1. Editorial Well here we are at the start of the 'true' Millennium. I know we claimed that our Millennium issue containing the first scientific paper to appear in 2000 appeared at the start of the third Millennium, but several people have sent Christmas cards making it clear that they at least think 2001 is the important year. No, it doesn't matter really, but I suppose if you want to be pedantic, you can argue for 2001 rather than 2000. Just after the last edition we have a request from a reader about the relationship between the universally used strong and weak descriptions of bands and the absorbance. I took advice and have tried to come up with an answer. Very recently, I was talking to one of the postgrads at Southampton about his work on the use of infrared in identifying fibres in old tapestries. I suggested diamond ATR ad the use of polarisation. I thought it might be of interest to write up my advice and this forms the second article in this edition. Perhaps some of you have more experience in this field - if so please tell. Our manufacturer's Announcement Section continues to improve. You will have noted that the announcements are very different from those in competitor paper journals. We offer manufacturer's space and encourage them to include many pictures and diagrams - if possible in colour. This can cause a problem - publicity managers often don't have the copy to generate the article, but when they do - the result can be really worthwhile. A good example is to be found in this edition - from two collaborating companies Netzsch and Bruker. In the form of a paper it gives original and rigorous applications. Come on you manufacturers - IJVS make NO CHARGE for this service - why not contact the Editorial Office and offer us an announcement? In the submitted papers section we have two papers, both from the States. The first from Dr Xhiyong Xu and his colleagues at Michigan Technological University in Houghton, on quantitative mineral analysis based on meticulous sample preparation and KBr discs. The second is very different - it describes Pattern recognition methods in identifying spectra and comes from a group at Cooper Union in New York. Following up on my rant in the last Edition about the use of the ENGLISH
language, I have corresponded with colleagues abroad. A German contact who has worked in
Scotland and speaks excellent English obviously doesn't see much of a problem.
Simplification of english spelling and useage is helpful he thinks and a contributor to
Dear Readers obviously agrees. I also asked him about a recent trend to drop the
"umlaut" in German replacing 'ü' with 'ue' etc and I was surprised that this
change seems to be acceptable in Germany. I suppose this is the point - we ENGLISH are an
arrogant lot!
I've tried it out and I didn't set anything on fire. Let's be honest preparing a meal for the beloved is far worse that submitting oneself to a PhD Viva but it is much more fun! Editor
The Editor Late last year we received an e-mail from Richard Duerst asking what relationship existed between absorbance and band intensities on the old and well established very weak/medium/very strong scales. I didn’t know, but I have enquired. I have spoken to several elderly spectroscopists and compared their thoughts with my own experience. In years gone by when the intensity scale was devised, everyone plotted infrared and other absorption spectra on a % transmission scale. If absorbances were required suitably scaled chart paper was used and the absorbance duly read off the chart. Relatively few instruments could display absorbance on a linear scale until computer outputting became common. On the % transmission scale everyone had their own way of describing bands - the whole subject was completely subjective. However, some rough and ready agreement can be reached by comparing experience. In the figure I draw a set of peaks and label them with verbal strength descriptions. I have checked these with Bill Maddams (an author of several feature articles in IJVS and a very experienced, but now retired spectroscopist). On the RH margin I have written in the absorbance scale using the well known formula.
Whence for a given spectrum where c & l are fixed throughout
The relationship between the absorbance and the % transmission is logarithmic and of course the negative sign inverts the spectrum. The conversion is that
Bands which absorb ‘completely’ are always described as very strong (sometimes very, very strong), but the meaning is as impossible to quantify as it is to compare the musical descriptions ff and fff! At the other extreme, the weakest bands people used to see were limited by the noise in the spectra - noise far worse that we see today. Two percentage points was about the reliable limit so these bands were described as very weak. Below this level imagination and or a high alcohol level in the blood stream became useful. I remember that when I was a Postgrad, we used a Hilger H800 Prism instrument. Below 700cm-1the noise was pretty awful, but my supervisor Don Powell was studying metal amine complexes. Now the nPt-N in platinum II amino complexes can be very weak indeed and Don announced one day he had "found the band". He showed me a Honewell chart and pointed to this world stopping feature - I couldn’t see a thing! Tipping the paper up and looking along the trace was the trick I was told very firmly. To be honest, I still couldn’t see "the band", but as we all know PhD Supervisors like customers are always right. Later on, Donald ‘got his act together’ and "the band" he saw was confirmed. Phew! So we get the descriptions in Figure 1 and hence can read off the
absorbance values as an equivalent scale. Figure 1. Graphical Representation of band strengths
As spectrometers (or to be pedantic interferometers) have such excellent stray light and S:N characteristics these days, the scale above which refers to the historic literature could well be extended to include Extremely Strong and Extremely Weak. REF: P.J.Hendra, Internet J. Vib. Spec.[www.ijvs.com]
5, 1, 2 (2001) Relatively few users of infrared have considered the use of polarised light. In a sense, this is a little odd because in some areas of optical science, polarisers are considered to be essential equipment. Thus, optical microscopy is almost always associated with the use of polarised light either in transmission or reflection. Petrographic microscopy on mineral sections, fracture surfaces in engineering - all use polarised light to improve the resolution or to enhance the analysis. Polarisers in infrared analysis are also really valuable but rarely used - Why? Background principles If a sample is oriented, specific vibrations have a directional component. Let us consider nylon as an example. An oriented film of nylon will have its molecular axes along the stretch (deformation) direction. The orientation will not be perfect but it certainly will be preferred.
