A further approach to concentrating compounds can be accomplished by using multiple injections into a GC instrument, but this time a Deans switch can divert just
single peaks or a narrow zone into a cryotrap. By making multiple injections, and reproducibly switching the microfluidic device for each injection, it is possible to collect the heart-cut compounds or zones and increase the amount of sample, in proportion to the number of injections made. The cryotrap is then allowed to warm up, the compounds are re-mobilised into the second column, and just these zones are then analysed on the second column. This has all the separation advantages of a MDGC analysis, but with greater mass of component(s). If a compound gives a very small response, e.g. in a MS detector, then the response can be significantly increased, and ideally the mass spectrometry match quality will increase, as total solute mass increases. This approach was reported for some major and minor essential oil components, and tested with a caffeine standard. See ref 244.
This idea can also apply to volatile sampling by using SPME (ref 256). Since this is a solvent-free method, it is possible to make serial injections from a SPME fibre or different SPME fibres into a GC injector, and collect the volatiles from each injection at a cryotrap located at the start of the GC column. By turning off the cryotrap fluid supply, the GC analysis now commences. Sampling of each fibre from a sample depends upon how long the extraction step takes. Desorbing the fibres into the GC injector normally takes only 2 min. So 5 desorptions take only 10 min sampling time into the injector. We use multiple fibre holders to permit multiple extractions, and so we can use the same fibre type or different fibres and so obtain an integrated volatile sampling of the headspace. We then analyse the sample by MDGC or GC×GC methods.