0:08

Okay, so let's move on.

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

Â how you convert between them.

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

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

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

Â So here we have our little, animated light source.

Â Let's see if we move on this slide.

Â Yeah.

Â 0:36

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

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

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

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

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

Â So we're not gonna get into instrumentation here.

Â We want to get the main principles.

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

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

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

Â Here's our cell here that contains our sample.

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

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

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

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

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

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

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

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

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

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

Â Or light is not absorbed by the sample is I.

Â 2:14

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

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

Â plastic, which you maybe have quartz for UV light.

Â So that's just a technical side.

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

Â We're not interested in radiation.

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

Â it comes out as having an I value.

Â 2:42

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

Â So we're gonna put these very qualitatively first.

Â The thicker the sample, the more absorption.

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

Â a particular sample.

Â 3:23

So it takes us a sample.

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

Â how much energy it can absorb.

Â So to get this in a more quantitative way,

Â it was a bit qualitative in the last slide,

Â 3:41

this was developed by Beer and Lambert.

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

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

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

Â So the absorbance of the sample depends on the concentration,

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

Â Now we're getting a bit more quantitative.

Â And we measure concentration in moles per liter.

Â That's moles liter to the minus 1.

Â Or sometimes you have moles per decimeter cubed.

Â Moles decimeter to the minus three, same thing.

Â 4:30

Then you have the length of the light path.

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

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

Â And it's usually quoted in centimeters.

Â 4:57

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

Â Usually the cells are just a few centimeters so

Â it's easier to talk about centimeters.

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

Â 5:08

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

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

Â it's given this Greek letter epsilon here.

Â 5:34

So the absorbents,

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

Â We defined the more absorbed coefficient multiplied by the concentration

Â multiplied by l.

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

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

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

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

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

Â 6:07

And then you have just the molar absorption coefficient,

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

Â absorbing a particular wave is here in the property.

Â And it's also known as the molar absorbtivity and

Â the extinction coefficient.

Â So there's a few names for it.

Â So we'll call it the molar absorption coefficient.

Â 6:32

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

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

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

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

Â We already defined something called the absorbance as epsilon cl,

Â 7:07

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

Â This is actually a first order rate law.

Â You can derive that.

Â We're not going to do that.

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

Â So I = I0, since the power minus epsilon,

Â which is more absorbed through coefficient,

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

Â 7:33

So that's another definition if you like.

Â We've already had the absorbance,

Â which is equal to epsilon cl, so

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

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

Â mathematics, how we can get to that.

Â 8:03

So we have I = I zero,

Â ten to the minus epsilon cl.

Â So if we go I Over I0 is equal to 10

Â to the minus epsilon cl.

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

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

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

Â So you know that a log is the base ten

Â 8:55

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

Â but that's just working from this equation here.

Â So you know that log of ten of I0 over I

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

Â So that's where you get that relationship

Â between the absorbents and the transmission.

Â So you have the incident light,

Â the transmitted light.

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

Â that the transmissions is equal to I over I zero.

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

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

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

Â Just remind you again that we worked it out here.

Â I over I0, that's the transmittence.

Â So again if you take the log of that,

Â you're going to get minus epsilon cl.

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

Â which is usually the best way.

Â Even if it's the long way.

Â