Quite obviously those vibrations of this molecule that have vectors predominantly along the chain will interact best with resonant infrared radiation polarised vertically. On the other hand, the C=O stretching mode has a vector perpendicular to this direction. In a polymer there is almost always only one preferred direction - the stretch direction i.e. the chain axis. In the diagram above, the z axis lies along the chain but x + y have no significance. There are exceptions - many examples of polyethylene terepthalate (PET) are 2 way oriented. In this case stretching occurs in two orthogonal directions and hence as in a crystal, directions x, y + z each have significance. Coke bottles are just such a case. A small 'pre-form' with a neck of the finished size and shape is gripped at the neck, the body of the pre-form is heated and then blown with hot air into a mould of size identical to the finished product. The PET sheet forming the walls deforms in 2 directions and hence the benzene rings lie roughly parallel to the wall surfaces. There are many other examples of oriented materials from minerals to fibres, films, some coatings, liquid crystals etc, etc. Orientation can even be present and be an unwanted nuisance. In all these cases infrared can be pressed into service. The principle is to introduce a polariser into the optical system and study the spectrum with the sample oriented in different ways with respect to the polariser. In the case of a film or fibre two measurements are obvious.
If 3 way orientation is to be monitored a third measurement can be made.
i.e. the film is inclined with respect to the beam at 45º. In II the spectrum will be predominantly that of modes with vectors
parallel to the molecular axis whilst the -
we will set x as the out-of-plane axis. If the sample is PET the out of plane CH deformations of the arometic rings will lie along the axis. The third experiment shown in diagram IV attempts to exploit this. The method goes back to Harry Willis's pioneering work on the vibrational spectra of polymers at ICI in Welwyn Garden City, UK. The amount of interaction between the vector and the polarisation direction varies as Cosq.
If the angle of inclination is known in experiment IV and it is, the component in the spectrum due to the contribution of modes to dipole vectors along axis x can be unscrambled from those vibrating along y and z. Experimental Perhaps there are two reasons why people so rarely use polarised light -
Over the years, a wide range of polarisers has been used. Early ones were based on stacks of sheets of optically transmission material set at Brewter's angle to the beam. Silver chloride was popular when I was a student. These polarisers were very clumsy and were replaced by "wire grid" devices many years ago. These consist of a piece of optical window (usually ZnSe these days) vacuum coated with a gold grid consisting of fine gold strips separated by gaps narrower than the radiation to be passed through the device. These polarisers are highly efficient but both expensive (~$1500E) and extremely fragile. Touching the gold wire grid with anything will wipe off the gold and ruin the device. Some manufacturers arrange a polariser stand or filter wheel inside the sealed part of the system enabling the user to insert and remove the polariser under software control. This is ideal - no-one other than the service engineer will touch the polariser surface and it stays clean, dry and free from dust. Where such a facility is not available, the user must position the polariser in the sample area. Suppliers of polarisers usually mount them in a ring (often 25mm in diameter and ~5mm thick). They invariably provide a mark or dot to indicate the electric vector direction. To use the polariser in the sample area it is not adequate to tape it into position or prop the thing on a lump of blue tack or play dough. No-you need a mount, I draw below one we had at Southampton - very simple, completely effective and it held the precious polariser firmly and safely.
As I have shown in diagrams II and III we need two spectra in orthogonal directions. It would appear to be simplest to rotate the polariser and leave the sample fixed especially if experiment V is to be included but no - the rule is, leave the polariser fixed and rotate the specimen. The reason is that diffraction instruments and interferometers are themselves polarising filters # i.e. they have a preferred orientation and the preference changes with wave length (cm-1). In an interferometer, the trick is to set the polariser usually with the vector vertical or horizontal, run the background through the polariser and use this for all subsequent experiments. Some experts prefer the set the polariser tilted at 45º to the vertical, but this can cause confusion and mistakes in use. # Interferometers are usually for less polarising than diffraction instruments. In some instruments the transmission is almost identical in horizontal and vertical directions but it is unsafe to rely on this. ATR Years ago, when Harry Willis was a regular visitor to my group at Southampton, he got me to set up an ATR system based on a truncated square pyramid crystal. The idea was to clamp the sample on one of the flat surfaces, then run the polarised spectrum and rotate the sample and crystal sandwich by 90º before repeating. I wondered more recently if diamond ATR would work with a polariser. The answer somewhat to my surprise is 'yes'! Again, the polariser is inserted in the instrument and left fixed, the sample is placed in the diamond ATR either parallel or perpendicular to the beam direction. In the figure below a polypropylene filament was used. The two experiments were -
You can see the effect very well - some bands are stronger in the
II spectrum, others in the -
For some polymer applications it is essential to monitor the degree of orientation because it is a first class indicator of the mechanical performance of the material. Fine I hear you say, but in diamond ATR the sample is crushed onto the
horizontal surface of the diamond. Surely any measurement made on such a mangled surface
is hardly typical of the real material. I disagree - it is well known that oriented
materials are either fibrillar or behave as such. If you crush a fibrillar material e.g.
timber, the material retains its orientations as it crushes and de-fibrillates. Conclusion Any method of increasing analytical data enhances the reliability or specificity of the analysis. As so many specimens have at least a small degree of orientation - a preferred direction, the use of polarised radiation will improve the analysis. As I pointed out above, orientation even slight orientation can have serious consequences e.g. in service, long period distortion or enhanced risk of cracking or breakage in materials. So - go and get a polariser and see what fun you can have. Only
don't touch the surface! |
spectrum will emphasise those vibrating
across the chain.
