Refractive index, gradient

Working with different samples of the same polymer, other researchers have published conflicting values for the refractive index gradient in these solvents ... 

Using the original Hc2/r values, recalculate M using the various refractive index gradients. On the basis of self-consistency, estimate the molecular weight of this polymer and select the best value of dn/dc2 in each solvent. Criticize or defend the following proposition Since the extension of the Debye theory to large particles requires that the difference between n for solute and solvent be small, this difference should routinely be minimized for best results. 

Renn, C. N. and Synovec, R. E., Refractive index gradient detection of biopolymers separated by high-temperature liquid chromatography, /. Chromatogr., 536, 289, 1991. 

Fig. 2.14 The scheme of the cylindrical lens method for diffusion coefficient measurement (1) the source with the horizontal slit (2) the condenser supplying a handle of parallel beams (3) the cuvette with a refraction index gradient where the beams are deflected (4) the objective lens focusing the parallel beams to a single point (5) the optical member with an oblique slit and a cylindrical lens (6) the photosensitive material...

When acetic acid is diffusing from a 1.9 iV solution in water into benzene, spontaneous emulsion forms on the aqueous side of the interface, accompanied by a little interfacial turbulence. Results can be obtained with this system, however, if in analysing the refractive index gradient near the surface a correction is made for the spontaneous emulsion the rate of transfer is then in excellent agreement (57) with Eq. (20) (Fig. 6). Consequently there is no appreciable energy barrier due to re-solvation of the acetic acid molecules at the interface, nor does the spontaneous emulsion affect the transfer. With a monolayer of sodium lauryl... 

Hancock DO, Synovec RE. Microbore liquid-chromatography and refractive-index gradient detection of low-nanogram and low-ppm quantities of carbohydrates. Journal of Chromatography 464, 83-91, 1989. 

Since this optical system measures changes in the refractive index gradients in the solutions arising from concentration gradients of any of the solutes, standard diffusion... 

However, one anomalous feature of this technique is that there occurs an apparent rapid removal of material from the concentration gradient at the boundary, as evidenced by a reduction in the area under the Schlieren curve. For the standard PVP/dextran T10 system, we observed a reduction of 20 % during the 10 min after the formation of the initial boundary no further changes in the area occurred after this initial event. This reduction in area is not accompanied by the appearance of refractive index gradients elsewhere in the cell47 . The redistribution of material within the cell has been shown to occur by monitoring PVP 360 directly using absorption optics at 237 nm. 

The schlieren system of optics is an analytical method that is particularly well suited to following the location of a chemical boundary with time. It is routinely employed in ultracentrifuges and also in electrophoresis experiments, as we see in Chapter 12. Schlieren optics produces an effect that depends on the way the refractive index varies with position, that is, the refractive index gradient rather than on the refractive index itself. Therefore, the schlieren effect is the same at all locations along the axis of sedimentation, except at any place where the refractive index is changing. In such a region, it will produce an optical effect that is proportional to the refractive index gradient. The boundary between two layers is thus per-... 

The refractive index gradient (dn/dc). This is simply the local slope of a plot of the refractive index of a solution versus its concentration. 

The actual measurement of the refractive index of the solution poses no difficulty, but the evaluation of the refractive index gradient is more troublesome. The assumptions of the derivation of Equation (23) restrict its applicability to dilute solutions. The refractive index of a dilute solution changes very gradually with concentration hence a plot of n versus c, the slope of which equals dn/dc, will be nearly horizontal. Since the intensity ratio depends on the square of dn/dc, it is clear that successful interpretation of Equation (23) depends on the accuracy with which this small quantity is evaluated. Measuring the absolute refractive indices of various solutions and determining dn/dc by difference or graphically would introduce an unacceptable error. A more precise method must be used to measure this quantity. 

It was assumed, as in the first-order theory for the Gouy interference phenomenon (6), that as a first approximation only rays passing through equal values of the refractive index gradient on either side of the boundary center would arrive together at the slit image plane. Thus pairs of positions with equal dn/dx values (see upper part of Figure 1) were... 

Technically, the heterogeneity of skin and the presence of refractive index gradients likely impose some constraints to the accurate determination of imaging parameters. However, uncertainties in the determination of spatial resolution and axial location in transparent samples from which spectra are extracted, such as those described by Everall [21-23] and others [24, 25], are probably not important for highly opaque skin samples. We estimate the axial resolution to be 2-3 pm with the 785 nm excitation wavelength used in the current measurements. A study from this lab has suggested that errors in depth measurements are less than 15% which is probably adequate for most current purposes. 

The density profile (upper part of Figure 2) can be determined by measuring the refractive index gradient along the cell (lower part of Figure 2) and the correlation between the refractive index and the density for the gradient system (Figure 3)- This yields the density profile with an accuracy of about + 1 per thousand. 

Figure 2. Refractive index gradient and density profile in the cell of the ultracentrifuge in rapid density gradient centrifugation (N = 40.000 min1, centrifugation time t — to = 4 min, schlieren angle a = 65°)...

The change in the refractive index gradient (RIG) between adjacent laminar flow streams is employed as a MW sensor, as shown in Figure 7.19. A sample was mixed with a mobile phase in a PDMS chip. The two laminar flow streams... 

Costin, C.D., Synovec, R.E., A microscale-molecular weight sensor probing molecular diffusion between adjacent laminar flows by refractive index gradient detection. Anal. Chem. 2002, 74, 4558 1565. 

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