Instrumental dispersion, effect

Other laboratory studies conducted at IRCOM have compared the coupler and integrated optics in the visible (670 nm) in the frame of the ISTROG instrument (Huss et al., 2001 Fig. 18). The high dispersion effect in this spectral... 

Rotational viscometers often were not considered for highly accurate measurements because of problems with gap and end effects. However, corrections can be made, and very accurate measurements are possible. Operating under steady-state conditions, they can closely approximate industrial process conditions such as stirring, dispersing, pumping, and metering. They are widely used for routine evaluations and quality control measurements. The commercial instruments are effective over a wide range of viscosities and shear rates (Table 7). 

For a more accurate representation of the instrumental distortion effects see Section II.G of Chapter 2. The approximation given by Eq. (14) is convenient and for high-dispersion instruments observing weakly absorbing spectral lines it is often accurate enough for a number of experimental uses. 

Figure 4.21. Effect of instrumental dispersion on gradient analysis on a short 2-mm column, (a) Analysis on a standard HPLC system, (b) Analysis of the same sample on a low-dispersion system (Waters Acquity). Diagram courtesy of Waters Corporation.

Photomultipliers are used to measure the intensity of the scattered light. The output is compared to that of a second photocell located in the light trap which measures the intensity of the incident beam. In this way the ratio [J q is measured directly with built-in compensation for any variations in the source. When filters are used for measuring depolarization, their effect on the sensitivity of the photomultiplier and its output must also be considered. Instrument calibration can be accomplished using well-characterized polymer solutions, dispersions of colloidal silica, or opalescent glass as standards. 

From this equation it can be seen that the depth of penetration depends on the angle of incidence of the infrared radiation, the refractive indices of the ATR element and the sample, and the wavelength of the radiation. As a consequence of lower penetration at higher wavenumber (shorter wavelength), bands are relatively weaker compared to a transmission spectrum, but surface specificity is higher. It has to be kept in mind that the refractive index of a medium may change in the vicinity of an absorption band. This is especially the case for strong bands for which this variation (anomalous dispersion) can distort the band shape and shift the peak maxima, but mathematical models can be applied that correct for this effect, and these are made available as software commands by some instrument manufacturers. 

Electrostatics in Non-Aqueous Media. A popular misconception in studies of non-aqueous dispersions concerns electrostatic effects. Because these are more difficult to measure than in aqueous media, there has been a general tendency to ignore them completely. However, the few investigators who have measured zeta-potentials or electrodeposition with these systems have become convinced of their importance. With the advent of modern commercial instrumentation for zeta-potentials in non-aqueous media it is to hoped that these effects will be measured rather than ignored. 

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