OK, so we've now looked at a range of techniques to explore elemental analysis, and it's worth just spending a little time looking at.

Molecular analysis as well.

And.

By that I mean looking at how we can get a bit more information.

About the structure structure of the materials that we're looking at.

And.

We mentioned XRXRDX Ray Diffraction which allows us to identify particular crystal structures.

It can be really useful, but when we're dealing with sort of.

Non crystalline materials. It can be, it can be. It becomes increasingly difficult to use those kinds of techniques and SRD as well often requires or traditionally at least requires quite a lot of material with which to work.

And there's a variety of other techniques which available which are partially orally or fully non destructive.

And allow us to.

Assess the composition of materials.

In a way which users.

Excitation energies, so principally light and infrared.

Radiation so electromagnetic radiation.

To excite.

Our sample in.

A slightly different way.

From the way we've seen atoms becoming excited before and that's and we'll talk about what that how that works. But it's certainly come parable.

You can think of it in the same ways.

Process of excitation to higher energy States and the need to relax back and release that energy.

Is at least at least at least broadly speaking, compatible with the way in which atoms atomic excitation is operating.

And so if we think about atomic absorption and emission as a convenient comparison.

We can think about this process.

Our.

Of sort of molecular excitation if you like as being quite similar.

With atomic absorption we had energy in light energy going in and we are observing what is absorbed or this or we are observing how much less of that light is received after it's passed through our sample.

With atomic emission where exciting our sample with.

By adding energy to it and re observing how.

It admits characteristic energies as it relaxes back into its UN excited state.

Sort of visualized like this, you can see it's sort of that shifting between energy levels happening in a variety of different ways.

And that process of emission and detection with a light source and detector or a.

Excitation source and detector as being sort of Comperable, at least broadly speaking, to what we want to talk about today, which is molecular absorption and emission using infrared and ramen spectroscopy.

And we're talking here about vibrational energy states.

And.

With infrared spectroscopy, we're usually working with absorption. We're looking at how.

Infrared radiation at various wavelengths, various frequencies so useful to think about frequencies 'cause we're talking about vibrations.

Is absorbed in amount that's proportional to the amount of a particular molecule or molecular bond or molecular structure that's present in our sample.

And the frequencies absorbed, which of course are related to the wavelengths that are absorbed.

Are characteristic of particular vibrational States and therefore particular.

Forms of bonding and of particular bonds between different elements in a structure.

And that can give us quite a lot of information as we'll see.

Roman spectroscopy is a little bit different.

We're using a monochromatic laser to illuminate our sample.

Which will.

Result in a sort of virtual excitation of.

Molecules and molecular structures within.

Or sample.

And as a result of this excitation, they will subsequently relax back into their an excited state and release that energy in small amounts in the form of light.

That his scattered.

In scattered using as a result of the Roman effect. This is cool. This is so it's a very small percentage of scattering occurs in this way, but it can still be measured.

And the emitted light.

Is of a different wavelength to the laser from which it's by, which is the original sample was excited and the wavelength or the wavelength of that light will be different.

And we will also be characteristic of.

The structures or molecules that excite that were it became excited in this process.

So when we're dealing with infrared spectroscopy, we're dealing with relatively low energy.

Photons which can interact with molecular bonds if they have the right frequency.

And if the molecular that motion involves a shift in what's called the permanent dipole moment.

Of the molecule, which is one of its electrical properties.

If they can interact then we will be able to measure that interaction as.

Missing frequencies because frequencies where the malaak molecule is able to interact with that changing electromagnetic signal will be absorbed by that motion.

So Rama Spectroscopy uses a slightly different principle but also responds to vibrational modes and it's able to respond to a different range of vibration modes 'cause it's it deals with what's known as the polarizability.

All of that molecular system. That molecule that series of bonds.

And.

It results in the results. Is scattering as a result of the changes in molecular energy level and various ways.

So we've talked about vibrational modes already.

But let's just have a think about what that means.

And when we're talking about here, you have a molecule of carbon dioxide. For example, with two double bonded oxygen around a carbon molecule carbon Atom.

