1. Editorial This edition is very late. Edition I should have appeared in late February, but at that time we had absolutely no copy - not a scrap, no contributed articles, no features and virtually no contributions to Dear Readers. I therefore decided that as Editor 'I had better do what Editors do' and write some stuff myself. You will find a 'feature' AND a 'contributed' piece and I hope that you find them interesting. Fortunately, some other material has rolled in and so we can now offer Edition I. What's more - Edition II has just arrived - a superb special edition produced by Professor Z-Q Tian of the University of Xiamen in China. So, once desk-editing and formatting is completed, Edition II will be on-line in a few weeks from now, thus catching up our production rate. This edition - I have been aware for some time that many of the younger
folk who use infrared have had little or no instruction in technique. I suppose I didn't
either, but after 40 years or so, you pick up the odd tip or two. You will find in this
edition a rather simple-minded but 'none-the-less useful for that' piece on polishing
infrared windows. Why? I knew you'd all ask. In many infrared analyses you need to use
good quality windows - liquid cells, gas cells, liquid parafin mulls etc, etc. The problem
is that KBr and Rock Salt windows deteriorate very rapidly because they are water
sensitive, soft and very fragile. As a result users should store them properly and keep
them polished. In the article I give you all a really detailed description of how to grind
and polish windows and how to keep them polished and useful. In a near future edition I
will tell you all about the liquid parafin mull technique. In the submitted paper section you will find a paper on low temperature Raman cells in which we describe a new design. The only snag is that the glassblowers have to border on genius. Be that as it may - the new cell works a devil of a lot better than the older standard ones. Have a look! Geoff Dent told me recently of a long collection of Raman spectra of inorganic compounds recorded in his laboratory by Rose Keepax, an undergraduate student visiting his lab at Avecia last summer. The spectra are of excellent quality and we were hoping to include about half of them in this edition, but the "love bug" virus put paid to that. So we plan to get them to you as a special mini-issue between this and the next edition. More will follow soon after. Several people have again raised the question of the quality of the
spectra we publish. As I have reported before, many authors want their spectra to appear
in their articles to illustrate points, but they are not at all happy that their data is
available to readers in full digital form. We have respected this view to date, but we
have decided to resolve the problem by offering high quality digital data when the author
agrees. Louise will explain the full details below. We will try to persuade all
authors to let us publish digital quality spectral data if they wish. You never know,
perhaps most will agree! I have tried to set the scene by including all the spectra of our
two articles in this edition as .pdf files [see Spectra Index]. After escaping writing anything for a few editions, Patrick has decreed
that I 'run through' everything about pdf files and spectra. So now "busy mother of
one four-month old girlie who has decided to start sprout teeth(!)" also has to deal
with the job of presenting spectra properly!? I am using the latest version of Adobe Acrobat/Reader (Version 4.0)
etc, so you may find that you need to down-load the latest Adobe Reader from the Adobe
website [www.adobe.com].
