Excitation purity

Another method of expressing color is in terms of luminance, dominant wavelength, and excitation purity. These latter are roughly equivalent to the three recognizable attributes of color lightness, hue, and saturation. Lightness is associated with the relative luminous flux, reflected or transmitted. Hue is associated with the sense of redness, yellowness, blueness, and so forth. Saturation is associated with the strength of hue or the relative admixture with white. The combination of hue and saturation can be described as chromaticity. 

Fig. 3-3. Color designation in the CIE-Yxy system. Colors lie within the solid lines resembling a shoe sole. Colors within that plane may be given either by the Cartesian coordinates x and y, or by the dominant wavelength, [nm], and the excitation purity, Pg. The lightness, Y, is normal to the plane. (Scheinost and Schweitmann 1999 with permission)...

Purity, colorimetric n. Ratio of the luminance of the spectrum light, in mixture with the specified achromatic light required to match the light being described, to the luminance of the color itself. It is distinguished from excitation purity by the abbreviation Pc-... 

Purity, excitation n. Ratio of the straight line distance on a CIE chromaticity diagram between the chromaticity point of the sample and the achromatic or illuminant point on the diagram, to the linear distance between the point of intersection of this line with the spectrum locus and the illuminant point. It is properly designated as Pg, but is frequently abbreviation simply as P The excitation purity, then, describes the relative distance from the neutral point and roughly corresponds in concept to the psychological description of saturation or chroma. 

Pigment /titanium dioxide ratio 10 90. Tristimulus value Chromatidty coordinates Munsd renotation [nmj Excitation purity pq... 

Quality Control. The spectrometer is the most suitable instmment for determining most low level residual impurities. ASTM E414 is the standard method for the measurement of impurities in copper by the briquette dc-arc technique (65). In this method, the sample in the form of chips, drillings, or powder is briquetted and excited in a d-c arc opposite a high purity copper rod. Impurities in the ranges noted can be measured ... 

Requirements of Standards. The general requirements for luminescence standards have been discussed extensively (3,7-9) and include stability, purity, no overlap between excitation and emission spectra, no oxygen quenching, and a high, constant qtiantum yield independent of excitation wavelength. Specific system parameters--such as the broad or narrow excitation and emission spectra, isotropic or anisotropic emission, solubility in a specific solvent, stability (standard relative to sample), and concentration--almost require the standard to be in the same chemical and physical environment as the sample. 

The most exciting enantioselective photochemical conversion of a a-oxoamide to a P-lactam has been found in the case of N,N-diisopropylbenzoylformamide (96) which gives P-lactam 97. In the photocyclization of plain 96 in the solid state, optically active P-lactam 97 of high optical purity was obtained in high chemical yield. Thus no optically active host compound is necessary for the enantioselective reaction 48>. 

In order to obtain nearly absolute purity of the spectra of these xanthophylls, it was necessary to calculate the difference Raman spectra. Therefore, for zeaxanthin, two spectra of samples, one containing violaxanthin and the other enriched in zeaxanthin, were measured at 514.5 nm excitation. After their normalization using chlorophyll a bands at 1354 or 1389 cm-1, a deepoxidized-minus-epoxidized difference spectrum has for the first time been calculated to produce a pure resonance Raman spectrum of zeaxanthin in vivo (Figure 7.10b). A similar procedure was used for the calculation of the pure spectrum for violaxanthin. The only difference is that the 488.0nm excitation wavelength and epoxidized-minus-deepoxidized order of spectra have been applied in the calculation. The spectra produced using this approach have remarkable similarity to the spectra of xanthophyll cycle carotenoids in pure solvents (Ruban et al., 2001). The v, peaks of violaxanthin and zeaxanthin spectra are 7 cm 1 apart and in correspondence to the maxima of this band for isolated zeaxanthin and violaxanthin, respectively. The v3 band for zeaxanthin is positioned at 1003 cm-1, while the one for violaxanthin is upshifted toward 1006 cm-1. 

FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much more sensitive than the ultraviolet method but tends to be prone to error and is less convenient to use. Small quantities of impurities may lead to serious distortions of both emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and fluorescence excitation spectra can yield useful information on the purity of an FBA. Different samples of an analytically pure FBA will show identical absorption and excitation spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been developed for the quantitative estimation of FBAs and other fluorescent additives present on a textile substrate. The procedure was demonstrated by measuring the fluorescence intensity at various excitation wavelengths of moving nylon woven fabrics treated with various concentrations of an FBA and an anionic sizing agent. It is possible to detect remarkably small differences in concentrations of the absorbed materials present [67]. 

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