[Skiznibbler]

I still don't quite understand the purpose of chromotography. From what I gather its a method of analyzing mixtures to identify their constituents. Can thin layer chromotography identify the constituents of a completely anonymous substance?

If I had a mixture and I had absolutely no idea what compounds were present in it could I find out whats in the substance with TLC?

[Borek]

More like separate then identify, although identifaction usually follows separation...

[Skiznibbler]

Can TLC separate every compound in a mixture regardless of how many compounds are present? For example say I have some crude oil could I use TLC to separate the methane, ethane, hexane, benzene and whatever else is present?

Also are there certain compounds that TLC won't separate?

I'm guessing the method of identification after TLC would be testing the melting points or boiling points of each compound separated.

[Arkcon]

I wouldn't use the standard TLC plate for a mixture as complex as that.  Here're two examples for a TLC plate:

1). A pharmaceutical, freshly prepared, and another sample of the same pharmaceutical, extracted from some pills left in a cupboard above the oven for six months.  I'd spot them side-by-side on the TLC plate with the same solvent system, and see if they travel the same, without a lumpy shape, or extra spots for the cooked one.  And yes, this is a application noted in the US Pharmacopoeia.  Notice, I mentioned a pre-purification step for the pills, not a grind some up and smear.  And yes, people do do that to their medicine, and yes, pharmaceutical companies do have to test the pills afterward.

2). On one plate, I'd plate out a sample of malic acid, lactic acid and a several samples of wine, from different time points in it's fermentation.  As time passes, the wine samples will show less and less of the malic acid spot, and more and more of the lactic acid spot.  And yes, some wines do gain a flavor benefit from this type of fermentation.

For your complex petroleum mixture, I would still use chromatography, just not Thin Layer Chromatography, it lacks the resolution of gas chromatography, and the ability to handle large volumes like liquid chromatography.

Something people often fail to realize is that no one gives you a sample and says, "You the chemist, you figure out what it is and how to analyze it."  There's generally some idea of what you're looking for.