Adobe Reader is available FREE from this site. 2. Grinding and Polishing We started IJVS with an article [Int. J. Vib. Spect., [www.ijvs.com] 1, 1, 2 (1996)] by Geoff Dent telling readers exactly how to make good KBr disks and what to look out for if they had done a job badly. The article has proved very popular because as we all know it' far easier to make a lousy KBr disk than it is to produce a good one. Some time before Christmas I encountered an undergraduate trying to record a spectrum of a nujol mull† in the organic teaching lab. The spectrum was hideous for two reasons - the student had no idea how to make a decent mull and the two KBr flats she was using looked as if they had been through hell and high water - probably literally. Cross-examination revealed that she had not been taught how to make a mull - article in a future edition - and the "technicians polished the flats". So - I thought some of you might find it useful to learn how to polish windows properly. † Grinding solid compounds with liquid parafin and squeezing the paste produced between rock salt or KBr windows was the classic method of examining the mir spectra of solids. KBr disks began to rival the "mull technique" as it was called, only in the 1960's. Elderly spectroscopists still refer to the method as the Nujol Mull method although these days everyone describes it as the liquid parafin mull procedure. Why "NUJOL"?. Back in the pre 1960's, a high quality liquid parafin to British Pharmaceutical Standard was on the market called Nujol - recommended for the ease of constipation. At that time Nujol gave a clearer spectrum than non-branded parafins so the use of this particular brand of laxative was always recommended. Simple, ain't it? Can I suggest you find a battered old KBr or NaCl window and then follow what I tell you to do. Do the work near the spectrometer and check progress as I indicate. Let us assume that the 'optical flats' we have available are neither optically clear or flat - a lump of optical material cut into a round or square shape but looking as if some idiot had run it under the tap! Laugh not - I have actually seen a postgrad do just that! I searched around and found myself a 30mm diameter x 5mm thick KBr window which satisfied the description - very badly corroded, light transmission not too bad but the surfaces clearly uneven. I decided to grind and polish this particular window as follows: 1. Record the mid ir spectrum of the window -
2. Grind the surfaces of the window flat. The best way to do this is to use carborundum (SiC.) powder on glass. a) Find a scrap piece of glass preferably 5mm thick or more. 150mm square is quite big enough. Carefully wash the glass under water (if dirty, use a domestic cream cleaner), rinse and carefully dry with a cloth. Put the plate on the bench with a new sheet of white paper underneath it. Pass your hand over the glass (an excellent way of removing the last trace of grit). b) Obtain a supply of SiC of coarse and fine grades. I used grades 160 coarse and 600 fine acquired from the glass shop at the University. Mechanical workshops often have these grades of carborundum. Put about 10mg of the coarse SiC on the plate and wet it fairly heavily with ethanol. Grip the flat between the tips of your thumb and index finger, now using a circular motion alternating clockwise with anti-clockwise grind one surface of the flat. After 2 clock/2 anti-clockwise movements, turn the flat by 30º under your grip and repeat….
Don't press hard - use very gently pressure or the surface you produce will not be flat. After about 1 minute wipe the surface with a tissue and see how you are doing. Eventually you will produce a completely uniform white opaque surface.
When you have wiped the surface with the tissue, drop the tissue onto the floor. If by mistake you re-used a tissue, it might have a crystal or two of grit on it and this could ruin all your subsequent efforts. c) Turn over and repeat with the second surface. Using a new piece of tissue soaked in alcohol carefully clean the surfaces and sides of the flat. Do so at least twice each time dropping the tissue at your feet and using a new piece. I checked the i.r spectrum of the flat at this stage and got the following -
d) Remove the glass plate and re-wash it in warm water. Wash your hands and wipe over the bench. Replace the dried plate on new paper on the bench and wipe your hand over the plate. Why the ritual? We are going to grind the surfaces with much finer SiC than we used at first. One rogue coarse crystal of SiC will ruin our efforts - thus the precautions. e) Put on rubber gloves. Put ~10mg of the fine SiC on the plate, add ethanol and do exactly as you did before in b). No need to grind each surface for more than 1 minute. I checked the ir at this next stage and found that the transmission was a bit better than before particularly at larger wavelengths. Very much as I expected. You now have two good-looking clean uniform flat surfaces. Polishing You will need to have access to a commercial polishing kit. All the accessory makers offer these - they are all very similar and simple to use and highly effective. A kit was kindly loaned to me by Specac Ltd of St. Mary Cray in Kent, UK. The Specac kit consists essentially of a "smoothing lap" and a "polishing lap" an optical flat and various spares and supplies. The smoothing lap is a heavy piece of glass about 80 x110mm in size, covered on one side with an abrasive sheet. The sheet is adhesive backed, replacements are supplied and the abrasive looks very like fine "wet & dry" abrasive paper. Before you start - wipe the bench down, remove the silicon carbide, used tissues etc. Make sure no grit could contaminate the polishing kit or you will be wasting your time. Finally wash you hands, put on rubber gloves and re-clean the flats with tissue wetted with alcohol. Again, drop them onto the floor once used. a) Put out the polishing lap. Wet the part furthest from you with alcohol. Gently drop the window onto the alcohol and use the same motion as you did before - round and round clock and anti-clockwise moving the window in your fingers after 4 rotations. DO NOT PRESS HARD. After about 1 minute, lift and dry the surface. At a glancing angle you will see that it is slightly shiny i.e. a new much smoother surface is being produced. I recommend repeating with a little more alcohol i.e. spending 2 minutes per side. b) Repeat exactly as before on the second surface. Using the section of the lap nearest you that you didn’t use before. Wipe clean and run the spectrum. I got -
The polishing lap consists of a very fine velvet-like cloth (one of the trade names is Selvyt) stuck onto one surface of another heavy glass plate. The principle is that if a polishing abrasive is put on the cloth and wetted with alcohol, abrasion will remove a thin layer of material from the flat and produce a polished finish. The problem is that the cloth will crush under pressure hence the optical window will polish away more at the edges then at the centre.
