Aroma analysis of vegemite 2. Separation of vegemite aroma chemicals by gas chromatographyPosted: June 13, 2013
This is Part 2 of my vegemite aroma analysis series. If you haven’t read Part 1, you can do so here.
Now that the vegemite aroma compounds have been extracted from the sample, and onto the SPME fibre, they will be separated using gas chromatography (GC). GC is a versatile analytical technique with many applications in areas like environmental science, forensics and petrochemicals. Separation of mixtures is a really useful way of finding out what is in them by splitting them up into their component parts. Lots of different kinds of samples of varying complexity can be analysed with GC, the main limitation being that they must have a boiling point below about 400°C. GC enables the analysis of many complex mixtures by separating them out into their individual component chemicals. The separation happens in a very long (usually 15 – 100 metres) and thin tube called a column. The column is housed in an oven which increases temperature over time and this facilitates one mode of separation. The temperature ramping up over time causes chemicals to travel through the column faster if they have a lower boiling point and slower if they have a higher boiling point. The other influencer of separation is a polymer coating applied to the inside of the column walls (very similar to the coatings on SPME fibres mentioned in Part 1). Different chemicals will interact differently with particular coatings and this also affects the separation. These two interactions give the analyst very useful information with respect to the chemical properties of the mixture components.
A gas chromatograph, with the oven door open. Green arrow = injection port, blue arrow = column, red arrow = outlet to detector.
Now, back to the vegemite! In order to get the aroma chemicals trapped on the SPME fibre into the GC, the fibre is inserted into the heated injection port.
The high temperature of the injector causes the aroma compounds to become gaseous and desorb from the SPME fibre. A flow of helium gas sweeps the molecules out of the injector and onto the GC column for separation. The output of the GC following separation of the vegemite aroma is below.
There are maybe four main peaks in the chromatogram, representing the compounds which were most concentrated on the SPME fibre. But a zoomed in view of the chromatogram reveals many, many more peaks. Each peak represents at least one different chemical comprising the aroma of vegemite.
In the next post, I will discuss how we go about discovering which chemicals these peaks in the chromatogram are, and what contribution they might have to the distinctive aroma of Australia’s favourite spread.