40
February / March 2012
A Matter of Taste.... Flavour Profiling by GCxGC-qMS/FID
Diane Turner 1,2 , Dr G.H. Morgan 1 , Bryan White 3
1CEPSAR, The Open University, Milton Keynes, MK7 6AA 2Anthias Consulting Ltd, Papworth Everard, CB23 3UG 3 JSB, Maidenhead, SL6 3LW
The analysis of flavour compounds in many industries is important for a number of reasons. This can include the differentiation between genuine products and those of lower grade materials produced using different ingredients; for quality control purposes to compare different production batches to detect possible changes; and also in identifying any off-tastes or off-odours caused by aging and storage. The samples are often complex, containing many hundreds of organic compounds of varying polarities which may or may not contribute to the flavour. Separation of the compounds can be difficult on a single type of stationary phase and frequently, strong flavours are given by very low concentration analytes which may co-elute with high concentration analytes on several different stationary phases and, therefore, go undetected.
Introduction Comprehensive two-dimensional gas chromatography, GCxGC, has become a common technique in the research world over the past decade. Rather than analytes in a complex sample being separated on a single stationary phase; or a ‘heartcut’ of co-eluting peaks from one column being transferred to a second column containing a different type of stationary phase for separation; every peak transferred onto the column is separated on two different columns containing two different stationary phases. This multiplies the resolving power of one gas chromatograph by that of another, resulting in the possible separation of thousands of analytes.
The interface between the two columns, enabling the re-injection of co-eluting analytes from the first column onto the second column, is known as the modulator. The modulator has two main purposes: firstly to collect a cut, usually 1-10 seconds in length, of the eluent from the first column and secondly to re-inject this cut onto the second column in a tight sample band. The first column is usually of standard dimensions, typically 0.18-0.25mm i.d. and 25-60m long, commonly it contains a non- polar stationary phase. Separation is usually slow with a flow rate of 0.8-1mL/min; this enables a peak to be “cut up” into several slices for separation on the second column and help to prevent overloading.
The second column commonly contains a more polar stationary phase. To enable a fast
Figure 1: Zoex loop thermal modular. (a) trap with the cold jet; 9b0 reinject with the hot jet
Figure 2: Agilent Capillary Flow Modulator. (a) Load Position; (b) Inject position
separation within the modulation time from the first column, depending on the modulator type, the second column is shorter, typically between 0.5-2m long with a narrower i.d. of 0.1-0.15mm or has a standard i.d. at 0.18-0.25mm and is 3-10m long but with a very fast flow rate at around 20mL/min.
By introducing the cut in a tight sample band sharp peaks are obtained that are resolved and eluted from the smaller second column very quickly before the next cut is introduced. These second dimension peaks are around 0.03-0.1 seconds wide and result in an increase in sensitivity.
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