Compensators, polarized light

Fig. 1. Typical locations for CAM components, showing the photometer, 1 filter wheel, 2 monochromator, 3 shutter and aperture unit, 4 beam splitter, 5 accessories for polarized light such as a rotary analyzer and a compensator, 6 beam splitter for epi-excitation fluorescence, 7 objective lens, 8 stage, 9 substage condenser, 10 condenser aperture, 11 polarizer, 12 field aperture for photometry, 13 shutter, 14 primary illuminator, 15 arc lamp, 16 shutter, 17 monochromator, 18 filter wheel, 19 and ocular, 20.

This particular example represents one class of stereoisomers known as enantiomers, which may be defined as two molecules that are mirror images but are nonetheless nonsuperimposable. Such molecules are said to possess opposite configuration. If these isomers are separated (resolved), the separate enantiomers have been found to rotate the plane of plane-polarized light. This phenomenon of optical activity has been known for well over a century. A 50-50 mixture of two enantiomers is optically inactive or racemic, since the rotation of light by one enantiomer is precisely compensated by the rotation of tight in the opposite direction by the other enantiomer. 

If fibers remain unidentified after examination by phase contrast, polarized light, compensators, or measurement of angle of extinction, the fibers can be removed from the membrane for dispersion stain analysis by ashing, particle picking, or dissolving the membrane. 

Asbestos, quartz or other minerals can be analyzed by consideration of mineralogical principles and crystal systems. Polarized light, compensation plates, measurement of angles of extinction and dispersion staining are useful techniques. Optical behavior of a mineral is related to the internal crystal structure of the mineral. Tables of optical constants are useful for mineral identification. The microscope is a powerful tool for analysis that should not be overlooked by the industrial hygiene chemist. 

The plane-polarized light pulses characteristic of mode-locked lasers also provide an ideal excitation source for time-dependent fluorescence depolarization studies although conventional excitation sources can be used. If the rotational relaxation time of the excited molecule is comparable to its fluorescence decay time, then the vertical (I ) and horizontal (Ix) components of the fluorescence decay observed through suitable polarizers following excitation by polarized li t pulses, may be analysed to provide information concerning the size and motion of die molecule and Sect. 5. However, if only the true fluorescence decay characteristics are of interest it is necessary to compensate for these emission anisotropy effects Perhaps the simplest technique is to analyse only that component of fluorescence emitted at 54.7° to the direction of pdarization of the excitation source, the so-called magic-angle ... 

The convenhonal spectrometer consisted of a Coderg double monochromator equipped with a cooled PMT and was described in detail by Brooker et al. (1994). A 1 W laser was required to obtain spectra with adequate signal to noise ratio. A half-wave plate controlled the polarization of the incident beam. The 90° scattered light was analyzed with Polaroid films with accepted parallel or perpendicular polarized light. A quarter wave-plate in front of the entrance slit served to compensate for grating polarization preference. 

Figure 17.1.13 Schematic layout of one type of ellipsometer. Linearly polarized light (P) is incident on the sample (5). Reflection produces elliptic polarization (E), which is restored to linear polarization (A ) by the compensator (C). The analyzer (A) is adjusted to achieve extinction. [From R. H. Muller, Adv, Electrochem. Electrochem. Engr., 9, 167 (1973), with permission.]...

Several difl ereni types of cllipsometers arc available commercially. The earliest type was the null-type ellipsometer in which a circularly polarized incident beam was reflected off the sample surface onto an analyzer. The incident-beam polarization stale was chosen by a polarizer and compensator so that linearly polarized light was obtained after reflection. 1 he analyzer was then rotated until it was perpendicular to the polarization axis of the light coming from the sample as indicated by a minimum in the light iniciisiiy. Some instruments today still use the null principle, bui they are computer controlled and have charge-coupled-device (C (-I)f cameras as detectors. 

When two enantiomers are present in equal quantities, that is in a 1 1 stoichiometry, it is termed a racemic compound or racemic mixture. Such a mixture has no effect on polarized light since, through compensation, each enantiomer rotates the plane of polarization by the same amplitude and in the opposite sense. A racemic mixture can occur, particularly in the crystalline state, in three different forms. This is an important point since the possibility, or not, of directly separating the two enantiomers depends on the nature of the racemic mixture in the solid state. This separation of enantiomers, or resolution, is an essential task for the chemist who is interested in chirality and obtaining enantiopure compounds. 

This achiral material is termed a meso compound. This situation had been predicted by Le Bel. He considered that two identical chiral groups could compensate for each other and so cancel out the effect of rotation of plane polarized light. 

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