And we can think about those bonds as Springs.

And this is a useful way of conceiving.

Of the various helps to visualize how this various modes of vibration can occur.

And we're thinking about in terms of the different different directions vibration can occur in. We can think about in the terms of the different planes the vibration can occur in.

And so we talk people talk about in and out of plane.

Stretching, bending, wagging.

Rocking and so forth.

Scissoring

and the various modes. You can look at them in a variety of ways.

You can sort of visualize, but really what's important to know?

Is that these happen?

Something like a symmetric stretch will often be invisible too.

To infrared spectroscopy, it's IR inactive.

Because they involve involves no change in that sort of dipole moment. Depending on the molecule and the shape of that molecule.

Other forms will be rohmann inactive.

Ann will be invisible to ramans spectroscopy, so this, as I say this, is a complementary processor.

OK, so just an example of sort of relatively simple one.

Here's water.

Oxygen and two hydrogens bonded to it and you can see that there's a range of different.

Vibrational sort of activities being registered here in the spectrum.

We've got only one kind of bond in this molecule on RH bond, and you can see.

This OH stretching region here.

So somewhere between 300 three 1200 wave number 3200 and 5:50 ish.

Here's someone that's region.

You've got this. What's called the H stretching region where these.

Bonds are stretching out and vibrating this way.

Further up the spectrum, there's not much else happening here. Further up the spectrum here you've got.

A sharp peak.

Corresponding with

the bending vibration of the H2O molecule as a whole, those two.

Bonds working in parallel.

And again in that's in plane bending and then you've got wagging out of plane.

Further up again, a sort of broad peak here.

So you can see the principle of that and.

If you imagine what will happen with a very complicated molecule, you can see that this would get.

You'd still end up with a much wider range of peaks across this spectrum, and if you have a material which is not a single thing but a complicated material made of multiple different kinds of compounds, you can see this can get quite difficult to interpret after pretty quickly really.

And you can visualize it in something in a way like this, where you have your infrared radiation passing over the molecule and causing all of these different movements.

And that this kind of interaction is going to lead to the formation if we measure those different movements and their absorbance.

As a result of them, we can see these complicated would like to get some sort of complicated spectrum.

It's useful when interpreting spectrum to realize that higher higher wave numbers.

Generally refer to individual baby of individual functional groups, whereas lower wavenumbers we're looking at something which generally helps us to fingerprint the molecule as a whole, and so when you're dealing with organic molecules particularly, it's usually described as anything below 1800 ish. Is this what's called the diagnostic region? And anything above that is referred to as the fingerprint region?

For archaeologists, we're dealing with complicated mixed materials. As we said, this is can be really challenging because the.

Complexity of these plots and the ability to resolve individual peaks.

From amongst them, and more importantly, to attribute those individual peaks to a particular.

Type of vibrational sort of activity.

Can be really difficult.

Libraries of comparative data, however, can really help, and for certain things.

This sort of analysis can be really valuable.

So how do we go about measuring?

Absorbance of infrared. Well, theoretically we could use a dispersive system and separate out our wavelengths of infrared light and observe how they are individually.

Being absorbed, but in reality that is not a technique which has been used really very commonly since.

Probably since the 1970s eighties.

The availability of.

High-quality computing enables us to make calculations.

And.

Calculate.

The absorbance at a whole range of wavelengths all at once using something called Fourier Transform.

Spectrometry

and what we're doing is we are exposing our sample to.

A modulated signal.

From a continuous light source of infrared.

And we are observing power variations in that Modulated signal at relatively low frequency.

Because of the way that that is being done.

Using what's called an interfere ometer.

And we are measuring power variations in that signal before and after it goes through our sample.

So we're measuring effectively a background signal.

And then we're observing differences from my background signal.

And we can convert that through what's called a Fourier transform from the frequency domain into the wavelength domain, and we can talk about there for about wave numbers and we talk about wave numbers because they are. If you remember proportional to frequency.

OK.

So that's the sort of simple way we can understand that we have in our sample we have incident. We have our interfere ometer or sample and then our detector.

