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NEWS & REVIEW

Editorial

1. Oh no! I hear you say - not ANOTHER journal! Well - the INTERNET JOURNAL OF VIBRATIONAL SPECTROSCOPY is unique - a periodical initially planned to appear six times a year, aimed to assist you, the user of vibrational methods, hopefully full of useful information and discussion - and FREE.

Geoff Dent and myself perceived some time ago that many people who use vibrational spectroscopy, frequently and on a routine basis, are equipped with very little background information. At university or college they may have had very little formal instruction in infrared spectroscopy, probably even less on near infrared and Raman, and have had to pick up bits and pieces as they go along.

When I was a student in the 50's, infrared and Raman were hot issues and we were taught about them thoroughly, but since then the centre of interest has moved to NMR or mass spectrometry so that now we teach little or nothing about infrared or Raman at the undergraduate level and haven't done so for a long time. The IJVS is aimed at helping to fill the gap.

Each edition will carry three or four feature articles of graded sophistication. The first two are fairly basic and cover relatively routine but crucially important matters such as sampling or sample preparation, good laboratory practice, spectral interpretation, or basic data processing. The remaining articles are more specialised and most readers will find that they contain information new to them.

In addition to news items, we then include two unusual offerings - The Spectroscopists Bookshelf - suggestions of books to have handy or ones containing really useful information, and the other we call Hot Sources. How often has someone asked you to recommend a reference which explains the infrared spectrum of this or the vibrational behaviour of that? Well, Hot Sources will be the answer as the database builds up and it will be simply and reliably machine searchable.

To complete each edition we then offer contributed articles, but these are not like those in any other journal. The idea is to offer papers with real novelty on subjects with a reasonably broad interest, carefully written and edited to make them easy to read. We want these articles to describe a development and to provide routes to more detailed information for those who are interested - sort of micro reviews. You won't find a contribution on "Fermi Resonance as a possible explanation for the multiplet structure...", but you may well find something on new methods of sampling or the quantitative analysis of an important pharmaceutical. Much of the material in which we are interested will have appeared, or is about to appear, elsewhere; the Editorial team will encourage authors to offer an appropriately revamped, snappier version for us.

One of the advantages of Internet publication is speed. We plan to process manuscripts exclusively by electronic means - e-mail or FAX - and will avoid snail mail.

It is almost inevitable that there will be hitches in transmission, or problems with the quality of reception and downloading. You will also have comments about the journal itself or want to raise an issue for discussion.

Please give us a shout - send an e-mail or FAX - even telephone. We really MUST hear from you, the readers, and we promise to respond however rude you are!

So - please print the Journal and read it at your leisure. We hope you will find it useful and easy to read.

PATRICK HENDRA.

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Feature Article



2. Preparation of Samples for IR Spectroscopy as KBr Disks

Geoffrey Dent

ZENECA Specialties, P.O.Box 42, Hexagon House, Blackley, Manchester M9 8ZS
E-Mail Geoffrey.Dent@UKBLA71.ZENECA.COM


Powders, being examined by Infrared Spectroscopy, in transmission, are generally prepared by mulling in liquid paraffin (Nujol), or by grinding with potassium bromide (KBr) powder. The latter is then pressed into a disk. The method of preparation of a powder sample is generally determined by the information required or the chemical/physical stability of the sample. If information on the physical state, e.g. polymorphism, is required then grinding may change the state and mulling is preferable. Some substances, such as base hydrochloride, may exchange halogen with potassium bromide powder, again mulling is preferable. However most mulling agents contain bands in the spectrum which may mask bands in the sample spectrum. KBr does not contain bands in the mid-IR region of the spectrum, and therefore preparation as halide disks potentially loses less information. Samples dispersed in halide powder must be homogenously dispersed, with a particle size small enough not to cause scatter (theorectically < 2 microns). The strength of an IR absorption spectrum is dependant on the number of molecules in the beam. With a KBr disk the strength will be dependant on the amount and homogeneity of the sample dispersed in the KBr powder. The amounts stated below are for guidance only, the bulk density of the sample or other diluents may require these to be varied. They will also have to be varied according to the diameter of the disk required. The weights quoted are for a 16 mm diameter disk. Approximately half should be used for the 13 mm diameter disks.


