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THE SPECTRO-PAEDIA

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absorbance
adsorption
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bandwidth
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bioluminescence
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chromatography
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luminescence
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Light Pipes
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Atomic Spectroscopy
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Time of Flight Mass Spectrometer
Optical RotationCircular Dichroism
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Polarization Effects in Raman Spectroscopy

If examined with a polarimeter, some lines of the scattered radiation (not all) may be polarized and, furthermore, they will be polarized to different extents. The reason for this is not immediately obvious and can be best explained using the following simple case of Raman scattering. Consider a molecule, the polarizable ellipsoid of which is spherical (typified by the molecule of methane ) which is depicted in figure 13. The spherical form of a polarizable ellipsoid is given the term spherical top.

 

Figure 13. The Spherical Top Molecule methane .

There are various forms of vibration that methane can take and as the polarizable ellipsoid, in this case is spherical, one form of the vibration (known as the symmetric stretch) is where all four C-H bond distances increase and decrease in phase. Thus, the ellipsoid shape remains spherical (for obvious reasons the frequency of these vibrations is sometimes referred to as the ‘breathing frequency’) and the molecule in this form will plainly be Raman active throughout the vibration. Consider a beam of unpolarized radiation travelling in the direction of the (x) axis falling on the molecule. Since the polarizable ellipsoid is a sphere it follows that it is equally polarizable in all directions and any induced dipole will be along the greatest electric vector of the radiation (i.e. along the (xy) plane). This situation is depicted in figure 14. In figure 14, two incident beams are shown striking the molecule but it should be noted that induced dipole is in the (xy) plane for both. A non-polarized beam will contain components having all possible values of (α)

Figure 14. Raman Scattering from a ‘Spherical Top’ Molecule Vibrating at its Breathing Frequency.

As the induced dipole lies in the (xy) plane the oscillating dipole will emit radiation that is plane polarized. Thus, for the breathing frequency the Raman line will be completely polarized. For any other type of vibration the polarization ellipsoid will not be spherical and the axis will be randomly oriented and, thus, the scattered radiation will no longer be polarized. This phenomenon can be observed and measured employing the apparatus shown in figure 15.

Figure 13. A Raman spectrometer Fitted with Polarization Facilities.

The apparatus is very similar to the Raman instrument previously described except that a polarizing unit is placed in line with the laser radiation to produce light polarized in the plane of the paper. In addition a polarizing filter is placed in line with the scattered light that can pass either light polarized in the plane of the paper or set at right angles and only accept light that is not polarized in the plane of the paper. As a consequence, in the first setting the total strength of the radiation polarized in the plane of the paper plus all other scattered light (IT) will be measured. In the second setting only scattered light that is not polarized in the plane of the paper (IS) will be measured. Thus, taking the previous example, the proportion of scattered light that arises form the breathing frequency (α) will be given by,

α = IS/IT

and this will be a measure of the degree of polarization of that line. This principal of identifying and measuring certain types of energy can be extended to other substances and those interested and requiring further information the books listed in the references are recommended.

Raman techniques are recommended over infrared techniques for vibrational measurements as the scattered light occurs in the UV and Visible region. Consequently, simple construction materials such as quartz and glass can be used and the more difficult materials such as sodium chloride and other IR transparent materials are avoided. In addition water strongly absorbs in the IR whereas water only produce weak Raman scattering and so aqueous solutions can be easily examined. As result Raman Spectroscopy lends it self to the examination of biological materials.

An example of the application of Raman Spectroscopy is given by the Spectrum of myoglobin depicted in figures 14 and 15.

Figure 14. The Raman Spectrum of myoglobin .

There are a number of Raman modes exhibited arising mostly from the protein chains. myoglobin contains iron that is capable of reversibly bonding oxygen and carbon dioxide but more strongly carbon monoxide. Consequently myobloblin can carry gasses around the body for biosynthetic purposes and carry waste gases away. It follows that myoglobin is essential in natural muscle function. It is also responsible for the toxic properties of carbon monoxide. Details of the iron-carbon stretching region for myoglobin bound to carbon monoxide (between 400 and 560 cm-1) are shown in figure 15.

Figure 15. Details of the Iron-Carbon Stretching Region for myoglobin bound to Carbon Monoxide

The dotted curves are the result of the computer deconvolution of the main envelope and exposes the presence of several peaks which indicate that carbon monoxide may exist in different forms in different myoglobin molecules.


 

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.

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