INDEX effects

Figure 7.8 A scattering map in reciprocal space. Equal intensity contours are shown schematically, and the Ewald sphere is represented as a plane near reciprocal lattice points 0 and h. The dynamical diffraction from the specimen is displaced slightly from the relp and from the centre of the diffuse scatter by the refractive index effect...

This expression describes the corrections to the refractive index effect due to the cosmological expansion. For small z < 1, expression (35) yields... 

Measurements taken at wave lengths near 740 m/i and 860 are also uncertain, because of the abrupt changes in refractive index associated with such absorption bands. Instrumental inadequacies magnify the anomalous refractive-index effects associated with these bands. As a practical method, most laboratories determine an absorbancy-index curve similar to that shown in Fig. 2, which, thereafter, is used for correcting measurements (made against water in the reference cell) to the colorless sucrose standard. This curve is actually also a calibration of the instrument, and, as the two effects may not always be applicable, occasional anomalies may be encountered. 

Fused silica capillaries are the most popular capillaries and are commercially available with internal diameters ranging from 10 to 200 pun, although the most commonly used capillary dimensions are 25, 50, and 75 /xm I.D. and 350-400 /xm O.D. To minimize distorting refractive index effects and light scattering with on-column optical detection, the ratio of the outer... 

Viscosity and refractive index effects In-line degassing Flow programming Summary References Acknowledgment... 

FIGURE 7-29. Refractive index effects of water-methanol gradient. Column /aBondapak C]8,3.9 mm ID x 30 cm. Mobile phase 0-100% methanol in water with a linear change over 30 min. Flow rate 3 mL/min. Detector (a) UV at 254 nm, 0.08 AUFS (b) UV at 254 nm, 0.05 AUFS. 

The previous discussion is quantitatively valid only for the angles and indices of refraction presented, but the analysis and conclusions would qualitatively hold for a general combination of angles and indices of refraction. Also, in practice the uncertainty introduced by the refractive index effect requires that a range of ratio combinations be used in the three-angle discrimination test rather than two exact ratios, but the advantages are still present. The method can also determine refractive indices when inversed. 

Flow-injection analysis (FIA) is a technique for automating chemical analyses. The principles of FIA are reviewed here. Methods for applying FIA to the anayses of nitrate, nitrite, phosphate, silicate, and total amino acids in seawater are examined. Analyses of other nutrients, metals, and carbonate system components are also discussed. Various techniques to eliminate the refractive index effect are reviewed. Finally, several examples of the application of FIA to oceanographic problems are presented. 

It is important to note that the measurement should operate at least a few degrees above the critical angle in order to avoid a strong dispersion of the refractive index effect to the spectrum. 

The Schlieren effect in flow analysis has often been referred to as a refractive index effect, but both reflection and refraction are involved (Fig. 4.6). Indeed, it is a phenomenon more related to light reflection. 

G. Ham, Refractive Index Effect in Flow Injection Analysis. Anal. Proc., 18 (1981) 69. 

Because of refractive-index effects, an unretained solvent used to dissolve the sample— if different from the chromatographic mobile phase—often deflects the base-line when passing through an ultraviolet detector cell. This indicates the void volume or the void time. Consider the chromatogram in Figure 21.19. (a) Determine the capacity factors for each nitroaniline isomer, (b) Determine the selectivity factor for the m- and p-substituted isomers relative to the o-nitro-aniline. 

Although applications for column separation or preconcentration systems coupled to chemiluminescence determinations are few, published reports show no particular difficulties in such applications, except for the requirement of an adjustment of the chemical conditions of the eluate to suit the chemiluminescence reaction. Interferences due to refractive index effects are not likely to occur, owing to the often used spiral shape of the chemiluminescence flow-cell and to the fact that light emission is measured perpendicular to the direction of the flow. Therefore, column washing is usually not as important as for spectrophotometric applications, so that time-based sample loading manifolds such as those used for flame AAS may be used to advantage for improving the concentration, efficiencies. 

Leon-Gonzalez et al.[31] proposed an FI spectrophotometric method for the determination of Triton-type non-ionic surfactants based on their reaction with alizarin fluorine blue. An on-line ion-exchange column was incorporated in the system to eliminate interferences from ionic and amphoteric surfactants. In case of interferences from non-ionic surfactants, an on-line Amberlite XAD-4 adsorption column was used to retain selectively the Triton-type surfactant, which was subsequently eluted by ethanol. However, no information was given regarding interferences from refractive index effects at the ethanol/aqueous interface and their elimination. 

Note that there is a multiparameter trade-off between grating-based (interferometric) evaluation of displacement and direct interferometric evaluation. Typical grating-based systems have relatively short non-common paths, resulting in reduced sensitivity to time-varying index effects. There is, however, increased sensitivity to cyclic effects in grating production and Imig-range effects of coefficient of thermal expansion. A detailed discussion of this trade-off is beyond the scope of this article. 

It should be noted that eqn [57] strictly only applies to dilute gases. In condensed phases refractive index effects become important. 

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