Recommended Method

1. Transfer weighed amounts of sample, approx 2 mg, and KBr powder, approx. 300 mg into an agate mortar. The KBr powder must be of spectroscopic grade purity, and be spectroscopically dry.
   
   

2. Grind the powders together, with an agate pestle, until the sample is well dispersed and the mixture has the consistency of fine flour. With some very hard or crystalline powders this may not be possible by hand. If necessary, use mechanical or low temperature (liquid nitrogen cooled) grinding accessories.
   
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   Figure 1, KBr die assembly
   
   

3. Assemble the die, with the lower pellet polished face up.
   
   
4. Transfer the ground mixture into the cylinder bore so that it is evenly distributed across the polished face of the lower pellet. Gently inserting the plunger and lightly swivelling can often achieve a flat, even surface
   
   
5. Insert the second pellet, polished face towards the mixture, into the bore followed by the plunger.
   
   
6. Place the die assembly into a hydraulic press, between the ram and the piston.
   
   
7. Ensure that the die is firmly held in the press.
   
   
8. Connect a vacuum tube and switch on a High Vacuum pump.
   
   
9. Leave the die assembly under vacuum for approximately 2 mins. This removes air from the disk.. (Some spectroscopists claim loose water is removed from the KBr and/or solvent from the sample. Others dispute this. Either way good vacuum leads to good disks)
   
   
10. Increase pressure in the press to 15 tons (10 tons for 13 mm die, follow manufacturers instructions for max pressure with other diameter dies).
    
    
11. After approximately 1 minute, slowly release the pressure.
    
    
12. Carefully release the vacuum, and remove the die from the press.
    
    
13. Dismantle the die, and transfer the KBr disk to a spectrometer disk holder. Avoid touching the faces of the disk.
    
    
14. Check that the disk is translucent and that the sample is homogeneously distributed in the disk.
    
    
15. Mount the disk holder in the spectrometer.
    

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The spectrum quality is affected by the quality of the disk. The flatness of the baseline is dependent on the particle size and dispersion of the sample in the KBr powder. Check the disk and spectrum for the following faults :-

If the disk breaks on removal from the die, this indicates that the disk is too thin caused by too little powder, or too much pressure for too long. Remedy this effect by increasing the sample load. Also check that the correct pressure is used.

If the disk is not translucent, this can have numerous causes.

1. Uneven distribution of powder in die
   
2. Too much sample
   
3. Too much KBr powder
   
4. Poorly dispersed sample
   
5. Water in disk
   
6. Pressed at too low pressure or for too short a time
   

All except the last fault can be remedied by re-grinding and pressing with adjusted amounts. Water in the disk can be present due to a wet sample or wet KBr powder. Small amounts can be removed by the High Vacuum step (9). Heating the disk at approx 100°C for a few minutes and repressing the disk will sometimes remove residual water.

The disk turns brown. This could be due to the sample being an oxidising agent. Check the spectrum for halide degradation and re-examine as a mull if possible.

Truncated Bands. If the spectrum contains bands which have a flattened turning point and do not reach 0 % T, this is caused by a poorly dispersed sample or holes in the disk. Check the disk visually and if necessary repeat the preparation.

Sloping baseline. This is usually due to a poorly dispersed sample. Some substances are too hard (polymers) or too crystalline (e.g. Anthraquinone) to disperse properly. The latter can also cause bands to appear like a first derivative spectrum. This is due to refractive index changes and is known as the Christiansen Effect

The faults listed above have been commonly found , sometimes in publications, but is not necessarily exhaustive. Use of other halide salts can overcome some effects, or extend the range of the spectrum examined, e.g. CsI extends the lower wavenumber range from 400 to 200 cm ^-1. The use of these will be discussed in a later Edition. Poor sample preparation can lead to avoidable errors in interpretation of the resultant spectrum. A little care can avoid the need for repititous sample preparartion or embarrassing errors in results.