I exaggerate of course! To counter this effect, the glass under the smoothing lap is often deliberately made convex so that after smoothing the surface is slightly concave. Polishing then flattens this out - in theory! c) Put ~5mg of polishing powder (usually alumina and often called jewellers rouge although it is white not red!) on the lap, wet with alcohol and proceed as you did for smoothing. At this stage you must wear rubber gloves. Use very light pressure, move quickly and wipe the flat on a dry section of the polishing cloth before inverting and inspecting. I found ~40 seconds polishing give quite a good result. d) Repeat on the second surface, gently polish the surfaces with dry tissue and run the spectrum. You should get something like this -
For most purposes these windows are fine. You may however need to make the surfaces really flat. This is a job for the professionals but with care (just the right amount of smoothing and then polishing) you can achieve a surface to ~4-10 'fringes' i.e. with an error from flatness of about 2-5 microns. To check flatness, you need the optical flat provided in the kit. Clean it thoroughly and then pass the palm of your hand over it. Place the window on it and gently press out the air (remember to put on rubber gloves before you do this). Hold as shown in the diagram below -
You should see a set of blue/green rings at the interface between the glass and the KBr. They are very feint and hard to see. The number of rings you see indicates the flatness. They arise from interference of the blue/green lines from the fluorescent strip-light as the radiation passes through the thin air layer between the surfaces. Don't move the window across the flat as you will certainly scratch the soft KBr surface. Storage There are many myths about storing KBr and NaCl. If kept warm, they
will survive quite high humidity so keeping them in a warmed cupboard is fine in temporate
or dry climates or in air-conditioned rooms. [NOT in the tropics]. For
long term storage or in really humid environments, store in a dessicator over silica or in
a sealed jar or can again over silica. In humid labs - ALWAYS warm the
dessicator or can under an infrared lamp before opening. What kills these materials is
condensation on the surface.
HOW LONG DOES ALL THIS POLISHING TAKE? I hear you plaintively ask!! A flat in reasonable condition can be improved, ready for use in less than a minute by using just the polishing lap. So, I recommend having the polishing kit ready for repeated use. Grinding and polishing a roughly cut window or ruined one takes about 15-20 minutes per window. If you look in the catalogues you will see that rough cut windows are MUCH cheaper than polished ones - is the difference in price worth more or less than 15 minutes of your time? I leave you (or your boss) to judge. Good Luck. Editor B J McGrattan, R A Spragg and H M Wilson Abstract FT-IR is proving valuable in the analysis of materials during solid phase synthesis. It has the great advantage of being able to characterise reaction products without first cleaving them from the support. The principal aims are to confirm the identity of products and to determine the degree of completion of the reaction. Although virtually all the usual sampling techniques have been tried, direct transmission measurements have emerged as the preferred approach. These involve some form of compression cell as the pathlength through the original beads is too great. We describe a new cell designed specifically for these measurements. It can provide a controlled pathlength while eliminating the problem of interference fringes associated with windows made of a high refractive index material. The highest quality spectra are those obtained from single beads, but these require an IR microscope. For rapid routine measurements it is far simpler to look at a group of beads, and indeed this reduces any problems associated with bead-to-bead variation. However it introduces other problems resulting from non-uniform pathlengths and gaps between the individual beads. The impact of this on spectral quality will be discussed, with regard to both spectral subtraction and quantitation. We have developed software to monitor solid phase reactions from the
spectra of compressed beads. This automatically generates the difference between spectra
of beads before and after reaction, correcting for stray light and differences in
pathlength. A database of IR band frequency correlations simplifies interpretation of the
results.
