1. Editorial This edition is a lot thinner than the last one and definitely less sophisticated. Many readers use IR to analyse wide ranges of materials often by searching libraries or perhaps more frequently relying on group frequency correlations. Presenting liquids for transmission analysis is pretty nearly impossible to get wrong - a drop of analyte between a couple of KBr windows but solids are more of a problem. In our first ever edition Geoff Dent told us all how to make a decent mull - in this edition we do the same for liquid paraffin mulls. Some of you only use KBr disks and perhaps have never made a mull - this is a mistake. Equally folk like me who never use disks are missing out as well. So I hope you will have a look at our article.My co-author Fabrice Birembaut from Southampton University did all the work - of course! As many of you will know, F-T Raman is now an accessory offered by instrument companies to fit on their FTIR benches. The method can be really valuable as an analytical tool particularly if the two complimentary spectra - infrared and Raman are recorded and used together. Just as infrared has its sampling problems so to a limited extent does Raman. In FTR there is a tendency to heat the sample with the laser source - a problem which can easily be tamed. In the second article if this edition I have run through the various ruses available to F-T Raman users. We have as usual some contributed offerings…… - Rapid Quantitative Analysis of Organophosphorous Pesticide Formulations by FT Raman spectroscopy - Constantinos Georgious - Conformational and vibrational analysis of N-3 Pyridinythmethanesulfonamide - Nicolay Dodoff Over the last few months several people have talked to me about catalysis and the value of IR and Raman in following reactions. I find that in this very specialised area, few people are real experts but the vast majority is bewildered by the experimental complexities. So - for the next edition, I promise a micro review based on my experience. Turning now to more bureaucratic matters - the readership continues to
grow, but your literary effort declines! We must have more copy from you. PhD
students - all of you will write an introductory chapter as part of your thesis
surveying the field in which you are doing your research. Many if not most supervisors ask
you to write this chapter first so that you can collect the history, survey the literature
and collect your ideas. Why don't you ask your supervisor if you can offer a shortened
version to IJVS? It certainly wouldn't do your CV any harm especially if you persuade your
supervisor to let you do it on your own. You folks are young - your fellow readers are
young - get it? Patrick Hendra The decision by we, the scientific community is rather odd. English is full of illogicalities, but to replace some by others seems to me to be crazy. Thus, SULPHUR becomes SULFUR, but PHOSPHORUS is not to change. Why not FOSFORUS? Because of Fluorine perhaps? Why do we not drop the use of sodium and use the German useage Natrium? And what about Aluminium? This unique element enjoys the mis-spelling Aluminum in the US whereas all other elements ending in "UM" keep their "I" throughout the whole World! No - I know it's totally arrogant, but we the English will define how our language is to be used, just as the French define their beautiful tongue, the Spanish theirs etc, etc,. If the colonists could not spell and spoke lousy English, they should have been corrected years ago. They were not and it is far too late now. Thus there are two related languages - ENGLISH and AMERICAN. Let us leave it at that. Perhaps Sir Winston Churchill hit the nail on it's tip, when he commented that Britain and America were two countries separated by the same language!
In 1985 a fund was established as a memorial to Gordon Kirkbright in recognition of his contributions to analytical spectroscopy and to science in general. The fund is administered by the Committee of the Association of British Spectroscopists (ABS) and by the ABS Trust. The award enables promising young scientists of any nation to attend a recognised scientific meeting or visit a place of learning. Applications are invited for 2001 Gordon Kirkbright Bursaries. The award is not
restricted to spectroscopists.
