Hello, welcome to today's lesson.

My name's Mrs. Clark.

You might have come across chromatography before.

Today we're going to look at different methods of chromatography, particularly thin layer and gas chromatography.

We're going to compare those techniques too.

Now chromatography uses very small amounts of samples and it's often used in forensic science to identify different components in samples.

This lesson is part of the unit Separating Substances.

Let's start the lesson and here are the outcomes for today's lesson.

So by the end of it, you should be able to identify features of the different types of chromatography and be able to describe the techniques that are used and how they separate and identify components of the mixture.

Here are the key words for today's lesson.

Stationary phase, mobile phase, chromatogram, thin layer chromatography and gas chromatography.

And here are those words written into a sentence.

You might like to pause the video and take some notes about these words to help you to refer back to during the lesson.

Today's lesson is broken into two parts.

We're going to look at paper chromatography and compare that to thin layer chromatography first of all, and then we'll spend some time on gas chromatography.

So let's get started on the first part of our lesson.

Chromatography is a technique for separating and analysing samples.

It enables us to distinguish between substances which are pure and those which are not pure.

So sample a for example here on the chromatogram is pure because it only produces one spot during the process of chromatography.

And sample b is impure because there's more than one spot.

There's multiple spots.

All types of chromatography have a stationary phase and a mobile phase.

So the stationary phase is that bit which doesn't move with the components in the sample.

So in this case it would be the tissue paper.

The mobile phase is the movement of the solvent along with any of the dissolved components of our sample mixture.

And how well a component in a sample interacts with the stationary phase and the mobile phase will impact how well we get a sample separating.

Let's just remind ourselves about the equipment set up for paper chromatography.

So we've got the mobile phase or the solvent which is usually water at the bottom.

We've got the sample line which we draw in pencil so it doesn't interact with the chromatography process.

And we've got the stationary phase, which is the layer of filter paper and we usually do this in a beaker.

And then the pattern that's produced is what we call the chromatogram, and here we've got a chromatogram that's been completed.

You can see the pencil line and the cross where the sample was placed and the components have moved up the paper.

And those that have got a strong attraction to the mobile phase will move further along the chromatogram and components with a strong attraction to the stationary phase will remain much lower down on the chromatogram, so those like the yellowy sort of component there.

If we change the mobile phase or the stationary phase, that can affect the quality of separation and our ease of analysis.

We could change the solvent or we could change the type of paper that we're using.

Let's have a quick check.

A component with a strong attraction to the stationary phase is found further up the chromatogram.

Is that true or is that false? Well done if you said false.

And now can you justify your answer.

And well done if you've said a, components with a strong attraction to the stationary phase do not move further along the chromatogram.

The setup for both paper chromatography and thin layer chromatography are quite similar.

So if we look at paper chromatography first, our stationary phase we've talked about might be filter paper or some type of absorbent paper, and our mobile phase is usually water.

And then in thin layer chromatography, we've got an inert substance spread very thinly on a non-reactive surface.

The mobile phase is usually an organic solvent.

Let's look a little bit more at thin layer chromatography or TLC.

We usually call the stationary phase a TLC plate because it's usually a little bit more solid in TLC.

It's usually a layer of silica or alumina spread thinly over a glass or plastic surface.

And we have to be quite careful with TLC plates to make sure that we don't actually scratch the silica or alumina on the surface or that we don't get grease from our fingers or anything like that onto the plate because anything like that will actually disrupt how well the chromatogram develops.

Some of the solvents that we might use for TLC are propanone, hexane and ethanol, and they're all volatile which means they evaporate really easily.

So we need to make sure that when we're using thin layer chromatography, we place a lid over the top, and actually thin layer chromatography works much better when the atmosphere inside the beaker is saturated with the solvent.

So putting a lid on there would help with that.

It also stops or minimises the amount that those solvents are getting into the atmosphere and for people breathing them in 'cause sometimes they're toxic.

Let's have a quick check.

So what substances would be found on a TLC plate? Well done if you said both of those, alumina or silica.

Now we sometimes use a developing agent with thin layer chromatography before we have a look at the chromatograms. And the reason for this being is that some of the spots sometimes are not as easy to see.

They're colourless, but they're still important.

We need to know that they're there, so we use what's called a developing agent such as UV light or iodine or ninhydrin to make those spots much more visible for us.

Once we've got all the components identified on a chromatogram, we can calculate a retention factor.

We often just call that the RF value.

And we do this by measuring the distance that's been moved by each of the components or substances, divided by the distance moved by the solvent.