The role of IR spectroscopy The principal advantage is that IR spectra can be obtained without cleaving compounds from their solid phase substrates. Although the spectra include a contribution from the support this often does not obscure the bands that characterise the compound of interest. There are two situations where IR spectra are most often used. One is simply to confirm that a reaction has proceeded as expected. The second is during method development, monitoring the degree of reaction in order to optimise reaction conditions. The measurement can be rapid, requires a very little sample, and is non-destructive, making this a very attractive technique. However obtaining any quantitative information is not straightforward. Why use transmission spectra? In general the aim is to determine the changes in relative band intensities as a reaction progresses. The requirements are therefore the same as for quantitative analysis or for good spectral subtraction. The samples consist of beads ranging in diameter from 50 to several hundred micrometers, too large for direct transmission measurements. However methods requiring no sample preparation are unsatisfactory. Reflection spectra are unsuitable because they give a varying mixture of surface and diffuse reflection where band intensities are not readily interpreted. In ATR spectra the problem is that imperfect contact between the beads and the crystal makes it difficult to obtain reproducible spectra. Transmission spectra can be obtained either by making KBr pellets or by direct measurements of compressed beads. Pivonka has shown excellent spectra from single beads that have been compressed after swelling with a solvent. However to measure a single bead requires an IR microscope. The solvent softens the bead so that it is readily compressed to a fixed pathlength. Here we describe measurements on clusters of beads that have been compressed without using any solvent. Contrary to some suggestions many beads can be compressed to a suitable thickness without shattering. However the resulting sample does not consist of a uniform film. At best it can be regarded as incomplete but otherwise uniform film. We show a typical cluster of compressed beads. Although virtually all the material can be compressed to uniform thickness there will be two sources of distortion in the spectrum. One is that some radiation passes through gaps where none will be absorbed. The other is that there will be a distribution of different effective pathlengths in the regions at the edges of the beads. However as can be seen the beads can be compressed to the point where these regions are only a small part of the sample. The compression cell We have built a compression cell and matching beam condenser specifically for these samples. The cell has diamond windows with a clear aperture of 2mm. In earlier designs there have been problems from interference fringes caused by reflections between or within the windows. Our design minimises these fringes by avoiding parallel surfaces. Sample preparation consists simply of placing a suitable quantity on one window, putting the two halves of the cell together, and tightening. Typical samples consist of fewer than a hundred beads, which readily form a single layer. The path length can be controlled by inserting metal foil spacers. Otherwise it depends on the quantity of sample and the pressure applied. The cell is designed to have a minimum path length of 25µm.
The potential problems with the spectra are stray light,
uncontrolled pathlength, and interference fringes. Stray light passing through the gaps in
the sample distorts relative band intensities, reducing the apparent absorbance of
stronger bands. This distortion is greatest for the most intense bands as can be seen from
the plots in below. Although stray light can be kept below 5% by loading the sample cell
very carefully, this requirement detracts from the simplicity and convenience that make
the IR approach attractive.
