Thin layer chromotography

- [Instructor] 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.

And the first step is to think about,

is it just one substance

or is it a mixture of multiple substances?

And the focus of this video is a technique

to separate out the substances

to understand at least how many there are,

and this technique generally
is called chromatography,

but we'll focus on thin
layer chromatography

which is the most common
that you might see,

but other variations of chromatography

like paper chromatography

operate on very similar principles.

So 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.

And 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.

And 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.

Now, usually this is going
to be a very shallow amount

of this solution, which, as we'll see,

will be something of a solvent.

And you usually want to put
it in a closed container

like this

so that this fluid down
here doesn't evaporate out.

And 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, 'cause 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

and it's picking up some
of this mystery substance

and it's transporting it.

And 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.

The chroma is referring
to the various colors,

but 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.

And 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 attractive

to the silica gel, which is stationary,

and there is a way to quantify
how far these things traveled

relative to your solvent.

And that's called a retention factor.

Retention factor.

Which the shorthand is R subscript f.

And it's just defined
as the distance traveled

by the solute divided
by the distance traveled

by the solvent.

And 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.

And let me label that as the origin.

So what would it be in this situation?

Well, to help us there, we
would have to get out a ruler.

So 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.

And so that is going to be,

we see it traveled five
centimeters past the origin.

So one centimeter over five centimeters,

which is the same thing as 0.2.

And 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,

past where we put the
sample right over there.

Five 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,

and so the things that are
more polar were harder to move

by the less polar solvent.