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The synchrotron is a relatively novel source of IR radiation. The basic principle of the synchotron is depicted in figure 8.
A beam of electrons is generated in a linear accelerator that then enters a ‘booster ring’ where the electrons are accelerated to close to the speed of light. The electrons are then injected into a larger ring (the storage ring) where the electrons continue to circulate at relativistic velocities. Any charged particle, travelling at these velocities and traversing a curved path will emit intense broad-band electromagnetic radiation across the whole Spectrum from X-rays to the far infrared. Modern synchrotons utilize magnetic devices called wrigglers”, and “undulators” round the ring to increase the intensity of the radiation. The radiation is emitted tangentially to the electron path and so any port incorporated in the ring will provide an intense radiation beam. The beam is then sent through an optical system where the wavelengths of interest are separated and made available for experiments. Most experiments carried out with radiation from a sychrotron involve X-rays but some work on infrared has been carried out. Time can be rented from the synchrotron establishment but it may be many years before such facilities become generally available. Such a source does hold promise for vastly improved results from Raman Spectroscopy. In figure 9 the performance of a typical FT-Raman instrument is shown by the spectra of indene.
The excitation energy was 1 W and data was acquired for two seconds in the lower Spectrum and for thirty seconds in the upper Spectrum. The two spectra appear identical which demonstrates that no thermal degradation occurred with the extended exposure to radiation for thirty seconds. The majority of samples can cope with high intensity near-IR without decomposition as the absorption is involved with combinations of vibrational modes and overtones and, therefore, are rather weak.
The main advantage of working with near-infrared radiation is the lack of Fluorescence that can seriously interfere with the quality of the resulting Spectrum. This is clearly demonstrated by the two spectra of anthracene obtained employing radiation in the Visible region (5145Å) and in the near-infra red (1.06μm) shown in figure 10. The upper Spectrum produced by irradiation with Visible light shows the spectra superimposed on a broad ‘hump’ of Fluorescence almost completely obscuring the Raman Spectrum, whereas the lower Spectrum obtained by near-infrared irradiation gives a clean, virtually Fluorescence-free Spectrum.
Admittedly, this may well be one of the worst-case ‘scenarios’; nevertheless the spectra clearly demonstrate the advantages of employing near-infrared radiation in the production of Raman spectra. A possibly even more impressive advantage of the use of the near infrared for the excitation to eliminate Fluorescence is shown in figure 11.
In this case the Raman spectra is being taken of a strongly fluorescent material but employing radiation of 106 mm as the radiation with a 60 second measurement time a Raman Spectrum of a useful form could still be obtained.
Employing near-infrared frequencies as the exciting radiation in conjunction with FTIR techniques provides yet another advantage. The ease and accuracy of spectra addition and subtraction employing the FTRI system used in conjunction with spectra that have extremely low Fluorescence interference allows solvent spectra subtraction techniques to be extremely accurate.
Spectrum B obtained from pure toluene.
As both spectra are (in a sense) extremely ‘pure’, subtracting the solvent Spectrum from the mixed Spectrum provides a very accurate Spectrum of the solute alone that can be used with confidence for both identification purposes and also for structure elucidation.
The spectra given in figure 12 clearly demonstrate this advantage. The Spectrum obtained after subtracting the Spectrum of the solvent from the Spectrum of the solution of the solute in the solvent is virtually identical to the Spectrum obtained from a sample of pure toluene. In fact the difference is almost indistinguishable.
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.