Correcting for Stray Light The gross errors introduced by gaps between the beads can be largely removed by a very simple procedure that is readily automated. This is based on the observation that the thickness of the samples is always sufficient for some bands to be totally absorbing. For example the polystyrene bands near 2920, 1450 and 700cm-1 can be regarded as having essentially zero transmission. An approximation to the stray light contribution can be obtained by constructing a smooth curve through the transmission values at these points. This is subtracted from the initial spectrum. The spectrum is then rescaled to restore the baseline level. This procedure is illustrated for a spectrum with approximately 50% stray light. Tentagel beads have suitable bands at 2900 and 1100cm-1. The procedure was applied to several spectra with different levels of
stray light resulting from different amounts of the same material. If the original spectra
are converted directly to absorbance the intensities of the polystyrene bands in the
region 2000 – 1800cm-1 differ by a factor greater than 2.5. However after
correction the intensities vary by only +/- 10%.
Shown here are spectra of a cluster of beads in the compression cell,
and of a single bead in the cluster measured in an IR microscope. After correcting for the
15% stray light the band intensities in the two spectra agree very well. The difference is
dominated by interference fringes in the microscope spectrum, corresponding to a
pathlength of 29µm. These fringes are not seen in the normal spectrum because there is a
small angle between the windows. This angle is not large enough to avoid fringes when the
aperture is restricted to the 80µm used in the microscope.
Correcting for pathlength variations In order to compare band intensities in different spectra it is important to correct for variations in pathlength. This can be done by normalising the intensities of bands that are common to all spectra. It is best to use relatively weak bands as these are least affected by stray light or differing pathlengths within the sample. For polystyrene beads the bands in the 2000 – 1800cm-1 region are very convenient. The spectra of Tentagel beads can be normalised on the aromatic C-H stretching bands around 3100 – 3000cm-1.
Following a reaction These corrections were applied to data from hydrolysis of a sulphonic
acid on Tentagel. This is not an easy example as the major sulphonic acid bands at 1250
– 1200 cm-1 overlap with other strong absorptions. The spectra were
obtained at approximately 40 µm pathlength and showed up to 10% stray light. The true
transmission at 1250cm-1 in some of these samples is only about 1%. After stray
light correction and normalisation on the aromatic CH stretching bands the changes in band
intensities are evident. By plotting the band intensities against time it can be seen that
the reaction is 90% complete after one hour.
Software The software is intended to simplify comparing spectra from beads at different stages of a reaction. It incorporates a routine that can correct for stray light automatically. The subtraction routine is designed to compensate for path differences by cancelling common bands. Two aids to spectral interpretation are provided. The user can specify functional groups associated with the starting material or expected product. The software then displays regions where characteristic bands occur. Alternatively the user can position a cursor over a band and the software will display a list of functional groups expected to absorb in that region. Typical screen displays from an esterification reaction are shown on another page.[Click to see screen at full size] Spectra of starting material and product
Difference
spectrum with cursor at 1720cm-1 Although virtually all the usual sampling techniques have been tried, direct transmission mir measurements have emerged as the preferred approach. These involve some form of compression cell as the pathlength through the original beads is too great. We have described a new cell designed specifically for these measurements. It can provide a controlled pathlength while eliminating the problem of interference fringes associated with windows made of a high refractive index material. The highest quality spectra are those obtained from single beads, but these require an IR microscope. For rapid routine measurements it is far simpler to look at a group of beads, and indeed this reduces any problems associated with bead-to-bead variation. However it introduces other well known problems resulting from non-uniform pathlengths and gaps between the individual beads. The impact of this on spectral quality have been discussed, with regard to both spectral subtraction and quantitation. We have developed software to monitor solid phase reactions from the spectra of
compressed beads. This automatically generates the difference between spectra of beads
before and after reaction, correcting for stray light and differences in pathlength. A
database of IR band frequency correlations simplifies interpretation of the results.
Fortunately these data bases are widely available.
|
) the
light is less scattered than it is at 7800cm-1(1.2 µ wavelength).