The Editor Recently, Bowie, Chase & Griffiths [1] have contributed feature articles in Applied Spectroscopy describing sampling in Raman Spectroscopy. The articles are to be recommended because they survey in a most readable form, the advantages and disadvantages associated with Raman measurements. Quite correctly, Chase & Griffiths point out that one of the snags with Raman spectroscopy and particularly in the F-T technique is sample heating, but they offer very little information about controlling it. So I thought I would help! F-T Raman spectroscopy has made a real impact on analytical chemistry. The method relies on illuminating the sample with a near infrared laser and processing the scattered light on a good quality FTIR bench. Most of the leading FTIR manufacturers offer a 'Raman Accessory' and sales have been brisk for at least 10 years. With a Raman set-up on an FTIR you can record the two IR and Raman complimentary spectra on your sample. Raman has, in the past, failed to gain widespread acceptance within the analytical community because sample colour and fluorescence have made it almost impossible to use it as a ROUTINE tool. Infrared is (or rather was) much more versatile in that the vast majority of a 'basket' of samples will give an infrared spectrum of sorts. In Raman, this was definitely not the case. When Chase demonstrated that NIR/FTR was feasible back in the mid 1980's [2], this restriction was largely removed - probably 85% of a random bunch of samples will give an F-T Raman spectrum. However, one major cause of failure is sample heating. In F-T Raman, laser powers of ½w are typical. The laser is focussed
into a patch on the sample around 0.5mm in diameter hence the brightness of illumination
is Strictly, the brightness and any absorption are not in themselves a problem - the absorption of the laser is an energy term, which becomes serious when multiplied by time. Put another way, the sample heats at a rate dependent on the brightness of illumination and the absorption coefficient at the laser wavelength [1.064µ]. As it turns out, a huge number of samples do indeed have a very small but significant coefficient near 1 µ (particularly samples that are wet, have water of crystallisation, contain OH and or NH groups), so some degree of heating is very widespread. In most cases, this causes little or no problem, but it can be a headache in a low melting material. An example is shown in Figure 1, the sample has melted but the analyst wanted the spectrum of the solid!
Quite obviously, the rate of heating and of course, heat
loss vary from sample to sample and due to differences in design will vary from instrument
to instrument but in my experience a 5K rise in temperature is routine and in heavy
absorbers several hundreds of Kelvin definitely occur.
Let us consider each in turn. Heat dissipation The principle here is simple - dilute the sample with an efficient conductor of heat and provide a pathway for the energy to dissipate and hey presto - the problem is solved - maybe! The alkali halides have thermal conductivities as high as metals and so two Raman solutions have been reported. 1) use a near infrared type KBr disk and 2) try a high concentration liquid paraffin mull squeezed between KBr flats. The KBr disks recommended by reporters tend to be thicker and more concentrated than for IR absorption measurements and similarly the thickness of a mull for Raman work is certainly greater than is typical of IR usage. Both methods work well and FT Raman spectra of graphite - and you can't get much blacker than that - have been reported. A good convenient source of further information will be found in IJVS [3]. An article about mulls follows this one. Quite obviously, this is attractive because with a little bit of forethought the IR and Raman spectra can be recorded on the same sample very easily. However, this method does have its limitations - use too much laser power and an unacceptable level of heating will still happen, alternatively wait forever for the results! The dark colour usual in these cases (vide graphite) reduces the optical efficiency of the overall process thus long scan times are inevitable. Sample cooling and cold cells If the sample is heated by the laser, one almost trivial way of controlling the problem is to cool it to cryogenic temperatures. The sample will then tolerate a rise in temperature of a couple of hundred K and still be far from overheated. A temperature rise of 150 or 200K is unlikely to precipitate a disaster. So we need a cryocell - what is available? This subject has been covered before in IJVS [4], but at the risk of
boring you, I will go over some of the material again. There are in effect, three types of
cryogenic cells - 'cold finger', streaming cold gas and 'transfer gas cells'. All are
drawn in Figure 2.
