Optical Configurations

Photoluminescence finds its greatest strengths as a qualitative and semiquantitative probe. Quantification based on absolute or relative intensities is difficult, although it is useful in applications where the sample and optical configurations may be carefully controlled. The necessary conditions are most easily met for analytical applica-... 

In normal carbonium-ion chemistry, reaction proceeds from a precursor with a tetrahedral carbon capable of asymmetry hence, the stereochemistry of displacement in an aliphatic system can be ascertained by observation of the fate of the chiral center from reactant to product. An ethylenic system, of course, has no such chiral center, and hence there can be no change in optical configuration as the reaction proceeds. However, the stereochemistry of vinylic displacement and hence the symmetry and geometry of the intermediate can be... 

A subsequent detailed analysis of the permanganate oxidation of the tertiary hydrogen atom of 4-phenylvaleric acid in 2.5 M potassium hydroxide solution supports the caged radical mechanism. The reaction order is two overall, A h/ d is ca. 11.5, ring substitution has little elfect on the rate (p 0) and the oxidation proceeds with a net 30-40 % retention of optical configuration. 

V. Galiatsatos, R.O. Neaffer, S. Sen and B.J. Sherman, Refractive index, stress optical coefficient, and optical configuration parameter of polymers. In J.E. Mark (Ed.), Physical Properties of Polymers Handbook, Springer-Verlag, New York, 1996, p. 535. 

The biosensing configuration described above is referred to as a sandwich assay7 but a similar optical configuration can be used in a variety of other biosensing applications, e.g., monitoring of DNA hybridisation events. 

Compounds having the same optical configuration show similar Cotton effects. If the absolute configuration is known (for example, from x-ray diffraction) for one optically active compound, a similar Cotton effect exhibited by another compound indicates that it has the same optical configuration as the known. In other words, if two compounds give electronic transitions that show Cotton effects that... 

In this reaction, the nitronate is transformed into the 3 -hydroxycarbonyl fragment with complete retention of the stereo- and optical configuration. If the generality of this procedure will be demonstrated, it will play an important role in chemistry of cyclic nitronates. 

Optical computation, 6 67-68 Optical configuration, amino acids, 2 564-565... 

Figure 1 Diagrams showing the essential electron-optical configurations used for various imaging modes in CTEM and STEM as seen by two points A and B on the sample, (a) CTEM axial bright field, (b) CTEM tilted dark field, (c) CTEM hollow cone dark field, and (d) STEM with bright field and annular dark field detectors.

The proportionality factor k depends on several parameters, in particular on the optical configuration for observation (i.e. the solid angle through which the instrument collects fluorescence, which is in fact emitted in all directions) and on the bandwidth of the monochromators (i.e. the entrance and exit widths see Chapter 5). 

The light source can be a xenon lamp associated with a monochromator. The optical configuration should be carefully optimized because the electro-optic modulator (usually a Pockel s cell) must work with a parallel light beam. The advantages are the low cost of the system and the wide availability of excitation wavelengths. In terms of light intensity and modulation, it is preferable to use a cw laser, which costs less than mode-locked pulsed lasers. 

Fig. 10.44. Examples of optical configurations for passive optical sensors.

In the passive mode, the optical device measures the variation in fluorescence characteristics (intensity, lifetime, polarization) of an intrinsically fluorescent analyte. The optical device can have different optical configurations involving in most cases an optical fiber (passive optode) (Figure 10.44). 

In the active mode, the optical device uses for optical transduction the changes in the fluorescence characteristics of a fluorescent molecular sensor (as described in the preceding sections) resulting from the interaction with an analyte3. The two main optical configurations are ... 

Under the same optical configuration, FCS (Fluorescence Correlation Spectroscopy) measurements (see Section 11.3) can be carried out on samples at the singlemolecule level under conditions where the average number of fluorescent molecules in the excitation volume is less than 1. It should be noted that at low fluorophore concentrations, the time required to obtain satisfactory statistics for the fluctuations may become problematic in practical applications (e.g. for a concentration of 1 fM, a fluorophore crosses a confocal excitation volume every 15 min). 

Figure 7.8. TIRF optical configuration for an inverted microscope detail of the sample chamber region.

Regardless of the optical configuration chosen, the following suggestions may be helpful ... 

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