Fourier Transform Infrared Spectroscopy – An Introduction

In this interview, AZoM talks to Dr. Andrew Davies, an applications specialist at Specac Ltd., about Specac and their FTIR technologies.

Who is Specac and what work does the company carry out?

We are based on the edge of London. We were founded 49 years ago, and we are in the process of gearing up towards our 50th birthday celebrations this time next year. We have got 80 employees at offices in the UK, USA, China, and Singapore.

We design, manufacture, and dispatch from our purpose-built 33,000 square foot high-tech operation located in London. We are the market-leading provider of spectroscopic accessories to academic, industrial, and research institutes.

We were the very first to market a commercial ATR solution with the Golden Gate, which was launched more than 20 years ago, and it is still a stalwart of the labs around the world. We have got extensive experience of different IR sampling accessories, and in 2019, we moved into our new premises.

At the new facilities, we have got the Newton Demo Lab, giving us the ability to bring customers in for training and, if customers wanted to send samples in to test, and if our equipment will work for their application, we now can do that in house.

In 2018, we were delighted to be awarded the Queen’s Award for Enterprise, which is a highly prestigious award.

What products does Specac offer?

Our product portfolio can be divided up into accessories for FTIR applications. Our biggest one is in the ATR market. We also make accessories for transmission and more advanced products. We also have a range of sample preparation products, from presses and press accessories to a mill. We also produce infrared (IR) polarisers and near-infrared process cells for industrial applications.

An ATR accessory from Specac

What is FTIR?

Start by considering light. Light is a wave that consists of peaks, which have a wavelength, in this case, say 2000 microns. But spectroscopists normally talk in terms of wavenumber, which is just the number of peaks in a unit spacing. In this case, one centimeter.

If your wavelength is 2000 microns, you have five peaks in one centimeter. That is five wavenumbers. The different wavelengths of light correspond to the colors you see in the spectrum. If we go beyond violet, we have got ultraviolet ranging through the visible, and then beyond red, you go into the infrared. We cannot see it, but it is split up into near-infrared, mid-infrared, and far-infrared.

Typically, chemists tend to use the mid-IR more as a diagnostic technique and Specac’s products are mostly for the middle range, although we do produce accessories that can be used across UV to far-IR.

Spectroscopy is the study of the interaction of light and matter and infrared spectroscopy, in particular, is probing molecular vibrations. To understand what is meant by this, consider a molecule: all molecules vibrate. Some frequencies of infrared light are transmitted without interacting with the molecule. Other frequencies of infrared light are absorbed, causing an increase in the vibrational energy levels.

The frequencies that are absorbed are dependent upon the atoms and bonds. By studying which are absorbed and which are transmitted through the sample, we can begin to build a picture of what a sample consists of.

To collect an infrared spectrum, we have a simplified spectrometer consisting of a sample compartment and a detector. To use this, you would insert your accessory into the sample compartment and transmit some infrared light through the accessory.

You would start by recording the starting intensities of your light. You would then fill your cell with your sample and note down the new intensities of the light transmitted through your sample. You can then process this either as a percent transmission or into an absorbance scale, which is just a log transformation of that data.

This gives you information on your sample. From this, you obtain a chemical fingerprint and you can do qualitative and quantitative analysis.

What are the different techniques for sampling?

There is a range of different sampling techniques, the simplest of which is a transmission measurement in which you press your sample or contain your sample into a small volume space, and you transmit the light through the sample and record the intensity before and after you put your sample into the beam. It is suitable for gasses, liquids, and solids.

By controlling the path length of the beam through your sample, you can tune your peak intensities. This spectral interpretation is extremely simple, too. The alternative method to collect your spectrum is a reflection technique, but this is only suitable for liquids and solids because you are unable to reflect light off your gas and subdivided into different categories with different benefits. The major one is attenuated total reflection (ATR), along with a few others.

What accessories does Specac make for transmission FTIR?

For gasses in their pure form, we make single-path length gas cells, where you pass your beam through the accessory, which has a path length of about 10 centimeters. We make both non-heatable, and down at the bottom is a heatable version of this gas cell.

If you are interested in analyzing trace gases, for instance, if your gas is down at the parts per million or parts per billion levels, you are going to need to massively increase your path length on the scale of between 2.5 and 20 meters. This is not feasible to do in the lab; you cannot have a 20-meter gas cell in your spectrometer. So, what we make for this is a multi-reflection gas cell.

Moving on to how you would analyze the liquid through-transmission mode, Specac makes two different accessories for this. Our entry-level accessory is the Omnicell, which comes with either a fixed or adjustable path length, and a range of sealing options and window materials are available.

We make two different types of cells; sealed cells and demountable cells. Sealed cells are good for path length reproducibility. If you are doing quantitative analysis, then a sealed cell is the way to go. We also make demountable cells, which allow you to take the windows apart and replace the spacer in between them to change the path length. These types of cells give you flexibility and they are good for a general-purpose cell to have available in the lab.

