Thin-layer chromatography (TLC) | Intermolecular forces and properties | AP Chemistry | Khan Academy

So let's say that I have a vial of some mystery liquid right over here, and I want to start figuring out what's going on there. The first step is to think about, is it just one substance or is it a mixture of multiple substances? The focus of this video is a technique to separate out the substances to understand at least how many there are. This technique generally is called chromatography, but we'll focus on thin layer chromatography, which is the most common that you might see. Other variations of chromatography, like paper chromatography, operate on very similar principles.

What we're going to do is set up on top of something like glass or plastic. We're going to put a thin layer of a solid polar substance. Now, what you typically do is put a thin layer of silica gel; that's the most common solid polar substance that folks use. It's also porous, and the fact that it's porous is really important because we're going to want liquid to have capillary action and travel up through it.

Now, the silica gel, as I mentioned, this thing is very polar. Now what we're going to do is take some of our mystery substance—let's say it's this color right over here—and we're going to place a dot of it on that silica gel. You then want to take this plate that has the silica gel on it and that little dot of our mystery substance, and then you want to dip just one end of it in a solution.

What's really important is that the solution is less polar than the silica gel. Less polar here, and we'll talk a little bit about what happens depending on how polar this is. Usually, this is going to be a very shallow amount of this solution, which, as we'll see, will be something of a solvent. You usually want to put it in a closed container like this so that this fluid down here doesn't evaporate out.

Then what do you think is going to happen? Well, as I mentioned, this is a porous substance here, and so you're going to have capillary action. This fluid at the bottom is going to move upwards through the silica gel, through those little pores in the silica gel. This is the stationary phase. Why do we call it that? Well, because it's not moving, and you can imagine we would call this less polar solvent the mobile phase because that is traveling through the silica gel.

It's picking up some of this mystery substance, and it's transporting it. Let's say this mystery substance is made up of two different things. If something is more polar, that means it's going to be more attracted to the stationary phase, which is very polar, and so it's not going to travel that far. While the parts of our mystery substance that are less polar, they're not going to be attracted to the silica gel as much, so they're going to travel further with the solvent.

So maybe it might go like that, and you would run this until your mobile phase makes a good way to the top of your silica gel right over here. Now, just looking at this, and the reason why it was called chromatography is when they originally did this, they were actually separating out various tissues in vegetation that had different colors. Though "chroma" is referring to the various colors, it doesn't necessarily even have to refer to things that have different colors, or sometimes you might need a UV light to see them.

But when you run thin layer chromatography, you will see that your original dot will have traveled to various degrees with your solvent, and then will now be multiple dots depending on how many things were in your original mixture. As I just mentioned, this thing right over here, this is the less polar thing, is going to travel further than the more polar thing—more polar constituent substance—because the more polar thing is more attracted to silica gel, which is stationary.

There is a way to quantify how far these things traveled relative to your solvent, and that's called a retention factor. The retention factor, which the shorthand is r subscript f, is just defined as the distance traveled by the solute divided by the distance traveled by the solvent. We need to be clear; it's not the distance traveled by the solvent in total; it's the distance traveled by the solvent from this origin— from where we applied this dot right over here—so past the origin.

Let me label that as the origin. So what would it be in this situation where to help us there? We would have to get out a ruler. The retention factor for substance A right over here—so I'll put that dot there, label that A—would be equal to the distance traveled by the solute, which we can see it traveled one centimeter, one centimeter, over the distance traveled by the solvent past the origin. We see it traveled five centimeters past the origin, so one centimeter over five centimeters, which is the same thing as 0.2.

Then the retention factor for substance B is going to be equal to how far did it travel? Well, it traveled three centimeters out of a total of five centimeters for the solvent past this origin, plus where we put the sample right over there—5 centimeters—which is equal to 0.6. So notice in this situation, the more polar substance had a lower retention factor than the less polar substance, and that makes sense because our stationary phase is more polar than our solvent. The things that are more polar were harder to move by the less polar solvent.