So if you look at component a there, for example, you would measure the distance from the centre of that spot down to the sample line and that would be the distance moved by the substance.

And then you would measure the line from the solvent front down, and that would be the distance that's been moved by the solvent.

And the calculated RF value can then be compared to a database that's got lots and lots and lots of RF values for substances that undergone the same type of chromatography under the same conditions.

And when we match our figures to those figures, we can help identify the components that we've got.

Let's have a quick check.

Which of the following might be used as a developing agent in thin layer chromatography? Well done if you said iodine and UV light.

Let's have a look at some of the advantages now of thin layer chromatography over paper chromatography.

So we might use thin layer chromatography because we get a more efficient movement of the mobile phase across the stationary phase and so the chromatogram develops faster.

And we can also change our mobile phase or our stationary phase or both of them and get a much better component separation which makes it easier to analyse.

The stationary phase that we use for thin layer chromatography is actually much more sensitive than paper chromatography.

So the use of the alumina or the silica over the glass plate and plastic surface is much more sensitive, and that means we need less of the sample to carry out thin layer chromatography.

And that can be really useful if you've only got a small sample which is often the case in forensic science.

And actually we can even scrape the sample back off the TLC plate and use it for further analysis which could be really useful.

Let's have a quick check.

So which samples on this chromatogram are pure and impure? We've got a, b, or c.

And then could you explain why you've chosen those? Pause the video and come back when you're ready.

So well done If you said sample b for pure and the reason being the explanation, because it only has one spot or component.

And obviously a and c are therefore impure because they contain multiple spots or components.

Well done.

Question two.

So we've got a variety of statements on either side of the Venn diagram and I want you to place them in the right place on the Venn diagram for each type of chromatography.

And obviously in the centre the overlapping circles, that would be statements which are common to both paper and TLC.

Pause the video and come back when you completed it.

Okay, how did you do? Let's have a look.

Here are the answers, well done.

So absorbent paper is the only one common to paper chromatography, and TLC we've got glass/plastic surface, thin layer of silica, thin layer of alumina.

And then the common things to both paper and thin layer of chromatography, they both use a solvent, they're both separation techniques and they both have a stationary phase and a mobile phase.

Amazing job if you've got all of those in the right places.

Well done.

Question three.

Jacob and Aisha are discussing similarities and differences between paper and thin layer chromatography.

I want you to read through their statements and correct any errors that you might find.

Pause the video and come back when you've noticed them all and change them all.

Let's have a look at the answers.

So we've changed Jacob's statements to say TLC separates a sample more clearly or efficiently than paper chromatography, and paper chromatography is slower than TLC.

And for Aisha we've changed hers to say, paper chromatography is less sensitive than TLC and TLC uses a thin layer of silica or alumina on a glass or plastic surface for the stationary phase.

Amazing job if you've got all of those.

Now we're going onto the second part of our lesson where we're gonna focus on gas chromatography.

Gas chromatography is a technique that we use to separate samples of gases.

You'll notice that the equipment is more complicated and there are a variety of components which we'll have a look at in the next few slides.

So we've got the carrier, gas sample injector, the column and the detector, and a display and the column oven.

So the carrier gas must be very unreactive and so nitrogen or helium would commonly be used.

And the reason that they need to be unreactive is that they mustn't interfere with the actual sample.

The sample mixture that we're going to analyse is injected into the mobile phase, which is the carrier gas, that unreactive carrier gas such as nitrogen or helium, and that will carry it through the column.

And as it passes through the column, the sample mixture will separate into the different substances or components.

Let's have a quick check.

Oxygen can be used as a mobile phase.

In other words, the carrier gas and gas chromatography.

Is that true or is that false? Well done if you said false and what's your justification? Well done if you said is a reactive gas, well done.

So let's carry on looking at gas chromatography.

We've got to the column now and that contains the stationary phase and this is where separation of the mixture really occurs.

And the column is made of metal, and it's got inside it's got pack silica or alumina or a thin layer of a very unreactive liquid on a few silica support.

The oven is used to keep everything at the same constant temperature.

The detector then measures the amount of each component as they leave the column.

So substances with a greater affinity for the mobile phase will reach the detector at the end of the column much more quickly.

The detector then measures the amount of each component of the sample as they leave the column and substances with a much greater affinity or attraction for the mobile phase will reach the detector quicker.

Substances with a greater affinity for the stationary phase will move much slower through the column.

And then the information from the detector is used to create the chromatogram on the computer.

Let's have a quick check.

So where is the stationary phase located in a gas chromatography set up? Well done if you said the column.

