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The principle of Diffuse Reflectance IR Fourier transform spectrometry is depicted in figure 20.
When incident light strikes a surface, the light that penetrates is reflected in all directions and this is called Diffuse Reflectance. As the light that leaves the surface has passed through a thin layer of the reflecting material, its wavelength content will have been modified by the optical properties of the matrix. Consequently, the wavelength and intensity distribution of the reflected light will contain structural information on the substrate. It is clear that this process would be applicable to the study of surfaces and coatings and could obviously be employed to scan TLC plates. The use of the technique for scanning TLC plates was investigated by Zuber et al. .
The TLC plates were inserted directly into the spectrometer sample chamber and the Spectrum obtained by reflectance directly from the plate surface. A reference laser was used to aid in spot alignment, and the plate contributions to the background were subtracted from the Spectrum, which was obtained in the usual manner. It was found that spot identification was possible, providing reference spectra were available that had been obtained under the same operating conditions. The quality of the spectra and the useful size of the IR window available varied between different types of plate.
The preparation and care in handling both the sample and the background plates were extremely important to the success of the method. Although solvent selection was critical, providing the plate was completely dry, the solvent had no effect on the quality of the spectra produced. The Nicolet (Madison, WI) 6000 FTIR spectrometer equipped with a nitrogen cooled cadmium telluride detector was employed, and a Diffuse Reflectance attachment was used to run the spectra.
The interferometer was run at a mirror velocity of 0.586 cm/s and 2000 scans or less was found necessary to produce a good quality TLC/FTIR Spectrum. The TLC spot diameter varied between 2 and 8 mm, and the diameter of the infrared beam, focused on each plate, was 1 mm. The sample and background spectra were run, and the Spectrum of the sample obtained by difference. The analysis time was typically 15 minutes for the TLC separation, and about 30 minutes were needed to obtain the IR spectra. Examples of the results obtained from the tandem system are shown in figure 21.
It is seen that there is a distinct difference in the form of the spectra taken from the TLC plate compared with that from the KBr pellet. It follows that reference spectra that are to be used for solute identification should also be obtained from the TLC plate in the same manner. The mass of solute in each spot examined was about 10 μg but it was estimated that about 1 μg would be sufficient for a recognizable Spectrum to be obtained.
Direct measurements taken on the plate restricts the range of wavelengths that can be employed in the spectroscopic examination, whereas the removal of the solute from the plate allows the material to be examined over the normal range of wavelengths. Unfortunately, solute removal and recovery almost always involves losses, and sometimes the losses are accompanied by the decomposition or molecular rearrangement of labile materials.
Chalmers et al.  chose to use FTIR Diffuse Reflectance spectrometry in an off-line manner, by extracting the material from the spot before measurement.
The solute on the TLC plate was transferred to a KCl pellet, made directly from ball-milled potassium chloride. 0.7 g of the dried powder was pressed into a 13 mm disk die, at a pressure of 500 psi. The resulting pellet was about 4 mm high (± 0.5 mm). A metal backed TLC plate was employed for the separation, and the spot cut out and placed, metal backing downwards, in a tube 5 cm long and 17 mm I.D..
The KCl pellet was placed on the top of the disc, and about 2 ml of chloroform carefully pipetted down the inside of the tube.
The chloroform was allowed to evaporate at room temperature, and after about an hour, the pellets were removed for IR examination. An example of the spectra taken of some samples provided by the plastics industry is shown in figure 22. It is seen that good spectra were obtained, but the process was tedious, and considering the extraction and concentration processes that were involved, the overall methodology does not appear to have provided a very good sensitivity.
About the Author
RAYMOND PETER WILLIAM SCOTT was born on June 20 1924 in Erith, Kent, UK. He studied at the University of London, obtaining his B.Sc. degree in 1946 and his D.Sc. degree in 1960. After spending more than a decade at Benzole Producers, Ltd. Where he became head of the Physical Chemistry Laboratory, he moved to Unilever Research Laboratories as Manager of their Physical Chemistry department. In 1969 he became Director of Physical Chemistry at Hoffmann-La Roche, Nutley, NJ, U.S.A. and subsequently accepted the position of Director of the Applied Research Department at the Perkin-Elmer Corporation, Norwalk, CT, U.S.A.
In 1986 he became an independent consultant and was appointed Visiting Professor at Georgetown
University, Washington, DC, U.S.A. and at Berkbeck College of the University of London; in 1986 he retired but continues to write technical books dealing with various aspects of physical chemistry and physical chemical techniques. Dr. Scott has authored or co-authored over 200 peer reviewed scientific papers and authored, co-authored or edited over thirty books on various aspects of physical and analytical chemistry. Dr. Scott was a founding member of the British chromatography Society and received the American Chemical society Award in chromatography (1977), the M. S. Tswett chromatography Medal (1978), the Tswett chromatography Medal U.S.S.R., (1979), the A. J. P. Martin chromatography Award (1982) and the Royal Society of Chemistry Award in Analysis and Instrumentation (1988).
Dr. Scott’s activities in gas chromatography started at the inception of the technique, inventing the Heat of Combustion Detector (the precursor of the Flame Ionization Detector), pioneered work on high sensitivity detectors, high efficiency columns and presented fundamental treatments of the relationship between the theory and practice of the technique. He established the viability of the moving bed continuous preparative gas chromatography, examined both theoretically and experimentally those factors that controlled dispersion in packed beds and helped establish the gas chromatograph as a process monitoring instrument. Dr. Scott took and active part in the renaissance of liquid chromatography, was involved in the development of high performance liquid chromatography and invented the wire transport detector. He invented the liquid chromatography mass spectrometry transport interface, introduced micro-bore liquid chromatography columns and used them to provide columns of 750,000 theoretical plates and liquid chromatography separations in less than a second. Dr. Scott has always been a “hands-on” scientist with a remarkable record of accomplishments in chromatography ranging from hardware design to the development of fundamental theory. He has never shied away from questioning “conventional wisdom” and his original approach to problems has often produced significant breakthroughs.