That's the basic units of the process, and the interferometers is. We've talked about interference several times before, but interference is where in the interferometer is using a moving mirror.

To bring to to bring Anna station minute mirror to which is the beams from which are being recombined and the moving mirror is taking.

All of the wavelengths that are being used in our sample.

In and out of phase.

It's taking it's moving along and as it moves one beam, one path of light has to travel further.

And as a result, it moves slowly in and out of phase along with the mirror.

And we can then measure variation in the power of the signal at.

If we monitoring time and we know the distance that the mirror is traveling.

We can.

Understand.

How those power that para variation is changing and so are mirror. It's it's, you know there are a number of things that are really important in this process.

The first is ensuring we know our mirrors, so we need to measure the distance that the mirrors away. Very precisely. That's done using a laser.

We need to have advanced computing to be able to calculate back from an interferer ground the differences between interferograms to create our modified Spectra.

But you know, that's something which we can do today very effectively and it makes FDR a really very powerful and rapid technique for certain applications.

So just to show you what that looks like in principle, you have our mirror moving back and forward and you have sort of. If you had a monochromatic light source you would just see.

A modern modulation in power.

As a result of that, in and out of that movement in that phase.

OK, well you have two different wavelengths. You would start to see what's called beats.

As a result, for those two wavelengths are out of sync themselves coming in and out of phase in a regular pattern.

And as you get a complicated polychromatic source, that process becomes more more complicated, of course, but we can calculate out those differences again using complicated computing, which is now within the range of any computer really these days.

If we look at the arrangement of our sample inside the instruments, there's a variety of options that are open to us and traditionally, sample instruments have used what's called a potassium bromide pellet.

Where we would be pressing our sample mixed with potassium bromide into a sort of window through which we can shine our infrared light, and we can use very little sample material to do this.

And that makes this set this in principle the technique almost non destructive, which is in a real benefit.

The other benefit of course, is speed and performing potassium bromide's pallets. They're not particularly complicated is a process requires time, and it requires particular environments. It doesn't work very well if you're working in humid environments, for example because you get absorption of water pellets, and consequently you get water interfering with your patterns patterns. You're actually interested in that you want to see, so alternative systems are using what's called to attenuated total reflectance.

Where you have a crystal made of.

Automated diamond ore.

A number of other materials which is used as to which is used in as a sort of a prism to reflect the infrared light.

Within it and what you get at the surface here of that interaction point between your sample placed above it and the prism itself is what's known as an evanescent wave. Now you don't need to understand what that is, it's just something which happens and as a result we can use this system to actually measure in solid samples directly on the surface of this crystal, which means we are our preparation. Time is almost none at all. We can just gently press that sample.

Onto the surface. The crystal take our measurements and.

Prepare it for the next sample. So when we're looking at this technique.

We're looking at something which is brilliant for the identification of certain types of materials. It's particularly good at dealing with particular individual materials, so multi component systems it can deal with, so things like ceramics, but it has more challenges associated with it, it's.

As a quantitative technique, it's not widely used in archaeology, although is used semi quantitatively to compare differences. So compare changes through different processes like heating and so forth.

It can be quantified, but as I say.

In archaeology it usually isn't.

It's more commonly used as a qualitative technique to really identify things like pigments, particular minerals, or groups of minerals which are can be compared to known samples.

And it can be used for both inorganic and organic materials and for organic materials. It's very widely used for things like the identification of resins and so forth in a way which is very rapid and requires no or very limited sample preparation.

In comparison to something like will steal it this week looking about gas chromatography.

This can be a very rapid tool, although correspondingly it's a much less precise separation tool, so it doesn't allow us to deal with those complicated mixtures. I think that's the important thing to realize.

So an example of how this works in principle when we're looking at ceramics, we can use FDR facts spectroscopy for temperature firing, temperature estimation using a variety of different features of clay minerals and.

The kinds of high temperature phases which are formed as the firing progresses.

And just just really is just a quick example. You can see how certain peaks will correspond to certain types of minerals, so carbonates, sort of carbonate groups have a peek here, sort of about 1430.