The cold finger design of cell requires that the sample is clamped to a refrigerated surface and is in excellent thermal contact with it (particularly if the whole ides is to draw away the laser energy to minimise heating). In electronics, thermal transmission is maximised by using a special paste squeezed between solid state devices and heat sinks but this solution is not usually applicable in Raman because the paste has a spectrum (and fluorescence) all its own. The sample and cold finger are encased in an evacuated zone - again not really appropriate for our purpose because continuous pumping or at least semi-continuous evacuation is required. So we move on to the streaming gas cells. As you will see in Figure 2b, the sample simply dangles in a flow of nitrogen boiling off the liquid. These systems are simple, work well but it is hard to precisely control the temperature of the sample. You can increase the temperature of reducing the flow rate and vice versa, but its all a bit hit or miss. The third alternative, is the 'transfer gas' cell. As you will see in Figure 2c a gentle and continuous flow transfers heat from the sample to the refrigerant. The latter need not involve boiling gases - solid CO2/acetone or even ice/NaCl are just fine. The sample holder can contain a tiny heater and thermocouple and these can be used to accurately control the temperature at a selected value. The sample sits in a cold gas cell, not a vacuum so thermal conduction of the laser energy away from the sample is maximised and also samples can be changed rapidly and easily (no vacuum to release and re-pump). There is a further advantage with the transfer gas cell appropriate only to F-T Raman. Manufacturers of our instruments almost invariably seem to think we don't need any space around the sample zone - "small is beautiful" must be their collective motto! Further, the standard arrangements of the sample chamber (which is always interlocked with the laser) make it advantageous to minimise the tubes and cables connecting the cell to the outside laboratory. Now, the cold finger cells need vacuum hoses, the streaming gas cells require a large and insulated gas transfer tube. Both systems are hard to accommodate in a small sample area with a closed lid. Transfer gas cells are made from glass, the Dewar vessels sealed and they require only thermocouple and fine heater cables to pass to an external power supply. An example of a simple transfer gas cell will be found in Figure 3. I could say more about these cells and describe more sophisticated designs which have recently appeared in IJVS, but I will leave the matter there. For new designs see IJVS [5].
Increasing the area of illumination Two approaches are possible here - increase the patch illuminated by the laser, thus lowering the brightness and/or move the sample under the laser beam so that a larger area is illuminated and time is given for the heat to dissipate. Let's consider the simple ruse of de-focussing the laser beam. There is a problem. The instrument "looks" at a patch on the sample, which is a virtual de-magnified image of the entrance Jacquinot stop. See Figure 4. To put in some fairly typical figures - at analytical levels of resolution (2-4cm-1) manufacturers use a 'J' stop size around 6mm. Most magnify the sample at the 'J' stop plane by about 6x so the instrument sees a patch only about 1mm in diameter at the sample. This is why the laser is focussed down to a spot around ½mm in diameter - it drops easily into the viewed area [BioRad focus least of the suppliers]. Enlargement of the illuminated patch is permissible but only within the viewed area. So, the opportunity to reduce burning this way is very restricted and in my personal opinion not worth the trouble. To do the job, you must change the manufacturer's laser focussing lens and then carefully align the illuminated and viewed patches. OK, if you are good at "Do-It-Yourself", but otherwise fraught with problems over manufacturer's warranty, servicing etc!
The alternative is sample movement (usually rotation). The idea here is
to illuminate the sample as it moves so that each part is heated by the laser for only a
very short instant and it then given time to cool down before it is heated again. Simple
rotation cells have been used for some years and Nicolet owners have bought considerable
numbers. An example is shown in Figure 5. To prove that the method works, you will see in
Figure 6, a spectrum of solid lidocaine. This very low melting material is a liquid under
a stationary beam (Figure1) but not if it is kept moving.
You don't have to rotate alone - Franz Langkilde at
Astra Zeneca, in Mölndal in Sweden, demonstrated the value of rotation and translation
occurring together and this too have been developed/. See Figure 7. Samples held in 5mm
NMR tubes can be illuminated over a spiral path of height 10mm or more as they rotate, so
illumination can be spread over 200mm2very easily.
Pharmaceutical tablets are a rather special case.
Heating can indeed be a problem, but so also can inhomogeneity. Manufacturer's make their
tablets by impact compression of powder mixtures and often imprint identifying logos,
numbers or letters into their surfaces. Shape also is variable, round, lozenge shaped,
rounded triangular or cylindrical tablets are commonly found. The trick when examining
these is to rotate and scan up and down at the same time making sure that the instrument
examines a high proportion of the tablet's surface. Heating is then minimised and further,
the spectrum is that of the tablet AVERAGED over the viewed surface. Imprints are ignored.