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Spectra




Figure 2, Phenyl Acetic Acid (i) Too Weak (ii) Too Strong. [High-Resolution Spectra]




Figure 3, Correctly Prepared KBr Disk of Phenyl Acetic Acid. [High-Resolution Spectrum]

Editors Note: If you regularly use KBr disks, try Geoff's instuction carefully and check that you can reproduce Fig 3, then use the method you normally use and compare. Might be quite a surprise. The background theory to the article is now covered by Bill Maddams.

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REF: Int. J. Vib. Spect., [www.ijvs.com] 1, 1, 2 (1996)



Feature Article



3. The Background to Sample Preparation for Infrared Transmission Measurements on Solids

William Maddams

University of Southampton
Until recently Chief Vibrational Spectroscopist, BP Research, Sunbury on Thames, UK.


Measurement of the transmission infrared spectra of liquids poses no great problem from the sampling viewpoint. These materials may be squeezed between polished plates of sodium chloride, potassium bromide, or another suitably transparent material until absorption bands of measurable intensity are obtained. The thickness of such films is usually a few microns. Alternatively, if information of a quantitative or semi-quantitative nature is required the sample is made up in solution, at 5 or 10% concentration, in a solvent such as cyclohexane, carbon tetrachloride, chloroform or carbon disulphide and examined in a sealed cell whose path length is, typically, 100. Solids may also be examined in solution, given that a reasonably transparent infrared solvent can be found. It is also possible in some cases to cast thin films, a technique that is somewhat time consuming but is useful for some polymers.

However, when these approaches are not applicable because of sample insolubility and it is necessary to examine samples in powder form, problems arise for three reasons. Two of these relate to the particle size of the solid, in quite different contexts, and the third to the effect of sample refractive index. It is useful to have a basic understanding of why these problems occur and how their effects may be minimised in order to obtain good quality transmission spectra from solid samples.


1. Particle Size and Optimum Packing

It is a simple process to squeeze a liquid, however viscous, between optical flats and hence to make an acceptable thin layer. In contrast, the preparation of a continuous, homogeneous layer from a powdered solid poses considerable difficulties, not merely because the squeezing process is ineffective.

The problem may be understood by considering two model systems where the particles consist of (i) regular spheres and (ii) regular cubes. Fig.1a shows a cross-section of a group of regularly packed spheres of equal diameter. It is clear that there is considerable space between the spheres. Hence, if a collection of spherical particles of equal size is inserted into the beam of a spectrometer, a significant proportion of the radiation will not traverse the sample. If, as shown in Fig.1b, there is more than one layer of particles, it is possible for these to be arranged so that some of the gaps are closed. Alternatively, if the spherical particles cover a range of diameters, there is even less free space between them, as shown in Fig.1c. Again, multiple layers reduce still further the free space. However, the multilayer option is limited in practice; unless the particle size is below about one micron it will not be possible to achieve the optimum sample thickness we need - a few microns.

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In the case of cubic particles, these may pack wholly regularly if they are of equal size, as shown in Fig.2a, and there is then no free space. However, if the packing is irregular as in Fig.2b, there will be some and this is likely to be rather greater if the cubic particles are not of equal sizes.




In practice, real particles are likely to be of irregular and varying size and shape and the presence of some free space is almost inevitable. What are the consequences?

Suppose that the free space amounts to 10% of the area over the cross-section of the spectrometer beam at the point where it traverses the sample. Then, if the sample is absorbing all of the radiation falling upon it, the measured absorbance will be log₁₀ 100/0+10, i.e. 1.0 instead of log₁₀ 100/0, i.e. 00. Similarly, if the sample absorbs 90% of the incident radiation the measured absorbance will be log₁₀ 100/10+10 = 0.70, rather than log₁₀ 100/10 = 1.0.

For a sample absorbing 60% of the incident radiation, the measured absorbance will be log₁₀ 100/40+10, i.e. 0.30 instead of the correct value of log₁₀ 100/40, i.e. 0.40. So the spectrometer will always give absorbance values that are too low and the error will be greater the greater the absorbance being measured.

