Chromatography, an explanation

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Chromatography, an explanation: 

Chromatography is one of those subjects which is really quite simple in practice but has some fairly complex theory behind it which bears little or no resemblance to the ordinary science involved in the rest of the course. Therefore its an easy way to throw away marks if you don't know your stuff. Enter chromatography... 


Chromatography is a an analytical technique (something sciency to you and me)- used to separate compounds within a mixture. Like taking each ingredient at a time, out of a cake, until you just had what you started with. In your A2 course, you'll need to know about two types THIN LAYER CHROMATOGRAPHY and GAS CHROMATOGRAPHY which are both explained below. 

Cool, but how? 

Chromatography works on the understanding that compounds in a mixture will have different affinities (attractions) to what ever they are on. Two specific terms are used which take some understanding, but unfortunately are really crucial to understanding chromatography. Lucky you got me. 

-Stationary Phase this as unhelpfully put by textbooks is "the phase that doesn't move" really helpful right... 

-Mobile Phase "The phase which does move" 

So, that is really helpful. Clues in the name really, but we're no closer to knowing what that is supposed to mean. The trick to understanding it is to think of the two 'phases' as halves of your chromatography process. Its quite easy to think the 'phase' is a description of what the sample is doing, but it's in fact a description o where it is. It can be either in the mobile or stationary phase. So keep that at the top of your skull as we move on to the exact techniques used, because its hard to explain without reference to a particular technique. Keep the above in mind and it'll all click soon. 

Thin Layer Chromatography  

This is your cheapskate method of chromatography. The kind you've almost certainly done at primary/secondary school. Remember a little plate in a jam jar of water with moving coloured dots on it? Yeah, that was TLC (not tender loving care or what ever Activia claim it is). The idea is, although I recommend you read more about it either in your textbook or on the tinternet, is that a dot of a sample is placed on a plate of Adsorbent material and then is moved by a solvent, which climbs up the plate. 

So that's easy right, you've been there, done it and worn the t-shirt. Now you actually need to know WHY it happens the way it does. Mobile and Stationary phase. That's what counts. 

In TLC there are two 'Phases' a big word for bits. The actual plate on which the samples are places is the STATIONARY PHASE. The samples are placed on it and when the solvent leaves them behind they return to the plate. So really, when they are on the plate they don't move. Simply the sample starts and…


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