The cell is made up of the backplate and then a window and then a spacer. You change the thickness of the spacer to change the path length of the cell. Then, there is another window. You then inject your sample between these two windows through some injection ports on the front plate.

What is the best way to fill the cell?

We recommend using the two-syringe technique. In this technique, you attach a filled syringe to the bottom of the cell with your sample in, and you attach an empty syringe to the top of the cell. Then you draw on the top syringe to pull your sample through the cell, rather than injecting a sample by pushing on the bottom syringe. Using this technique, you prevent the formation of air bubbles in your cell, which can interfere with your measurement. You also reduce the build-up of pressure that can cause leaks in your cell.

What types of cell do Specac manufacture?

We manufacture the advanced liquid transmission cell called the Pearl. It is a fixed, reproducible path length cell ranging from 25 to 1000 microns. You have two window options, calcium fluoride, or ZnSe. We make two different types of cell,wedged or parallel. Wedged cells eliminate fringing artifacts in your spectra, so they are quite good for quantitative analysis. Parallel cells are for when you require this fringing to determine a very accurate path length.

The Pearl Liquid Transmission Accessory from Specac

The benefit of the Pearl over the Omnicell is that it is an extremely simple accessory to use and the operation is extremely fast. You just mount your cell up in the slide drawer, you drop your sample onto the bottom window, and then you will have inserted the top window which drops down by gravity to form a reproducible path length. Then, you close the drawer and record your spectrum.

To clean the cell, you just lift your top window away and then you have got full access to the cell. This massively decreases the time it takes to change over your samples, which enables you to maximize the efficiency of your spectrometer. It has also got huge benefits if you are using a particularly viscous sample, which can be extremely hard to get a decent, reproducible path length for the other types of liquid cells. With the Pearl cell, you just load your sample, press the top window down and you get the same path length every single time.

What advice do you have on caring for the windows?

In terms of storing and handling the windows, the important thing to note is that the windows are often extremely delicate and should be handled as little as possible and with as much care as you possibly can. You should always wear gloves when you are handling your windows, partly to protect the window from your skin oils and also to protect yourself if you are dealing with toxic windows.

In particular, KRS-5 is an extremely nasty substance, so you never want to be handling it bare-skinned. ZnSe is also a toxic window and you must avoid breathing in the dust if you ever damage these windows. A lot of infrared windows are sensitive to moisture and you mustn’t expose these to water in any way. Particularly, potassium bromide and sodium chloride windows will dissolve if you put them underwater. They will also fog up if you are breathing on them or exposing them to high-humidity environments.

To clean your windows, whichever one you have, you want to start by removing as much sample as possible. For example, if you are using an Omnicell, you can use an empty syringe to draw out as much of the sample as possible. You should then rinse the cell with a suitable cleaning solvent and normally, if you then just leave it to dry, or if you blow some air through the cell, or nitrogen if you are dealing with a moisture-sensitive window, normally that will be sufficient. But sometimes, you might require further cleaning. In this case, if required, you will need to disassemble the cell and then gently wipe with suitable tissue.

The best method to do this is to dampen a lens tissue with cleaning solvent, gently bring the tissue into contact with the window without pressing on the tissue itself, and then just drag the tissue across the window to clean it.

When it comes to storing these windows, if you have got a moisture-sensitive window, then we recommend that you store these windows in a vacuum desiccator to get the maximum life span out of your windows. Windows that are not moisture sensitive can be stored by wrapping lens tissue, placing it into a lint-free cotton sleeve, and then placing it into a protective box.

This is the same way the windows are supplied to customers. Essentially, it is possible to simply put them back into their original packaging to store them. Alternatively, if you want to store lots of windows, what I find extremely useful is to purchase a small plastic toolbox with window size compartments, and then I transfer the packing material from the box that the windows are supplied into the toolbox. That is how I store them. Ideally, I include a desiccant pack in the toolbox.

Could you talk us through the steps necessary for achieving transmission spectra through solids?

Solids present a bit more of a challenge when it comes to transmission methods. The KBr pellet method is one way that enables you to get a transmission spectrum through a solid. The way this works is by dispersing your solid material into a material that transmits infrared light through it.

Step one, you need to prepare your KBr powder by grinding it into a fine powder and then dry it at ideally 110 degrees C, and if possible, under vacuum for as long as possible. Step two is then to grind up your sample. The finer you make your particle size, the better the end spectrum you will get.

You then mix your sample with the dried KBr powder using the pestle but try to avoid grinding the KBr again, as this will create new crystal facets that could start to absorb water. You then load this mixed material into a pellet dye and press it into a disc.