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2. Particle Size and the Reflection and Scattering of Radiation

If we consider a beam of light moving in a particular direction as a travelling sinusoidal wave, we would expect secondary wavelets to radiate from all points along that line. This does not happen in empty space because, as Fresnel showed many many years ago, there is destructive interference of the secondary wavelets that arise at all points traversed by the beam. However, in practice, secondary wavelets are generated at disturbances such as when the beam encounters a discontinuity of any sort, and these do not interfere destructively. A fraction of the light is then scattered over all angles around the beam and this scatter is dependent upon the size of the discontinuity, the wavelength of the light and the refractive index of the medium through which it is passing.

In the present context the discontinuity is the particle of our solid sample upon which the beam of infrared radiation falls and it is convenient to consider the matter in somewhat more detail. For practical purposes we will look at three groups of particle sizes. They are: particles of a size appreciably greater than the wavelength of the radiation, particles whose dimensions are comparable with , and particles appreciably smaller than .

(a) Particle size greater than

Imagine a cubic crystal, whose dimensions are appreciably greater than , sited so that one of its faces is perpendicular to the beam of radiation passing through the spectrometer. Most of the radiation will pass into the crystal, some will be absorbed, and the remainder transmitted. However, a fraction of it will be reflected from the surface of the crystal and be lost. The size of this fraction is determined by the difference in refractive index between the crystal and the surrounding medium (which, in the case of air, is a very close approximation to 1.0). If the crystal has a refractive index of 1.5, a figure fairly representative of many organic compounds, the loss amounts to 4%. For diamond, which has a refractive index of 2.4, the value rises to 17% and this is responsible for the glitter and sparkle. The overall reflectivity of a girl's best friend is enhanced by cutting the diamond; the various facets reflect light at a range of angles and enhance the lustre and sparkle. This multiple reflectivity also occurs with a powder consisting of the cubic crystals considered above. Their faces will tend to be disposed randomly with respect to the beam of radiation. Since the reflection will take place specularly, the reflected radiation will leave the crystalline sample over a wide range of angles. There will be a substantial loss by scattering and this will occur for all types of crystals, irrespective of their symmetry. This reflection/scattering loss occurs for all particle sizes until they approach the wavelength of the radiation incident upon them. However, the smaller the refractive index difference between the particles and the surrounding medium, the smaller will be the radiation loss.

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(b) Particle size comparable with

The scattering process is more complex in this instance because diffraction effects also occur, and the intensity of scattering is considerably larger. Rather than struggle with the theory, let me explain a demonstration of the effect where the particle size is comparable to visible light wavelengths.

One of the most convincing of these involves small particles of sulphur. If a very dilute solution of sodium thiosulphate is acidified with a few drops of dilute sulphuric acid, the sulphur that is formed in the ensuing chemical reaction precipitates very slowly as small particles that gradually increase in size. When they reach about 0.4 to 0.5 in size, the reaction mixture appears blue, because of the strong scattering of blue-green light.

When the particles have almost doubled their size they scatter red light but rather less strongly, the latter because of the wavelength dependence factor in the equation for the scattering intensity.

It is clear therefore that it is highly desirable that solid samples for examination by infrared spectroscopy should not contain a significant proportion of particles whose sizes are comparable to the wavelength range of the spectrum to be measured. In analytical infrared spectroscopy the wavelength range is huge, 2.75-25 microns, so we have to be careful. Fortunately, there is normally no great difficulty in reducing the average particle size below 2.75 so that, on this count, the problem is much reduced. There is, however, another factor which produces scattering when the particle size is below and this will be considered below.

(c) Particle size smaller than

This type of scattering was studied in detail, both theoretically and experimentally, by Lord Rayleigh a century and a quarter ago. He showed that the scattering intensity is proportional to the sixth power of the particle diameter and inversely proportional to the fourth power of the wavelength (this latter also being the case for Raman scattering). These two terms tend to operate in opposite directions but, in practice, the diameter term is much the more important because the lower limit on is set by the wavelengths of the various vibrational frequencies we want to measure. There is also a practical limit to the fineness that may be achieved by intensive grinding; in many cases once the particle size falls appreciably below about 1 the particles start to stick together as the result of surface forces and behave optically as rather larger ones.

