, enabling effi cient transfer of analytes into the GC instrument. This technique is a powerful tool to look at the more minor components in a sample, due to the level of enrichment that can be achieved. Using an approach known as full evaporative technique (FET) also extends the range of compounds that can be observed. In this technique, a very small volume of sample is taken and heated to ensure all the analytes are in the gas phase, prior to dynamic headspace sampling. This enables compounds that would not partition readily into the headspace, such as more polar analytes, to be sampled as well as the more volatile/less hydrophilic and has an additional advantage that, due to the small volume taken, a drying step is generally not necessary. The advantage of this approach compared to a standard DHS method is illustrated in Figure 4 for analysis of fragrance in a cream.


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60