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  3. GCxGC

- [Oliver] So, we're gonna use 2-D GC, GC cross GC or whatever, however you want to call it. So the standard gas chromatograph works by sending a sample, a pulse of sample, through the column and when it hits the column, the light stuff basically stays in the flowing of carrier gas and the light stuff like methane will come right through with the carrier gas, just keep flowing around the 25 meter column, whatever it is and come out first. The heavy stuff tends to stick to the stationary phase, what's lining the column, for some period of time. And the bigger the stuff is, the more time it spends on the stationary phase so you separate the compounds generally in this column based on volatility of the material. In 2-D GC, you take the output of the first column and you send it through a second column. This column separates on a different chemical attribute by changing the lining of the column, in this case, we're gonna look at polarizability. So what is polarizable are the alkanes, actually it's related to decolorizing carbon that you use in cleaning up water supplies. It's a aromatic carbon, so aromatics. You would never think to put wax in a water purifier 'cause wax doesn't really attract much and that's because the alkanes are not polarizable, so they're gonna go through, the alkanes are gonna go through the second column fairly quickly. So what we then get is two retention times, that's on the volatility in the x axis and the polarizability on the y axis. So the alkanes separate nicely on volatility and then you can see the alkanes come out much more quickly on the second column than the single-ring aromatics and the two-ring aromatics and three-ring aromatics like that. So now you get chemical specificity. So, for example, if we look at an unbiodegraded oil, you can see all the n-alkanes here, here, here. There's n-alkane, n-alkane, n-alkane, so they're all there and then the isoprenoids, pristane and phytane, are here, so they're all there, whole complement. If you have lightly biodegraded oil, the n-alkanes start to disappear, they get weak, but the isoprenoids are there, so that's one way of representing them. You can see them more easily here. So there's the unbiodegraded oil, the n-alkanes are quite big and the isoprenoids are small. With biodegradation, the bugs eat the n-alkanes down to low levels and the isoprenoids, they don't like as much, so they don't eat these yet and so these become larger. So it's quite easy to see the effect of biodegradation.

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