Wave number, which you can another series of other peaks here corresponding different vibrational modes and you can see that as you heat up your sample. This is raw clay from a site thought to be a good candidate for the source of particular type of pottery and you can see that as you heat it up.

These carbonate peaks degrade.

Slowly they become less sharp, it becomes less well developed and ultimately as the carbonate decays as it is fired, carbonate will decompose.

You are seeing deformation and changes in the pigs.

You can see the same in the other pics as well. They rap, they vanish more more dramatically and more rapidly.

Still present as a shoulder peek there.

But almost invisible by that stage, as you would expect.

By 900 degrees, thermal decomposition of calcium carbonate should have occurred, so it's no longer there.



With Silicon.

Oxygen stretching region you're seeing again, you're seeing a series of changes in the shape of that peak.

Shifts in its position slightly.

As a result of.

Various changes during firing.

Alterations to those those molecules.

No subtle differences as we're getting more vitrified. Perhaps you're beginning to vitrify that material. These sort of bonding structures are changing.

Here you can see another example where in a region which in the raw clay is relatively clear of peaks.

You start to see.

At high temperature, the appearance of a series of new peace.

Lizia calcium silicates, which are forming above 850 and as a result of firing.

And so if we then go and take our clay, which we think is.

Like to be from this source and comparative archaeological samples of pottery. Sorry and compare it to these clay prepared clay samples. We can actually place it somewhere and in this worst case we can place it somewhere between 600 and raw, so we're looking at a low temperature firing below 600 degrees.

Where we're seeing a still a very well developed calcium carbonate backup, sorry carbonate peak.

You know much, much more development and we're seeing here in the 600 degrees sample.

And we're not seeing any of those.

New pics being formed so we know it's well below that already, but you know you can in your seeing still have that you're seeing some of these smaller peaks here as well. Which are, you know, were absent in other high temperature?

High temperatures, higher temperature firings, so you'll be able to see variation, and that's the basic point. So we can use it in that way to start to look at the decomposition of minerals when we expect Kayla nights to decompose and be no longer recognizable as crystal, their crystal structure and so forth.

So another technique which is used in similar applications to infrared spectroscopy is ramans spectroscopy.

And Roman spectroscopy is based on the observation of ramans scattering.

As a result of.

The an incident beam falling on a material.

And it's based on the principle of Ramen scattering, which was identified by Doctor Chandrasekar Rahman.

In the 1920s, who won the Nobel Prize for his discovery in 1930?

And he observed that.

Whilst almost all light which is scattered from a surface when it that surface is exposed to or when light is passed through a material, he observed that some of that light.

Was being scattered back at different wavelengths.

Now when we are observing infrared absorbance, what we're doing is we're exposing the sample to infrared, and we're observing which wavelengths are absorbed into that sample as molecules within that sample move from one vibrational energy level to another.

What we are is what what happens with Raleigh scattering is that Raleigh scattering doesn't introduce any change in the vibrational structure of our material.

What happens is that are.

That material is moved into a higher energy state as a result of exposure to that light, but that that energy is all returned.

Outwards so it scatters back the same wavelength of Light, the same energy.

And returns to its original position.

It's original energy state, so nothing has really happened.

In the sort of excepting that virtual moment, if you like.

With Rahman scattering.

What happens is that.

As the that although the light is in the light, induces this virtual vibrational energy state.

What happens when?

The material the molecule returns to return some of the energy is it doesn't return some return all of it. It retains some.

It retains it in as vibrational energy within its.

Molecular structure.

And that vibrational energy.

Is going to absorb some of that energy from that light so that what is returned out of the sample order scattered back.

Is light of a different energy of a different wavelength?

And.

The amount of energy that is taken up by the sample, so the and therefore the amount of energy which is we can measure as light which is being scattered by ramen scattering.

Will correspond to particular modes of vibration.

In the molecules and bonds of the molecules in that sample, and so we're measuring in a sort of slightly.

Indirect way. So whereas with IR absorbance with directly measuring absorbance with that material, we're doing the same thing with Rahman in principle, but we're doing it, sort of.

With an extra step, so indirect measurement, we're measuring it by difference of Energy.