How well the substances or components of the sample interact with the mobile and stationary phases will determine how easily they move along the column.

Very similar to thin layer and paper chromatography.

Here we can see that the red component of the sample travels much more slowly through the column and this substance has a greater affinity for the stationary phase in comparison to the blue substance.

Here's an example of a gas chromatogram that the display would produce.

You can see on the x-axis, retention time and on the y-axis, response.

Retention time is the amount of time it takes for the substance to travel through the column.

And you can see the red substance takes a longer time, so that has a longer retention factor compared to the blue substance.

A chromatogram produced by chromatography gives us key information about our sample.

So, the number peaks for example tell us the number of components in the sample.

Like with paper chromatography, the number of spots that we had, this is the number of peaks here.

So that tells us how many components our sample has, three in this case, and the area of the peaks peaks tell us the amount of component that's present in the sample.

So a larger peak means more is present, and we can use height as well to help us if the peaks are the same width.

And then the position of the peaks tell us the retention time on the column.

Remember retention time was how long it took the component to travel through the stationary phase and we've got three samples.

So sample a has the lowest retention time, it travelled the fastest, and sample c has the highest retention time and it travelled the slowest.

You might recall that we talked about RF values earlier on when we were looking at paper chromatography and thin layer chromatography and how we could compare those values, the values of our sample to a database and help identify what each component were.

Exactly the same thing happens for gas chromatography.

We can look at retention times of our sample and then look it up on a database and that would help us identify what that component actually was.

Let's have a quick check.

So how many components does this sample contain? And then there's an extension question, is this sample pure or impure? So I'll just give you a moment.

Well done if you said two components, there's two peaks.

And is the sample pure or impure, it would be impure and the reason, the explanation is because it contains more than one peak.

So let's have a look at another question.

So what's the identity of component a on this gas chromatogram? Use the table of data to help you.

Well done if you've said decane.

I think we're ready for task B now.

So I want you to match each description to the most appropriate keyword or phrase.

Pause the video and come back when you've completed it.

Okay, let's have a look at your answers.

So an inert gas such as nitrogen or helium would be suitable for the mobile phase.

A chromatogram shows the components as a peak or a spot.

The stationary phase can be found in the column of gas chromatography.

Gas chromatography is a technique used to separate a mixture of gases.

And so the number of components is equal to the number of peaks on a chromatogram.

Well done.

Let's have a look at question two.

I want you to give one similarity and one difference between thin layer chromatography and gas chromatography.

Let's have a look at the answers.

So a similarity would be that both have a mobile phase and a stationary phase, or you could have said that both can use silica or alumina in their stationary phase.

A difference would be that the mobile phases are different.

So in thin layer chromatography, we use a liquid state mobile phase, those organic solvents, hexane, propane, et cetera.

And gas chromatography uses a gas state mobile phase.

Remember those carrier gases, nitrogen or helium for example.

Let's have a look at question three.

Use the chromatogram to answer the questions and make sure you explain your answer to each of the three questions.

Pause the video and come back when you're ready.

Let's have a look at some of the answers.

So how many components have we got in the sample? We've got five because there are five peaks.

And so the explanation would be there are five peaks.

Which component was present in the smallest amount? That would be e and the explanation, it's the smallest peak and the largest amount, c, it's the largest peak, well done.

And which component had the shortest retention time? Is a, because it's the first peak or the peak furthest to the left.

Well done.

You can now interpret a gas chromatogram.

So we've come to the end of the lesson.

I've enjoyed learning this with you.

So let's have a look at our summary of learning today.

You've learned that there are multiple types of chromatography such as paper, thin layer, and gas chromatography, and all of them are used to distinguish between pure and impure samples.

All chromatographic processes use mobile and stationary phases.

And you should know that thin layer chromatography and gas chromatography both use silica or alumina as their stationary phase.

Remember thin layer chromatography, it was on a plate and in gas chromatography, it was in the metal column.

Thin layer chromatography uses locating agents or developing agents to develop chromatograms and aid our analysis of them.

Remember that some of the spots might be colourless or not visible with a naked eye, so we use things like iodine or ultraviolet light or ninhydrin to make those colourless spots visible so we could analyse the sample much better.

The chromatograms from gas chromatography also give us details about the amount of each of the components in our sample mixture.

And retention factors and retention times, both give us quantitative information about a sample and then we can compare each of those to a reference in order to identify the parts, the components in our mixture.

Brilliant job, well done.

Hopefully you are feeling a lot more confident about being able to compare and describe all the three types of chromatography we've looked at and understand more about the components involved in each of those.

Well done.

I'll see you next time.