New to GC - dull questions for those with patience.

[Absinthe Minded]

Hi all,

I'm new to Gas Chromatography, I have very little experience in this and no background in chemistry.

Is there anywhere on this forum that explains GC in layman's terms? If so, could you point me to it?

Alternatively, here's the little I know - any (and I mean any), additional information to this in plain English would be really helpful.

OK, as far as I know, different molecules move through the column at different speeds, that's how you tell what chemicals are in your sample (by the retention times). I've injected a binary mixture of MEK (butanone), and cyclohexane and managed to separate them. Then I can inject the samples separately and find out what the retention times are and compare these to my binary mixture.

Q1. The software I have integrates the peaks so I can find out the areas, these are proportional to the amounts of each component. How do I convert the areas to something meaningful, i.e. microlitres? I've heard Mol fraction mentioned but with my very limited knowlege in chemistry, I don't know that this would make sense to me. I do know how to make up a molar conc (mol wt in grams, in 1l water), but that's about it.

Q2. Suppose I inject and unknown sample (say, an oil), with lots of components. How do I identify these components from the retention times? Is there a resource that is used to find out a component from a retention time?

I'm not at work at the moment so I can't give the exact details of the instrument (but I doubt that's important). It's a Shimadzu FID GC with a 12 rack autosampler and a 30M (approx) BP capillary column. It's hooked up to a PC and running Shimadzu's GC solutions software.

Sorry for such basic questions, but I hope one of you can answer without boring yourself to death in the progress.

Thanks very much!

AM.

[Russ]

Some basics are explained in the FAQ section. Click on the tab at the top left of the page. You can also try:
http://www.itsjustabox.com

[Earl]

AM,

This is simplified, but I hope it can get you an idea of what your are doing.

Chromatography is the separation of mixtures of compounds either to measure the component compound concentrations or to isolate them or both.
Gas chromatography is the practice of separating mixtures of relatively volatile compounds by forcing them into the gas phase and separating them by driving them along a solid or liquid phase. Separation occurs based on boiling point, molecular size and shape, and polarity.

To make your areas into a concentration you must first calibrate the instrument. Analyze at least three levels of your binary mixture from low concentration to high (make the mixes in whatever makes sense to you ie mg/L or ppm or some molar equivalent). Then determine the quality of the curve (either in the GC software or build a graph in excel). Once you have determined if you have a good curve (R2 at least 0.99) use the equation of the line to solve for an unknown concentration. Use the area of the unknown as the y coordinate and solve for x, with x as the concentration. (plot x concentrations and y areas from the instrument)

To identify peaks in an oil you will be looking at a more complex issue. I do not know of a database that lists retention times for oils, but the retention time is relative to column phase, column length, carrier gas type and carrier gas flow rate, amongst other things. So you would need to find one that matches your parameters or adjust to fit the example chromatogram. Chances are you won't find the information published, so what you will have to do is get a mixture of known compounds (such as a characterized oil) and then id what peaks you can by retention time based on an example chromatogram which should be supplied with the characterized oil or other mixture of compounds.
If you have access to a GC/MS that will be your best bet for identifying the unknown peaks, otherwise you are looking at identifying some of them by known standards, or else finding all peaks by trial and error using your best guess compound to see if RT matches.


Feel free to post a follow up questions.