To do this, we have a range of different presses. We make a Manual Hydraulic Press that is very, very widely known. These things are in labs all over the world and quite often, they’re 20, 30, even 40 years old and they are still going strong. We also make electronic presses, such as the Atlas Autotouch Hydraulic Press that has a range of maximum tonnages that it can go to. With the Atlas Autotouch, you load your sample into a dye. Step one, you load your sample and you press it down into a pellet. You then invert the dye, load your demounting ring, and press again upside down, which ejects your sample from the top of the dye, and then you can just lift off the pressing pellets and lift your sample out of the dye.

We also have a mini pellet press, which is designed to be a little bit more portable, a little bit handheld, and it can go to two tons, which is typically all that is required to form a KBr pellet.

Could you tell us about the range of accessories that Specac offers?

Specac manufactures the Mini-Film Maker, which is capable of going to 250 degrees C with a load of two tons. For our other presses and adapter, we also make two different polymer presses. We have got the Constant Thickness Film Maker, which mounts inside of heated platens that then sits inside the press. This can go to 300 degrees C. We also make a High-Temperature Film Maker that is capable of going to a higher temperature of 400 degrees C.

Moving on to our more advanced transmission accessories, these are more for research rather than your day-to-day analysis accessories. The first one I would like to mention is our High-Temperature High-Pressure cell. This is capable of going to temperatures of up to 800 degrees C at pressures up to 1000 psi. There is a bit of a trade-off between the maximum temperature you can achieve with maximum pressure.

The High-Temperature High-Pressure Cell

This cell is capable of doing a transmission measurement where you just press your sample into a solid disc and pass the light through it. It has also got a decomposition mode where you analyze the gases that come off a sample as you heat it up. It has also got an adapter that transforms it into specular reflectance mode, so you would bounce your light off the sample disc to collect a reflection measurement. It has got multiple gas ports to connect up gasses to the cell and it has ZnSe windows.

You typically find the applications for this range from heterogeneous catalysis reactions, the monitoring of exhaust vapors from the thermal decomposition of solid fuels, to the study into the effect of elevated temperature and pressure on solids and gases.

We also make a variable temperature cell, and this accessory is capable of going from liquid nitrogen temperatures, so -190 degrees, all the way up to 250 degrees. For this accessory, you would load your sample into the cell at the bottom of the cold finger in the silver metal component, and then you would load that component into the blue vacuum shroud, and you would place this under vacuum.

You can then introduce a refrigerant into the cold finger, so you can use liquid nitrogen or dry ice, acetone mixtures, or ordinary ice or ice water, to get down to whichever low temperature you want. If you want to heat, you would just use the accessory without loading a refrigerant into it, and then you can heat up or cool down your sample to whatever temperature you need.

So, we offer two different configurations. Our cell holder option is designed to accept any of our liquid and solid cell holders. We also make a full port cuvette holder version that is suitable for liquids analysis and can be used for Raman, fluorescence, and UV/Vis spectroscopy.

Could you tell us more about how FTIR can be broken down into multiple techniques?

Reflection FTIR can be broken down into multiple techniques, the most important of which is ATR. In ATR, we have an internal reflection element and we shine our beam of infrared light into this element and reflect it off the surface through total internal reflection. This then generates an evanescent wave comprising the electronic component of the light, which penetrates your sample and can couple to the vibrations in your sample causing a reduction in the energy transmitted through the IRE.

What you do is you bounce your beam off the surface of the crystal, record your background, and then you add in your sample. If it is solid, you want to bring it into good contact with the crystal. You would normally apply a load at this point. If it is a liquid, you normally get that good contact without the load. Then you record your sample spectrum and you obtain an infrared spectrum as you normally would.

The one difference between ATR and transmission is the depth of penetration. This depth of penetration into your sample is controlled by the wavenumber, the refractive indexes of the IRE and the sample, and the angle of incidence.

In a single spectrum of a single sample, the last three of these are going to be fixed constants. However, the wavenumber changes depending on where you are in your spectrum. The penetration depth is going to change with your wavenumber. Typically, you will be getting about 0.5 to 2 microns, depending on where you are and which type of IRE you are looking at.

What are the advantages of ATR?

There is very, very little sample preparation. So, it is extremely easy. You do not need to prepare your sample with KBr or to grind it. You just load your sample on the crystal, click a button and your spectrum is there. You get superior reproducibility sample to sample compared to the KBr pellet method. There is very little user training required, too. As a result of this, and because you have not got any preparation involved, you are going to get a much higher sample throughput.

Sometimes you can have concerns over ion exchange between your sample and the KBr, which obviously if you have not got the KBr there, you have not got those concerns. Specac makes multiple different ATR accessories. Our bestselling unit is the Quest, and it has been designed to maximize the spectral throughput to provide an excellent signal to noise ratio. To achieve this, we have used high transmission throughput using all-reflective optics.

