Nov 16, 2009

Uncle OO's story time: How TLC works

Gather round younglings it's time for Uncle OO's story corner,

Today I will tell you a great story, of mystery and magic....ok so it's not so much that as chemistry and common sense. Basically, I'm going to give you my patented "How chromatography works" story. Hopefully somewhere along the line some cool cartoons can also be added but for now your imagination will have to suffice.

Ok so I detailed to you the way in which to run a TLC. But now for a nice little explanation of how they work.

Imagine a large forest of trees. This large forest of trees represents the silica on a plate or in a column. Now imagine that this forest (all of these trees are identical and evenly spaced) is very tightly packed all crammed in together. Now imagine you are a really skinny person and your friend is rather large (but for fairness let's say they are muscular). You, being skinny represent a non-polar molecule and your friend represents a polar molecule. You are both at the edge of the forest and want to race through.

Suddenly riot police jump out of nowhere (these riot police are of varying sizes some are as large as your friend and some skinnier than even you). Now these riot police represent a solvent system (once again large is polar and skinny is non-polar).

Still with me? Ok here is where it gets interesting. The riot police want to force through the trees to the other side but in doing so they scare the crap out of you and your friend. So you start to push through the forest. You, being skinny are able to move through the trees easier, you don't get stuck and the trees can't grab onto you as easily, unfortunately your mate is not so lucky, he is struggling through the forest but can't get very far as the trees can't move aside for him very much and they grab him along the way.

Now the riot police, are still pushing through the forest too and just like your mate the bigger guys are moving more slowly but their sheer numbers are pushing your mate through and the skinnier riot police are overtaking you to the other side of the forest. You keep running through and eventually make out to the other side.

Now the speed with which you and eventually your mate made it depends on the number and size of the riot police. More large riot police moving through the forest will clear the trees down and allow you to move through the forest more easily as a result but you and your mate will like both just rush out so fast you both can't tell who won.

If there are more skinny riot police they are liable to overtake you both and make you both feel like losers. So you will have to wait for all the riot police to pass. But you, being skinny, will be able to move through the forest more easily so you will win but not in good time.

If the riot police were all roughly the same size somewhere in between you and your friend (indicating a good polarity for the solvent system) you should make it out before your mate but only enough that he doesn't lose sight of you in the trees and hate you for the rest of your life. If this is the case you might end up with your friend stuck in the forest and have to call for a gang of riot police to flush him out. (sometimes compounds get stuck on the column and need to be flushed out with a polar solvent).

The same is basically true for chromatography, the more polar the solvent system the slower it will run through the column (it is the mobile phase). But the more it will take up your compounds and force them through the column. If the solvent system is not polar enough the solvent will run through the column but not interact with the compounds on the column and the compounds will remain on the column for longer. If your compound is polar it will stick to the column more and therefore elute (i.e. come off the column) later. Inversely, if your compound is non-polar then it will not stick at all to the column and move very quickly through as the mobile phase passes.

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