I just, I don’t get it. There’s a mobile phase and a stationary phase, but I’m looking at descriptions of paper chromatography and it’s saying that the stationary phase is the water in the paper and then the mixture is the thing at the bottom and then it just goes up the paper and separates? But like… wouldn’t it just all go up the paper? And also how else would you do this other than paper chromatography?
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Imagine a purple marker. In order to create that purple color, the creator had to use multiple dyes, let’s say some reds and some blues. Each individual dye has its own chemical structure. One dye may be really heavy. Another one may be really light. One may dissolve better in water. When you’re using paper chromatography, you are separating those elements. A heavier molecule isn’t going to move up as quickly as a lighter one. It’s going to slow down and potentially even stop moving way sooner than a lighter molecule.
This is why it separates. You’re seeing the different molecules that make up the dye get carried up by the water, but the heavier ones don’t make it as far.
There are other forms of chromatography like those that use a filtration system through a tube, called column chromatography. Chromatography is all about separating elements from something. For example if you take a substance and pour it into a tube filled with silica beads, smaller molecules will move farther. Bigger ones will get caught between the beads. So logically, if something is higher up in the tube, it’s bigger than the ones near the bottom. You can then collect them in separate tubes by filtering them through, the smaller molecules reaching the bottom first.
Edit to add: in column chromatography, size isn’t the only differentiating factor. Molecules can separate due to polarity or hydrophobicity too. Usually polarity is what they measure in column chromatography. Chemists usually know those details of the exact molecule they’re trying to get, so they can use that information to figure out where in the tube the molecule may be. There are a LOT of different types of chromatography and a LOT of different methods to isolate compounds, but the most important thing to remember is it involves separating parts of a whole based on their individual chemical properties.
Chromatography is primarily a way to separate the components of a sample, we have various types but since you talked about paper chromatography I’ll just focus on that.
Mobile phase – the one that moves (solvents)
Stationary phase – paper or silica plate (thin layer chrom)
Now you place your sample in the starting line, place it in your chamber then let it develop. As the mobile phase creeps up it carries the sample with it. Your sample would contain different parts. Each of this parts would have different attractions to the stationary phase hence what happens is some parts separate faster/lower and some parts separate lower/higher.
Okay, we now separated our sample into its components, what now? Now you’re going to compute for the rf value. it is a number from 0-1 that describes the part of the compound. Now if you want to identify the compound, you can match the rf values with standards performed using the same parameters and if you get the same value that’s probably the compound.
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