We also have a selection of interchangeable pucks. We make a range, including a flat puck and a liquids puck. If you have got a solid sample, particularly if you’ve got a large solid item, you can get that on a flat puck. If you have got a liquid, particularly if your liquid has either got a very low surface tension or is very runny, we make a liquids puck to help you contain your sample over the crystal and stop it running away.

The Quest requires minimal amounts of sample, so it is particularly suitable if you have only been able to synthesize a small amount of sample. Additionally, the crystal is securely fixed into the top plate using stainless steel support and indium sale. We find this gives us excellent mechanical and chemical robustness.

For solid samples, where you want to apply a load, we have got this loading feature that has a talk click feature, which gives a consistently reproducible load of 40 pounds. With this, what you do is just spin the black nob around clockwise until you feel it clicking under your hand. It gives a very satisfying click as it reaches the 40-pound load. Typically, we do advise that you ensure it has clicked at least twice, maybe three times, just to ensure that you have got this reproducible load.

Specac’s Golden Gate range for ATR is very established. Could you tell us more about what this range offers users?

For the more advanced ATR work, we offer our Golden Gate range, which has been on the market now for almost 30 years. Despite its age, we find that it is an extremely popular product that just goes and goes and goes. We make a variety of top plates for it.

The Golden Gate ATR

We have got a high-temperature top plate, which we recently updated so you can now control it from your computer software. We also make a low-temperature version that uses a refrigerant to cool it down to temperatures as low as -150 degrees. We produce a supercritical top plate that is capable of going to extremely high pressures up to 300 degrees C, and we also make a reaction cell for experiments into reactions.

We also make a multi-bounce ATR called the Gateway, which encompasses six reflections off the crystal. The benefit of this is that you are effectively multiplying your signal six-fold, so you get an improvement in your signal to noise ratio.

We make a large range of top plates for this accessory too. Whatever your experimental conditions are, we probably make a top plate that is suitable for them.

Are there alternative ways of collecting reflections measurements?

There are a couple of different methods. The first of these is diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). With this method, you scatter your light off of a rough surface. While some of the light is scattered, some of it is transmitted through to the next particle, which also scatters, and the light penetrates your sample. Every time it reflects, or it transmits, it absorbs some of the light.

The advantage of this technique is that it enables you to study catalysts and powders under real-world conditions. You do not have to compress them and change the surface properties; you can study your chemical reaction as it would be performed in industry. Often, to improve the amount of light that you get reflected to get a decent signal to noise ratio, you would dilute your sample in a non-absorbent material. Again, this would normally be KBr.

You don’t have to do this, however, and sometimes people will live with a poor signal to noise ratio to avoid having KBr present. The one downside to DRIFTS is that the spectral interpretation is less simple than either transmission or ATR. This is because the absorbance scale is no longer linear with sample concentration due to absorption scatter effects. However, the Kubelka-Munk correction has been developed to correct for this and give you a linear calibration. If you are using DRIFTS, you would normally use the Kubelka-Munk transformation, although there are exceptions to this rule.

To help you perform DRIFTS measurements, Specac manufactures the selected DRIFTS accessory that is suitable for studying catalytic powders. If you want to begin heating these powders, introduce gasses, if you want to do both, or if you want to heat and study heterogeneous reactions, we make an environmental chamber that mounts into this mirror system and enables you to study your samples at temperatures up to 800 degrees C, from vacuum to 500 psi.

The alternative measure of collecting a reflection measurement is specular reflection. For this, we manufacture a variable angle accessory that allows you to change the angle of the specular reflection that you are collecting. Again, the spectral interpretation is a bit more complicated than either transmission or ATR due to abnormal dispersion, but there is the Kramers-Kronig transformation that you can apply to produce a more transmission like a spectrum.

Where can our readers go to find out more information?

Specac produces a range of application notes that cover specific application areas and more general technical notes, such as our path length determination in window liquid cells. If you want any more information, these are a good place to start.

On our website, we also publish a range of news and guide articles. We have also now launched our infrared frequency lookup table, which enables you, if you have got a peak and you do not know what it is, to enter the peak position and get a suggestion on some of the possible assignments for that peak.

About Dr. Andrew Davies

Andrew studied for his master’s degree in chemistry at the University of Nottingham. He went on to study a Ph.D. under the supervision of Prof. Michael George investigating ultrafast time-resolved spectroscopy of heterogeneous catalytic reactions, and FTIR spectroscopy of gas uptake by metal-organic frameworks at cryogenic temperatures. He then worked as a postdoctoral research fellow in the Physics Department at the University of Nottingham where he used Raman spectroscopy to characterize Graphene/h-BN heterostructures grown using molecular beam epitaxy. Since 2018 he has applied his skills as the Applications Scientist at Specac helping to develop the next generation of spectroscopic accessories and providing guidance to its customers, helping them get the most out of their science.

 

 


Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.

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