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The modern Raman spectrometer is based on the use of a laser light source combined with a FTIR spectrometer. The laser light source must generate light of a suitable wavelength that does not produce significant Fluorescence and, consequently, as already discussed, the excitation wavelength is chosen to be about 1 micron. A diagram of the Raman attachment that is designed to fit a standard FTIR spectrometer is shown in figure 6.
Light from a near infrared laser source is focused onto the solid sample that is held on a support. The support also acts as a screen to prevent laser light directly entering the optical system of the FTIR spectrometer. The scattered light is then focused by means of a large ellipsoidal mirror and a collimating lens, so that it enters the FTIR optical system and strikes the Beam Splitter. From there on the optical system is very similar to that of the FTIR spectrometer. A commercially designed Raman FTIR system that provides the scattered light for subsequent resolution is shown in diagrammatic form in figure 7.
Figure 7. A Commercially Designed Raman FTIR System
Light from a suitable laser source is collimated onto a Diffraction Grating that selects the light of the wavelength to be used to produce Raman scattering (if a diode laser is employed this will not be required). Some of the light is focused onto a secondary photo sensor, the output of which is used to indicate the activity of the laser.
Light of the selected wavelength is directed by means of a mirror onto a reflecting system that also acts as a notch filter to remove any light of extraneous wavelength. The reflected light is focused onto the sample that generates the scattered light. The scattered light is directed back through the optical system and through a second notch filter that removes any traces of the incident light. The scattered light is collimated and is then ready to be resolved on a suitable Diffraction Grating to produce the required Raman Spectrum.
It should be noted that in order to render figure 7 virtually self explanatory, the incident excitation light is coloured vermillion and the scattered light is depicted in light blue. It is seen that the system can be constructed in a fairly compact form and be supplied as an additional module to a normal Fourier transform IR instrument.
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.