Rayleigh type scattering, as it is usually called, can occur with small molecules. Lord Rayleigh showed that the blue colour of the sky is the result of scattering by the oxygen and nitrogen molecules of the air. Although the scattering intensity from an individual molecule is very small, the thickness of the earth's atmosphere leads to a colouration readily detectable by the human eye. All transparent dust-free liquids show Rayleigh scattering. Logically, the scattering intensity for a liquid must be much stronger than for a gas. After all, there are about 1000 times more molecules per unit volume in a liquid than there are in a gas at one atmosphere pressure! In practice the intensity comparison is not 1000:1 but much less, at around Intensity liquid/Intensity gas = 50.

This apparent anomaly is nothing of the sort, since it is in agreement with theoretical predictions. Liquids are much closer to continuous media than gases so, as we saw above, Fresnel would point out that the secondary wavelets in liquids will tend to destructively interfere. Now, if our solid were to be surrounded by a liquid of the same refractive index, it too would approach continuity and as a result the scattering would be much less than it would be from a dry powder. That this is so is easily demonstrated. If a glass rod is dipped into Canada balsam or oil of cedar, it becomes practically invisible (The lads who repair stone damage to car windscreens use this principle). We exploit this effect in preparing solids for infrared analysis - grind the powder to a particle size smaller than the shortest wavelength we plan to use and immerse the analyte in a fluid of matching index of refraction.

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3. Practical Methods

So, we plan to coat the particles of a solid sample, preferably reduced in size to about 1, with a transparent liquid of comparable refractive index. How may this be done in practice? Apart from meeting the refractive index criterion, the liquid of choice must be as transparent as possible over the range 2.75-25. Inevitably, compromise is necessary, as is the case with solvents used for solution studies. Paraffinic hydrocarbons provide one such compromise, although they absorb rather strongly at the frequencies specific for the various types of C-H vibrations.

A high molecular weight paraffinic hydrocarbon is desirable because it will have a low volatility, a higher refractive index than the lighter paraffins, and a usefully high viscosity, thereby aiding the coating process. Medicinal liquid paraffin has been used for this purpose since the early days of analytical infrared, and the technique is universally known as the Nujol mull. Why Nujol? Many years ago a proprietary brand of constipation curer named NUJOL was widely available in pharmacies. It transpired that NUJOL was the best and most reliable source of liquid paraffin - as simple as that. Some skill and experience are required to produce good mulls consistently. It is not much of a problem to get the correct particle size of the solid but it is tricky to get the right proportions of nujol and solid for a stiff, homogeneous paste, without excess liquid. Nevertheless, this approach has been used with great success for more than half a century.

Since the wetting fluid has its own spectrum there is an interference problem. It is tempting to use the computer to subtract the spectrum of the nujol from that of the mull and a lot of people do just this, but it is really bad practice. Since your subtraction can never be perfect, you will never be certain that what you are left with is meaningful. You MUST make a mull with a material that does not interfere and then merge the data. In Fig.3 you will see the infrared spectra of nujol and an alternative mulling fluid, in this case hexachlorobutadine. Now, hexachlorobutadine H.B. is a relatively oily material of high index which has an intense infrared spectrum of its own but it contains only C-C and C-CL bonds. As a result, it does not absorb between 1250 and 1500 cm-1 or above 2000 cm-1 so it does not interfere where Nujol does. The problem with H.B. is that it is poisonous, so these days fluorocarbon oils have become popular.

Predictably, mulls provide an essentially qualitative approach. Nevertheless, they have been used to a limited degree for quantitative work, by incorporating an internal standard such as potassium thiocyanate, and utilising the characteristic peak in the vicinity of 2050 cm-1 as an intensity marker. There are problems associated with the reproducible grinding of both solids and the subsequent mixing but, with care, successful analyses are possible.

Liquids provide a reasonable approach to a continuous medium but some solids are even more perfect in this respect, and good quality of crystals such as quartz scatter less light than transparent liquids. In these crystals the molecules are arranged in a regular manner and this condition approaches that of a continuous medium from a mathematical standpoint. Hence, the destruction of secondary wavelets, as envisaged by Fresnel, is virtually complete. However, is it possible to disperse fine particulate samples uniformly in a homogeneous optically transparent solid? The answer is of course yes and it proves to be easier than might be envisaged.

