3/15/2023 0 Comments Nview spectra c![]() So – sorry – slim, young lady purchasers must ask their overweight male colleagues † to purchase the stuff! 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. I understand Nujol is no longer available, so we spectroscopic folk must resort to good quality liquid paraffin from their local pharmacists. 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”. The KBr technique is described in an article in Volume 1, Edition 1 of IJVS. The liquid has to satisfy several requirements – it must be free flowing, have a very simple and reproducible infrared spectrum and be readily available, cheap and stable. Here, the material is ground up and then suspended in an appropriate organic liquid as a paste. The KBr disk technique – where the suspension fluid is crystalline potassium bromide compressed so intensely that it behaves like a fluid and flows around and in optical contact with the particles and.This experimental situation can conveniently be achieved in two ways: 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. 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 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. On the other hand, intelligent choice could well result in a ‘close match’ and hence a low level of scatter. Thus, it is impossible to find a fluid in which to immerse a crystallite to acquire perfect index matching. Further, the index is orientation dependent AND the index changes as one goes through an absorption band. Quite obviously, each sample will have its own index of refraction. The greater the mis-match – the more intense the scatter. If the index match is close but not perfect somereflection (i.e.scatter) will occur. 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. 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. 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! Emission Spectroscopy – not sure we know much about this – perhaps we should know more AND.Reflection spectroscopy – we have talked about this, but perhaps we should look again.Diamond ATR -again described in IJVS, this time by David Coombes.The KBr disk method – described with examples in a piece by Geoff Dent in IJVS.A variety of methods exist for their study including – Most analytical samples submitted to the infrared spectroscopist for identification (i.e.
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