Have you ever watched
a TV show where to catch the criminal they take
a sample of the liquid found at the crime scene, run it
through this big fancy-looking box, and find out that
that liquid was actually some gasoline and are
able to suddenly trace the criminal back to
the nearest gas station? That fancy looking box you
saw is probably something that they were trying to
use for gas chromatography, but in real life,
gas chromatography doesn't really work like that. It's a slower process
for separating out compounds that have
different boiling points and a few other properties. But let's take a step
back and figure out how does the gas
chromatograph work. First, what you need to have is
a place to inject your sample. Even though you'll be injecting
it as a liquid, what happens is it gets to this box, and
it gets vaporized into a gas. When it's in a gas, let's say
that this particular mixture was made up of two
different kinds of gas. I'll show that as some green
dots and some orange gas particles. You can't really
see these though, because usually the
amount you're injecting is so small, on the order
of microliters, in fact. And in gas chromatography,
we've talked about how the mobile
phase is a gas, which means that you need to have an
inert carrier gas to push these through. And it's important
that this is inert, because you don't want
it to react with whatever it is that you're
trying to separate. Once it's passed through
that, it'll get heated up and then go through a long tube. In order to make it
fit into the box, they usually just coil
a long length of tube, and the longer the tube, the
better separation you'll get. And once it's finished
passing through the tube, there needs to be some kind
of detector that picks up how many particles of the
green compound were found versus how many particles of
the orange compound reached it. And they'll be
reaching the detector at different rates, which
I'll explain shortly. From there, the detector will
be able to take these signals and display them
in a way that you can analyze on your computer. Often what you'll get is
something that looks like this. This is known as a chromatogram,
which is just a way of saying, a graph for gas
chromatography, and we'll also be explaining this later on. So to recap, we injected
our liquid sample, which was vaporized
into gas, then it joined up with the
stream of inert gas that was already flowing and
was pushed onto the long column. But what's going on
inside that coiled column? Let's take a closer look. Pretend that this
is stretched out, just into a straight
column that's horizontal. It has some liquid
coating on the side, because the liquid is serving
as the stationary phase as the gas is
rushing through it. And what you would
observe, perhaps, is something that
resembles this. You might see the
green dots kind of hanging out on the sides,
while the orange dots are clustering more in the middle
and maybe even traveling a little bit further
than the green dots have. What does that mean? Well, we can't really imply
too much from that yet, so let's watch it for
a little bit longer. At the next time point,
what you might see is that, again, the green
dot, or the green compound, kind of staying
more to the sides. It's traveled a little
bit farther now, but this orange
compound has gotten pushed all the way
over here, the point that it's almost at
the detector already. You can already tell
that the orange one is going to reach
the detector first, meaning it will
produce the first peak. This would probably correspond
to this on the graph. But wait, what's that
tiny peak next to it? Usually, that
represents the solvent that you dissolved
your compound in. That solvent is
usually something with a pretty low
boiling point, so it gets pushed through first. But the second
peak that's bigger is usually the first peak
that actually represents a compound in your mixture. So that last peak
you see probably represents this green compound. But why are they coming out
at such different rates? What's the reason for this? And one of the reasons is
that in chromatography, it's always an interaction
between the two phases. Here it's the vapor
phase, or the gas phase, with the liquid phase, also
known as the stationary phase. So compounds like
this orange one that move really
fast, really, really like to interact with the gas. And this is because
they probably have pretty low boiling points
and are vaporized really readily. Whereas compounds
like the green one might have higher
boiling points, and prefer to spend their
time in the liquid phase, and are not quite as ready
to go into the gas phase as the compounds like
the orange compound that have lower boiling points. So separation by boiling
points is a big part of how gas chromatography works. But wait, there's actually
a few other things. What if the green
and orange compound had more similar boiling points? Could you still
distinguish them? Actually, you could. Let's take another example. If instead originally
what you had was something that looks like this, where
you had these tiny pink dots, those represent
tiny pink particles, along with large
purple particles. Again, these have the
same boiling point, but why is it that it looks
like the pink one is getting carried farther by the gas? That's because
it's really small. So just based on its size,
what would happen next is you'd see something
very similar to what we saw in the
second image before, where the purple dot
hasn't traveled very much, but the small pink ones
are just going so fast, they're almost at the detector. The way to visualize
this is, imagine the gas pushing through. Now picture that as
a really strong wind. If you have a tiny child
in a meadow where there's a strong wind, the kid
will feel like they're getting pushed
around pretty hard. But if you had a big
Sumo wrestler instead, they probably
wouldn't move too much no matter how hard
the wind blew. So in this case, the pink
dot's like the child, and the purple one's
like the Sumo wrestler. So we've talked about the
size of the particles, or the molecular
weight of the compound, along with the boiling
points as being ways to discern
between compounds in gas chromatography. But let's take a closer
look at that chromatogram you see on the computer screen. That chromatogram
is actually a plot of intensity on the y-axis,
representing how many particles are hitting the
detector at a time, versus time on the
x-axis shown here. So again, we saw something
that looked like this. We said that the very
first peak that comes out is probably just the solvent
that your sample was originally dissolved in. The next one represented
our first actual peak, and it represented
again the compound that traveled
faster and further. Let's call this compound A. The
second peak was the one that was a little bit slower,
so compound B. Just by looking at this
chromatograph, we can already know a little bit
about the relative properties of A versus B. Again, compound
A was probably smaller and had a lower boiling point, whereas
compound B was probably bigger and had a
higher boiling point. But that still doesn't
tell us anything about the identities of
these exact compounds. What you would really
need to do in lab is first run a reference,
meaning that earlier you could have run a graph that looked
like this and got two peaks. And if you knew that
your reference sample was a sample of hexane, and it
looked like they came out at about the same
time as compound A, you could probably infer
that compound A is hexane. Although, it's not
quite definitive, which is why gas
chromatography is usually coupled with other analytical
techniques that can give you even more information
about the compound. For example, techniques
like mass spectrometry can tell you about
the molecular weight, so that makes it even
easier to narrow down what the exact compound is. And I know that this can
be a pretty tricky process to figure out what
the compound is, but for analyzing these GC
graphs, what you'll mostly want to look at is the relative
difference between the peaks and try to compare
compounds qualitatively. Quantitatively, you can also
note that the area of each peak is directly proportional
to the amount of compound in the mixture. So next time you see on TV
that they're trying to use GC, you'll really know what
actually goes into it and that you really can't
catch a criminal quite that quickly using only this. You'd need to use a lot
of other lab techniques.