An example of such a cell is shown in Figure 8. Much more complex cells have also been
developed to study vapour absorption by powders - here the sample is exposed to a vapour
in a carrier gas e.g. wet air and the reaction that occurs as the vapour is absorbed is
monitored. Since these processes are temperature dependent, the vapour, gas and sample are
contained within a temperature controlled zone AND the sample rotates to minimise sample
heating. An example can be seen in Figure 9.
So, to conclude, to minimise laser heating in routine analytical
laboratories, try a KBr disk or liquid paraffin mull. Alternatively buy a simple rotator
and examine your samples uncontaminated in NMR tubes. If you want to be doubly sure,
purchase a 2 -axis rotator - round and round and up and down. Rotation is relatively
cheap, simple to use and requires little or no manipulative skill. If you want to lower
the temperature use a cryocell - more expensive - more difficult to use but these
accessories do often open up a range of experimental possibilities and you can look at
pieces of plastic, frozen liquids, bundles of fibres, rolled up films or lumps of rubber
equally easily. If a sample absorbs fairly strongly, cooling may not be enough - cooling
and rotation might be the answer [I'm working on this one!]. De-focussing the
laser - not a very good idea. Good luck!
Declaration of interest The accessories shown in Figures 3,
5, 7, 8 and 9 are supplied by Ventacon Wotton Ltd and I am involved in their design and
construction. I am not implying that these cells are the only ones available. It's just
laziness - I had the pictures to hand!
Editor and Fabrice Birembaut* Most analytical samples submitted to the infrared spectroscopist for identification (i.e. qualitative analysis) are solids. A variety of methods exist for their study including -
When your dear Editor was a postgraduate a very long time ago, his supervisor - the late Dr Don Powell considered that this method was the best available so it will not be a surprise, we suspect, if we still think it's worthwhile! The basic principles In order to record a transmission spectrum of a solid one needs to grind the particles to a size less than the wavelength of the transmitted wavelength and the particles should be immersed in a fluid with an index of refraction close to that of the solid micro-crystals. If a crystal is suspended in a fluid with a perfectly matched index, the crystal will disappear - there will be no effective optical interface and light will transmit freely - no scattering will occur. If the index match is close but not perfect some reflection (i.e.scatter) will occur. The greater the mis-match - the more intense the scatter. Quite obviously, each sample will have its own index of refraction. Further, the index is orientation dependent AND the index changes as one goes through an absorption band. Thus, it is impossible to find a fluid in which to immerse a crystallite to acquire perfect index matching. On the other hand, intelligent choice could well result in a 'close match' and hence a low level of scatter. If crystallites are suspended in air and are much smaller than the wavelength of the radiation passing around them, they cease to exist optically as much. If the size is very much less than the wavelength they behave almost as a liquid, if the size is a little less than the wavelength both scatter and some transmission will occur. If these particles are now immersed in fluid roughly matching the indices of refraction the scatter can be reduced and hence we have a true transmission spectrum. Diagrammatically we have -
This experimental situation can conveniently be achieved in two ways:
The KBr technique is described in an article in Volume 1, Edition 1 of IJVS. In the mull technique the fluid of original choice was a particular commercial grade of liquid paraffin renowned for its laxative properties - Nujol, now immortalised as the "Nujol Mull". I understand Nujol is no longer available, so we spectroscopic folk must resort to good quality liquid paraffin from their local pharmacists. This too is becoming hard to come by, because pharmacists are now instructed to be suspicious of customers purchasing same if there is any risk that they might be anorexic. So - sorry - slim, young lady purchasers must ask their overweight male colleagues † to purchase the stuff! † Neither of your esteemed authors are sylph like - hence they have not encountered this problem! The transmission spectrum of a thin film of liquid paraffin is shown below in Figure 2*. *Editor's Note - Don Powell would have told me to go to 'Boots the Chemist' to buy a new bottle of Nujol if he had seen this - it's dreadful. Such is progress!!
