Thank you bill and rolnor! I'll check out your link bill!
rolnor, understood completely about the boiling - their boil points are not THAT far apart so you will always get some kind of mix.
On the melting point/freezing route though, why do you say it is basically impossible? Take linolenic acid for example, its melting point is -5 degrees Celsius. Compare that to oleic acid, that looks like it has a melting point of around +14 degrees Celsius. That is almost a 20 degree separation. If one had a laboratory freezer that could hold accurate temps, and set it to 10 degrees Celsius, wouldn't that cause the oleic acid the crystalize, then you could pour out the liquid, leaving the solid oleic acid behind, and you would know what you poured out was essentially oleic acid free? I understand for some fatty acids that have melting points VERY close this would not work nearly as well.