0:08

Okay, so let's move on.

So again we talked about general spectroscopy, general units used,

how you convert between them.

And now we're gonna move on to the first spectroscopy.

We're gonna talk about UV, UV/visible spectroscopy.

[COUGH] In all spectroscopies, you have to have some light source.

So here we have our little, animated light source.

Let's see if we move on this slide.

Yeah.

0:36

So you're gonna get some UV radiation from that.

But usually you're going to get more than one, more than one wavelength, and for

spectroscopy purposes, for this purpose we want to just see one wavelength.

So you have a monochromator here, and the function of the monochromator is

to select a single wavelength on the wide range provided by the light source.

So we're not gonna get into instrumentation here.

We want to get the main principles.

But this monochromator selects a single wavelength and then you, so you have this

incident, this one wavelength and you had what we call incident light.

So this is a light that's going into the sample.

Here's our cell here that contains our sample.

So you have instant photon and [COUGH] what we give this,

we call this I, capital I, and subscript 0, so that's our I0,

and then it goes through the cell, and what we're trying to show here is,

you can see, it's thicker here than it is here.

So the idea is that some of it is absorbed and less of it comes out,

and then you have some detector that can detect that light there.

So that's the basic principle of any spectrometer, if you like.

You have a source, some kind of thing that will select wavelengths,

goes through a cell, and then you the detector, so

you have I0 going through, and the light comes out.

Or light is not absorbed by the sample is I.

2:14

So for this type of spectroscopy, again, just small points on technical,

you have a cell it's called a cuvette, a glass or

plastic, which you maybe have quartz for UV light.

So that's just a technical side.

So what we're interested in is what's going on.

We're not interested in radiation.

It goes in I zero and then some of it's absorbed and

it comes out as having an I value.

2:42

So there's three factors that'll govern the amount of [COUGH] absorbance.

So we're gonna put these very qualitatively first.

The thicker the sample, the more absorption.

That basically means that a big amount is in the cell if you like of

a particular sample.

3:23

So it takes us a sample.

Concentration of the sample, and then an inherent property of the actual molecule,

how much energy it can absorb.

So to get this in a more quantitative way,

it was a bit qualitative in the last slide,

3:41

this was developed by Beer and Lambert.

So it's called the Beer-Lambert law, and

we're also referring here to just at a single wavelength.

So it's the Beer-Lambert law, and it also always refers to a single wavelength.

So the absorbance of the sample depends on the concentration,

we've already mentioned this, of the absorbance species.

Now we're getting a bit more quantitative.

And we measure concentration in moles per liter.

That's moles liter to the minus 1.

Or sometimes you have moles per decimeter cubed.

Moles decimeter to the minus three, same thing.

4:30

Then you have the length of the light path.

We crudely mentioned that cuz of the thickness of the thing before.

But it's the length of the light path, l, through the cell.

And it's usually quoted in centimeters.

4:57

Again, because I think it's an easier unit.

Usually the cells are just a few centimeters so

it's easier to talk about centimeters.

But the length of path, l, is usually given in centimeters.

5:08

And then you have this, we show this inherent ability of the molecule to

absorb the light, and that's known as the molar absorption coefficient, and

it's given this Greek letter epsilon here.

5:34

So the absorbents,

we defined the absorbents of a sample with these three quantities.

We defined the more absorbed coefficient multiplied by the concentration

multiplied by l.

And strictly, you don't need to worry about this too much,

it's, we're talking about one wavelength.

So we're talking about the absorbance at a given wavelength is equal to epsilon at

that given wavelength, cuz that will change depending on the wave.

So anyways, so remember A is equal to epsilon cl.

6:07

And then you have just the molar absorption coefficient,

which this is inherent property and some molecules are better than others at

absorbing a particular wave is here in the property.

And it's also known as the molar absorbtivity and

the extinction coefficient.

So there's a few names for it.

So we'll call it the molar absorption coefficient.

6:32

All right, so let's move on to this, talk a little bit about this.

So we have our incident light and here is our cuvette.

So we have I zero coming in, we have I coming out, and

we need to know the relationship between the light coming out, I, and I0.

We already defined something called the absorbance as epsilon cl,

7:07

Now you can derive this, this is called a first order.

This is actually a first order rate law.

You can derive that.

We're not going to do that.

You just have to accept from me that that's what is given.

So I = I0, since the power minus epsilon,

which is more absorbed through coefficient,

l the length of path of the cell, and c, the concentration.

7:33

So that's another definition if you like.

We've already had the absorbance,

which is equal to epsilon cl, so

now we have that the absorption also is equal to this.

Now, if you know a bit of, let's see if we can do the math,

mathematics, how we can get to that.

8:03

So we have I = I zero,

ten to the minus epsilon cl.

So if we go I Over I0 is equal to 10

to the minus epsilon cl.

So now, if we go I0, if you know logs,

I0 over I is equal to 10 to the epsilon cl.

So just if you invert, invert one side and you change the sign here.

So you know that a log is the base ten

8:55

So I don't know how good your mathematics is,

but that's just working from this equation here.

So you know that log of ten of I0 over I

is equal to epsilon, that's equal to A.

So that's where you get that relationship

between the absorbents and the transmission.

So you have the incident light,

the transmitted light.

[COUGH] So this is just a carry on from the,

that the transmissions is equal to I over I zero.

The light comes out, the intensity of light comes out, divided by

intensity of light that goes in and therefore, going back to the last slide,

you can work out that it's A is equal to negative log ten of the transmittance.

Just remind you again that we worked it out here.

I over I0, that's the transmittence.

So again if you take the log of that,

you're going to get minus epsilon cl.

So you can either remember these or you can try to do the mathematics,

which is usually the best way.

Even if it's the long way.