Quite obviously, if you disperse a solid in this medium
you will always see the paraffin bands. These arise, of course, from u, d
& t CH motions near
2900, 1400 and 720cm-1respectively. Of these, the stretches and
deformations are the most persistent. We need an alternative dispersing agent (mulling
fluid). The classical one and I suspect still the best, is hexachlorobutadiene (HB). This
material is very oily, relatively non-volatile and has a usefully high index of
refraction. HB has, of course, a very considerable and intense absorption spectrum in the
mid i.r. but since it contains only C-C and C-Cl bonds it is devoid of bands above 1700cm-1and
also in the region 1500-1200cm-1. Thus, a combination of paraffin and an HB
mull reveals the whole spectrum. Unfortunately, hexachlorobutadiene is toxic hence
alternatives are recommended of which fluoro-carbon oils are popular. However, assuming
you don't take a bath in the stuff and/or breath in deeply of its rather pleasantly
scented vapour, we suspect HB is unlikely to bring any of you 'out in spots' and is
definitely an excellent mulling medium#. The spectrum of HB is shown in Figure 3.
Combining Figures 2 and 3, you can see the point - two mulls cover the whole useful spectrum.
Making Mulls You will need a small mortar and pestle made of agate. These are available from spectroscopic accessory makers. You also need a stainless-steel semi-micro spatula, two dropping bottles containing liquid paraffin and the alternative mulling agent of your choice (HB, Fluoro-carbon oil…) and plenty of tissues. The mull you make will be squeezed between two KBr flats so you will have to lay your hands on a pair in good condition. If they need polishing - see your Editor's recent piece on the subject [volume 4, edition 1]. The way you prepare a mull depends to some extent on whether your sample is organic or inorganic. Relatively soft materials - and these include the vast majority of organic materials, co-ordination compounds and some mineral type materials. Others are very hard e.g. ionic crystalline and mineral substances. Further, the index of refraction of MOST but not ALL organics will be around 1.4-1.6. Many inorganics have higher values so we have an unfortunate pairing of properties:
Thus, it is easy to make mulls from most organics and quite a bit harder for the hard materials.
Let's start with an organic. We chose at random benzophenone. A little less than 5mg of the material was transferred to the agate mortar. Dry grinding in this case does not work. The material simply forms a pad on the lower surface of the mortar. If this happens clean out the mortar, add new material and DON'T dry grind. Add 1 drop of mulling fluid (a drop is around 1/20ml) and gently grind the mixture. As you do so, the lightly ground material tends to form a ring around the outer surface of the pestle so you should touch this off as you grind to minimise the risk that some of the material will not be really well ground. Grind for about 30 seconds.
Scrape the mull into the centre of mortar and test its viscosity. The viscosity should be similar to that of tomato ketchup. If too viscous add a small further drop of mulling fluid and regrind for 15-20 seconds to mix. Again scrape the mull into the centre of the mortar and transfer to one of the KBr flats. Do not bother to try and transfer everything - you will only need about half. Add the second flat and then gently rotate and press downwards. Gently swirl the top plate over the bottom plate. DO NOT rotate. The motion is the same as the one used in polishing, but on a much smaller scale.
The mull will spread out into a roughly round layer. DO NOT press too hard. DO NOT slide the top flat too violently, be gentle or you will scratch the KBr flats. Put the flats as a sandwich into the spectrometer and run the spectrum. If you have ground the mull properly and the ratio of bezophenone to liquid paraffin is about right, you should achieve a spectrum like this.
You are unlikely to achieve this result first time. There are several problems you might find:
We suggest you now make a mull and try to match Figure 7. Once you have done it a couple of times you will "get the feel" and have no problem in the future.
Now clean the mortar and pestle with tissue. If you are in any doubt use some solvent - say acetone on the tissue. Clean the KBr flats. Use of some solvent here is strongly recommended. If the KBr flat is difficult to clean or has become scratched give it a polish before proceeding. Now make a second mull using the alternative mulling agent. We used HB and our effort is shown in Figure 10. In Figure 11 we combine the two mull spectra i.e. Figures 7 and 10 to produce the complete spectrum.