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The alkali halides, such as sodium chloride and potassium bromide, all have simple cubic structures. Cleavage along planes parallel to the crystal faces (and possibly along diagonal faces) should, in principle, be possible by the application of force. The ease of cleavage is determined by the surface energy of the crystal, and this may be calculated theoretically. The values so obtained are rather high but in practice it is found that the cleavage energies are two orders of magnitude smaller than expected. This considerable discrepancy probably arises from lattice imperfections, such as tiny surface cracks, which initiate cleavage. However, what is important for our purpose is that conventional laboratory grinding methods reduce the alkali halides to microcrystalline particles.

Because the cohesive energy is relatively low such powders, if subjected to a pressure readily attainable in a modest laboratory press, reform into clear homogeneous solids, a process often referred to as "cold sintering". The pressure required for the process is lowest for potassium bromide, followed by potassium chloride. If, therefore, an appropriate concentration of the solid whose spectrum is required, with a particle size of about 1, is dispersed into potassium bromide powder and pressed, a clear transparent disc is obtained, ready for insertion into the spectrometer. Since the alkali halides transmit infrared well except at long wavelengths, we do not have the interference problem typical of mulls, but remember - the particles of solid must be as fine as they are in a mull and the concentration of the analyte and its thickness must be just right to record a good spectrum.

The practical details of this process have been discussed above in depth by Dr. G. Dent and do not require amplification. Two points of detail do merit comment. The thickness of the alkali halide disc is set primarily by its mechanical strength, and the weight of potassium bromide powder for a 13mm die recommended by Dr. Dent will ensure that discs do not fracture during handling. With this thickness of disc, a sample concentration in the range 0.5% to 1.0% usually gives peaks of optimum intensity. It may be noted, in passing, that discs may also be prepared using caesium halides, particularly caesium iodide. This is transparent as far as 180cm-1 , whereas the cut-off for KBr is about 300cm-1. Discs may also be prepared from very finely powdered polyethylene, and these extend the working range still further into the far infrared region.

Dr. Dent refers, in passing, to the distorted bandshapes that may occur because of what is known as the Christiansen effect, and it is useful to briefly consider the background to this.

In the case of a material such as potassium bromide, which is transparent over the mid-infrared range of, say, 2.5 to 25, the refractive index decreases slowly with increasing wavelength. However, the situation is very different with a material having vibrational frequencies in this range. As an absorption band is approached from the high frequency side, the refractive index falls. Moving to the low frequency side of the vibrational frequency, the refractive index is anomalously high and then drops back to the value expected in the absence of an absorption. It may be noted that this effect also occurs strongly with electronic absorption bands and it was in this field that Christiansen did his pioneering work. This change in the refractive index distorts the shape of the absorption peaks in infrared spectra. In some cases the distortion is so marked that the short wavelength side of the peak may appear to be negative, giving it the appearance of a first derivative spectrum. This effect disappears when the particle size is smaller than ; hence, it should not appear if the sample preparation has been done properly. Some hard materials may be difficult to grind down to the required average particle size but fortunately they are in the minority.

One final point; the Christiansen effect applies in both alkali halide discs and in mulls.

A useful source on this subject is "Laboratory methods in Vibrational Spectroscopy", Ed. Willis, Van der Maas and Miller.

REF: Int. J. Vib. Spect., [www.ijvs.com] 1, 1, 3 (1996)

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Feature Article



4. Sampling for FT Raman Spectrometry

Patrick Hendra


In a real sense there is hardly any need to write this article, as FT Raman must be just about the most versatile and trivially easy non-destructive analytical procedure ever developed - you can get a spectrum from a screwdriver handle, a pill, a bottle of scotch (unopened) or a lump of cheese, with equal ease. In no case does one need to prepare a sample, just hold it firmly in the machine. Well, this is all very true, but to obtain best spectra there is a little more to it than brute force and ignorance.

All FTs have several features in common - they all illuminate the sample with near i.r. radiation from a Nd³⁺:YAG laser operating at 1.064. They all collect the scattered light in the reverse direction to illumination - so-called Back Scatter - and they all process the light with a modified F.T.I.R. instrument. The arrangement is shown below:




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Perhaps the only significant difference between the various competing products is that most collect the light with a lens (or lenses) but some, e.g. Nicolet, use an ellipsoidal mirror. Either way, scatter over a wide angle is collected and passed into the interferometer.

