Schlieren compensation

FIGURE 4.15 Schlieren compensation by dual-wavelength spectrophotometry. Top, middle and bottom sections refer to the analytical (512 nm), reference (800 nm) and net (AA) absorbance values respectively. The right and left columns refer to BCG + sucrose and pure sucrose solutions respectively. Other conditions are as in Table 4.4. Reprinted from Anal. Chim. Acta 234 (1990) 153, E.A.G. Zagatto, M.A.Z. Arruda, A.O. Jacintho, I.L. Mattos, Compensation of the Schlieren effect in flow-injection analysis by using dual-wavelength spectrophotometry, with permission from Elsevier (Ref. 128]). 

Although very effective for compensating the influence of the Schlieren effect, Eq. 4.7 holds only when the intensity of the Schlieren effect is not wavelength dependent, otherwise, correction factors should be added. As the transient mirrors established between fluid elements of different refractive indices are not ideal and the incident light is also partially refracted, the refraction angle is strongly wavelength dependent (Eq. 4.15). Hence, the use of Eq. 4.7 for Schlieren compensation may be subject to restrictions. Moreover, it cannot be directly applied for Schlieren compensation in flow systems with turbidimetric or nephelometric detection. 

E.A.G. Zagatto, M.A.Z. Arruda, A.O. Jacintho, I.L. Mattos, Compensation of the Schlieren effect in flow-injection analysis by using dual-wavelength spectrophotometry, Anal. Chim. Acta 234 (1990) 153. 

In a detailed study on the compensation of Schlieren effects using a diode-array spectrometer, Zagatto et al.[49 recommended the use of a dual-wavelength spectrophotometer as a best solution. Almost complete compensation of Schlieren signals as high as 1 absorbance has been achieved using this approach. The method has been used successfully by Ferreira et al.[28] in the spectrophotometric determination of zinc in plants, following an on-line column separation. This, however, calls for special instrumentation which is not always available in the laboratory. 

In most cases a reasonable degree of compensation of Schlieren effects may be achieved by taking the following measures ... 

Figure 10.20 shows polarizing microscopic photographs of the mixture of E44/chiral/w-azo-8 (80/9.8/10.2 wt%), changing to a Ch phase from a compensated nematic phase on UV irradiation. A schlieren texture in the initial state... 

To identify the antiferroelectric phase, texture observation of the homeotropic cells of racemic compounds is very effective. In the SmC phase, only the schlieren texture with four brushes is observable and that with two brushes is prohibited, because of the head-and-tail inequivalence of the C-director. In the SmCA phase, however, the schlieren texture with two brushes is sometimes seen, as shown in Figure 9.8 [18], [19]. The existence can be explained by taking into account a screw dislocation, as illustrated in Figure 9.9. The discontinuous change (7r-wall) of the C-director is compensated by the screw dislocation. This defect is a combined defect of a disclination and a dislocation, i.e., adispiration [18], [19]. 

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