Organic samples rich in OH or NH groups are subject to high levels of
hydrogen bonding. Such samples tend to give rather diffuse infrared spectra. H bonding
broadens the bands and they can frequently overlap making the spectrum appear to be poorly
resolved. There is no doubt that the quality of the sample preparation has a role here.
Now Inorganics Quite arbitrarily we chose potassium di-chromate. Remember, the problems here are that the material is much harder than is typical of organics AND the index of refraction of many (but not all) inorganics can be quite high. Proceed exactly as you did for benzopherone but emphasis must be put on grinding. At each stage be really careful that ALL the mull is ground finely. So, be very careful that the material that collects around the edge of the pestle is returned to the mortar several times. If you do the job properly, you should achieve a good mull and a spectrum like that shown below in Figure 15. If you don't grind properly and/or have too much solid, spectra like that shown in Figure 16 is typical.
Let us say your spectrum is too thick. The temptation is to press the flats closer together and to move the top flat around to smear out the sample. This works but is very likely to cause scratching of the KBr surfaces. A trick that sometimes works is to separate the flats, add a tiny drop of paraffin to one flat and rejoin. Gentle movement of one flat with respect to the other causes mixing, thinning and probably some grinding. The mull in Figure 17 was given this treatment and produced much better results. The index of refraction of HB is high, making it much better as a mulling agent than paraffin for inorganics. Very little effort was required to produce Figure 17.
So- it is easy and quick to make a mull and run it's spectrum. We estimate making the mull and putting it on the flats takes about as long as recording the spectrum. That is, the mulling method is certainly quicker than the KBr disk method. On the other hand, the mull always contains the bands due to the mulling agent. So - is it worthwhile particularly now that diamond ATR is so widely available? The answer is yes because, there is another excellent reason for using the mull method - reduced risk of damage to the sample. Inorganic chemists are well aware that exposing mixtures of their compounds with potassium bromide at extremely high pressures is seriously risking the possibility of reactions. Ionic compounds could re-equilibrate. Reactive chlorine or iodine atoms could well interchange with the massive excess of bromide. But - there is yet another hazard - polymorphism. Many materials exist in polymorphs i.e. they adopt more than one crystal structure. These structures inter-convert but many are meta-stable. Thus, although not the most stable form at room temperature, they either only slowly convert or they don't convert at all. Polymorphism is of vital interest in several areas of the fine chemical industry - dyes, additives and pharmaceuticals. The main reason is that the rate at which substances dissolve in solvents is governed by their polymorphism. Take the wrong polymorph of your pharmaceutical and it may do nothing! Unfortunately for analysts, the application of extreme pressure WILL re-equilibrate polymorphs so the use of the KBr disk or diamond ATR methods is risky. Both MAY be OK but both ARE RISKY. The mull procedure is far less likely to cause re-equilibration. Holding mulls in your spectrometer Most of the accessory manufacturers offer 'mull holder' consisting really of a simple clamp to hold the sandwich of KBr flats in the beam. In our opinion they are clumsy, relatively awkward to use and if used incorrectly the sandwich can become unstuck. There is a simple alternative - a simple support for the sandwich applying no clamping force, a solution you can make yourself. We show below two designs.
Both are shown on 2"x3" Industry Standard 'cards'. One simply consists of two appropriately cited stainless steel rods upon which round or square KBr flats can sit. We use 1/8" diameter rod (3mm is fine). The second uses a short section of aluminium 'angle' to support the sandwich. We recommend ¾" x ¾" x 1/8" material.
A reasonable competent "do-it-yourselfer" should have no problem. Oh yes - 'industry cards'. Most infrared labs have the odd card usually made out of stainless steel lying about. Good luck REF: P.J. Hendra & F. Birembaut,
Int. J. Vib. Spect., [www.ijvs.com] 4, 3,
3 (2000). |