Because the latter has a round hole as its entrance aperture (the so-called Jacquinot Stop), the instrument looks at a relatively large round patch of the sample and it is into this patch that the instrument maker focuses the laser.




The viewed VOLUME allowing for the depth of focus of the laser is of dimensions a cylinder 0.5-1mm in diameter and height around 2mm. Any sample placed inside this space will produce a spectrum.




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All the commercial instruments have interlocked lids so that the laser radiation, which is invisible anyway, cannot be damaging to the eyes. How do you "find" the sampled volume if the sample holding system is misaligned? Most manufacturers provide some sort of jig, but these tend to be unnecessarily cumbersome. It is a fact that Nd³⁺:YAG lasers melt black PVC tape, so the trick is to stick a layer of ordinary insulating tape over the sample holder and close the lid. Re-open it and - hey presto! - a tiny hole will be seen where the laser hit the tape.

A liquid can be viewed by containing it in a bottle, vertical tube, ampoule or capillary made of glass or quartz. Since the viewed cylinder has length, the thinner tubes may not be very efficient if viewed normal to their axis.

Clearly, better results would be obtained by viewing down the cylindrical axis but a flat window is then essential at the spectrometer end - all a bit tricky. There are a couple of ruses worth trying if a tube, small bottle or capillary are used. If you place a piece of clean aluminium foil behind the sample, the laser tends to be reflected back into the sample. Similarly, the scattered light leaving the sample away from the spectrometer is not then wasted but rather projected forward into the instrument. A much better technique is to silver the reverse outer surface of the bottle or tube. This is really easy and many recipes for silvering solutions exist. To protect the silver coat we use typists' correction fluid. Since the silver will then not rub off these, sample bottles/tubes can be used again and again.

Perkin-Elmer have taken this reflection idea further by using either a spherical reflector, inside which lies the sample enclosed in a tiny spherical flask 5mm in diameter, or in a capillary. Just as good results can be obtained on any machine by blowing a small bulb on the end of a boroscilicate tube and silvering the outer surface over about half of its surface.




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Solids

Lumps - not much you can do with these but beware focusing the laser too deeply inside the bulk. Best results seem to come if the viewed position is at or just beneath the surface. If the sample is glass clear, then move the sample forward and treat more like a liquid.

Powders - the normal procedure is to sample these in a thin glass tube (e.g. a melting point or NMR tube) but we prefer to use a self-supporting compressed pad. In fact, this is the normal procedure at Southampton.

Various cells have been devised




Our standard routine cells are home made 3mm (1/8") in diameter but we have made them down to only 0.6mm in diameter. The quality of the spectrum is not hole size dependent but the fine ones are difficult to fill and tricky to align. One word of caution - for years we made these cells from 1/2" brass rod because it is easy to drill and relatively corrosion resistant. It seems this was a bad choice - the thin coating of ZnO fluoresces like crazy, so if you miss the centre of the sample you can get a mysterious background. Better to ask your workshop to use aluminium alloy and have it anodized. The anodized surface is almost completely chemically resistant and very hard - ideal.

To grind or not to grind is indeed the question. Some powder specimens give excellent spectra, whatever their state, others improve if ground. The answer to the question is complex but in many but not all cases a bit of grinding can help. We do not actually grind the specimen as such, just move the specimen around as we compress the pad and/or rotate the pressure rod.

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Films

A relatively thin, free standing film such as a piece of wrapping material can often give a decent spectrum if it is rolled up and the roll is viewed on axis. The point is that the instrument looks into the sample in some depth. An alternative is to stack layers of the films. An easy and economical way to do this is to use a commercial paper punch (as used for preparing sheets for ring binders) and ask your workshop to make a simple metal holder. Making the bottom disc out of aluminium foil is a good idea.




What of films on surfaces? Not a strong point for F-T instruments. We have had luck with 2 thick films on reflecting surfaces but if a thin film overlies a dark or rough surface, you will have trouble unless you use a microscope (We will feature infrared and Raman microscopy in the very near future).