This principle is complicated by the fact that some of the material in a sample may already be excited, so it may already be at a vibrational energy state.

And in those situations, actually what can happen is that.

Light of a. We can receive backlight of a higher energy because we're actually that we're sort of combining those processes together that the light is just falling on the surface is taking up to a higher, even hire a virtual energy level and it will make them fall back down.

Two, it's unknown sort of UN excited state and that will release not only the energy of the photons of Light which fell upon it.

From the instant beam, but also with the addition of extra energy from.

The from the loss of that vibrational energy state.

So this is called the. These are called that the two ones are called the Anti Stokes and anti Stokes lines and for most purposes the Anti Stokes is usually less intense and because of the fact that we need to cut out.

The laser itself, the entire intensity light source from our analysis and avoid damaging our sensors and so forth. Anti Stokes lines are usually cut out as well.



Just for practical purposes. So instead of filtering out only that little notch here, which is requires quite an expensive type of filter, you would take this larger a larger sort of bandpass filter and just photo filter out all of this higher energy stuff.

Leaving only your lower energy.

Lights.

Which is what we're looking at, and we're talking about Rahman.

Scattering, OK, so this is what's really great about this. Is that what you're using is a?

High intensity laser which can be very finely focused onto a surface.

And within using, although I think FDR systems do exist.

What we're actually most commonly using is here a.

Monochromator similar to the ones we've seen in other analytical purposes and a CCD detector here.

And that means we can combine it with microscope effectively and we can very precisely pinpoint on a surface where we're going to analyze. The same is in principle possible with FDR, but it's been less common in general Rahman's been more widely used in this kind of application.

So what does that mean for archaeology? Well, there are some issues with this in as much as there's a variety of different choices, we can use it for organic and inorganic materials.

But with running the libraries that are available for the different tools are a little bit variable because different systems, different analytical laboratories, different analytical instruments will use slightly different wavelengths of laser and therefore the libraries which exists are sort of distributed across those varieties.

But nonetheless there are good libraries out there to make our comparisons.

Depending on the type of material we're going to be looking at, we're going to need to select different types of laser. So if we're looking at organic materials or pigments to some degree commonly used are.

Lasers, which will be relatively low wavelength, so sort of relatively long wavelength. Sorry relatively long wavelength, near infrared lasers which are going to induce less fluorescence in the material, so we avoid issues of competing physical processes.

For ceramics or corrosion products, you'll probably going to use a higher energy laser, although proportion of labs will still use lower engine letters for those analysis, so these are the kinds of things we need to consider, but ultimately the results are the same, and what we can use Rahman 4?

Is again it's a similar range of materials, both crystalline and Amorphis materials. Mixtures as well as single compounds.

And.

Up

what's really valuable about it again is it's non destructive, effectively and destructive, at least in most cases. It's can be performed directly in situ. It can be formed performed.

Under a microscope as well, so you can really get very good spatial resolution with these techniques.

And just to show you the kinds of things that you might do. So this was on the right here. Seeing the analysis of three areas on a little piece of.

Egyptian cotton aje from the wrappings of a mummy.

And.

The region of paint had three different colors, red, yellow and bluish green.

And a Roman analysis of each of these different areas shows a good match with library Spectra for.

In the top case here, here's the.

Measured some measured Spectra, and here's the reference. Mercury sulfide sulfide ore, cinnabar.

Here you've got a yellow region, which is corresponding quite nicely with what looks like arsenic sulfide.

And here again, is the library spectrum to compare with your seeing peaks.

Good matches for the peaks in every case pretty much.

And.

Here is slightly more noisy specimen, but showing a convincing match for calcium, copper silicates or Egyptian blue that blue pigment made manufactured pigment used widely in Egyptian art.

So this is just an example of how you might apply it, and as I say, it has very similar applications too.

IR spectroscopy.

And it's something which you'll find widely available in museums and also as portable devices and in microscopes. So it's a really useful and valuable technique as well.

OK, so next time we'll look further into organic materials and look at.

Chromatographic techniques for the study of lipids and proteins.

Thanks.