Limit Tests Infrared Spectrum

The ultraviolet-visible spectra of most compounds are of limited value for qualitative analysis and have been largely superseded by the more definitive infrared and mass spectroscopies. Qualitative analytical use of ultraviolet-visible spectra has largely involved describing compounds in terms of the positions and molar absorptivities of their absorption maxima, occasionally including their absorption minima. Indeed, some organic compounds are still characterized in terms of the number of peaks in the UV-visible spectrum and their absorbance ratios. This is usually the case in phytochemistry and photodiode array chromatography and when the analyst has a limited range of compounds to work with whose spectra are known to differ. In the pharmacopeias, however, absorbance ratios have found use in identity tests, and are referred to as Q-values in the U.S. Pharmacopia (USP). 

The application of high polymers as insulating materials at low temperatures, as is well known, is limited by their brittleness. Consequently, it is highly desirable to develop methods of checking quickly and simply if a certain polymer can be used at low temperatures or not. In this paper, we propose the use of spectroscopy as a testing procedure. From simple considerations of the molecular structure of polymers, it appears that spectroscopy in the low frequency range of lattice vibrations, i.e., the far infrared (FIR), or the low frequency Raman spectrum, is an appropriate tool for this purpose. 

Very recently, a tuneable CO2 laser has been combined with an AFM to form an aperture-less nearfield imaging system to obtain contrast in infrared absorption on a scale of about 100 nm [299]. However, the tuneable range of the CO2 laser is limited to a region of the IR spectrum that is not particularly informative for most IR chromophores ( 2300 cm ). For many applications coupling of a tuneable IR diode laser to an infrared microscope [300] is more attractive. Hammiche et al. [301] have used a Wollaston resistive thermometer as a photothermal probe to record IR spectra of polymers. Anderson [302] has indicated that an AFM/FTIR microscope without specialised tips can provide surface topography and chemical mapping at high spatial resolution. Direct infrared detection at a surface with the use of an AFM was tested both with filter and FTIR spectrometers (Fig. 5.6). Nowadays, IR spectroscopy at... 

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