There are two distinct problems with F-T Raman operating in back scatter. Both arise because the patch illuminated by the laser is relatively small (compared with the area sampled in an infrared absorption or reflection experiment). The first is burning and the second inhomogeneity, causing the signal strength to vary with position.

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Burning

All samples absorb the radiation incident upon them and, as a result, the sample heats. If a laser is focused onto the surface of a solid the brightness of illumination can be very high indeed (3000W cm^-2 are ordinary in FT Raman machines). If the sample has restricted thermal conductivity - not at all unusual in powders or polymers, a really significant temperature rise can occur. If heating causes darkening, the consequent heating effect can run away and the sample burns. Obviously, the brightness of illumination must be minimised and the thermal conductivity kept as high as possible. Since, as we have seen, the illuminated area must match the viewed, there is little opportunity to solve the heating problem by fiddling around with the illumination. The only real way to minimise the risk of damage is to keep the laser power down and accept poor signal intensity.

How can you check whether you have a problem? Obviously, if no Raman signal appears and there is a black patch on the sample, you have a problem. There are a few tests you can do. Try lowering the laser power dramatically (say to 50 mW) and attempt to run a spectrum. You may see evidence that the background is rising as you co-add the spectra. If your instrument has a "monitoring or fast low resolution mode", set up the sample, switch on the laser, and watch the result. The point here is that as the sample heats up, it will emit as a black body (hopefully, not literally!) and you will see the emission as a rising background at high shifts. A typical appearance would be




If the sample behaves itself at 50 mW, increase to, say, 100 mW and try again. Emission at high shifts will be evident if the sampled point reaches 120-130°C or higher.

So - how much heating does occur at the surface of a typical sample? It is very hard to measure but Dr. Yvonne West here at Southampton is something of an expert and offers a paper in the Contributed Articles section of this edition which you will certainly find useful.

Above, I pointed out that an alternative way to solve the heating problem is to keep the laser power normal but increase the thermal conductivity to allow the heat produced to diffuse away from the illuminated volume. Dr. Geoff Dent of Zeneca has come up with an excellent idea on this one and Dr. West's article in the Contributed Articles section covers the subject.

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Inhomogeneity

Simple consideration of this problem would lead you to the conclusion that two approaches might be worthwhile - view a larger patch of sample and/or move the sample with respect to the viewed patch so that can occur. There are snags to both solutions. If the optics are altered so that the viewed area is magnified, the solid angle collected by the instrument must be reduced (the f number at the sample will rise). So, the intensity of light collected per unit laser power will fall. Of course, one could increase the laser power to compensate for the fall but in a sense you are then asking for trouble through heating. There is a mitigating factor -




As the diameter 2r of the viewed patch is increased, its area rises of course as r². As a result, looking at a large area and defocusing the laser before increasing its power can achieve the results we need whilst keeping the brightness of illumination under control.

This is all very well, I hear you say - but does it work? Yes is the answer.

Using a special lens in front of the normal one on a P-E2000R we can achieve around 60% of the signal strength normally achieved and the sample point is moved by about 25mm.




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The illuminated and viewed patch is, of course, increased in size (to around 0.5mm diameter).

As an alternative, how about moving the sample under the beam? You might think that doing this would generate noise in the interferogram which the FT processor would translate to a background. It does, but well outside the audio bandwidth typical of spectra in the 1 - 1.7 region if you move the sample slowly.

Rotation of an NMR tube of powder at around 60 rpm is fine and this is frequently used on Nicolet machines, but it would not be a good idea to spin the sample at, for instance, 2000 rpm.




To conclude: F-T Raman is such a simple routine technique that you can get spectra with almost no sampling. On the other hand, it pays to try a few experimental tricks if you want the best results.

Next time, I will discuss heating and cooling, whilst in the future we will highlight infrared and Raman microscopy.

At several places I have described special sample holders, lenses, rotators or other sepcialised pieces of equipment. I am happy to provide full details if you send me a fax on +44-1962-776-390 or email me on ijvs@soton.ac.uk

REF: Int. J. Vib. Spect., [www.ijvs.com] 1, 1, 4 (1996)

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