Reflection refractometers

Differential refractometers detect the changes in refractive index of eluate when compared with the refractive index of the mobile phase. Four principles are exploited in the construction of differential refractometers deflection of light passing the boundary surface between the substances with different optical density deflexion refractometers), and subsequent light scattering in case of multiple refraction refractometers based on Christiansen effect), the Fresnel law of light reflection reflection refractometers) and, finally, the interference of light interferometers). 

In Abbe s double prism method a thin ho-meotropically oriented nematic film is placed between the hypotenuse faces of two rectangular prisms. Using light polarized parallel and perpendicular to the optical axis, respectively, the principal refractive indices and can be obtained directly from the critical angle of total reflection, provided that the refractive index of the prism is greater than that of the liquid crystal [2, 8-10]. In some cases the director of the nematic phase was oriented uniformly parallel to the prism planes [11-14]. Special variants of the total reflection method have been described [3, 4, 15, 16]. A total reflection refractometer of the Abbe type has also been used to determine n, n and An in the infrared region between 2.5 and 14 p.m [17]. 

The Fresnel refractometer which measures the change in the fractions of reflected and transmitted light at a glass-liquid interface as the refractive... 

Deviation refractometers are the most commonly used. This version of the DRI measures the deflection in the location of a light beam on the surface of a photodiode by the difference in refractive index between the polymer solution and pure solvent. The Fresnel-type refractometers operate on the principle that the intensity of light reflected from a glass-liquid interface is dependent on the incident angle and the RI difference between the two phases. The deviation and Fresnel detectors typically have cell volumes of 5 to 10 pi, detection limits of about 5 x 10-6 refractive index units (RIU), and a range of 10 7 to 10 3 RIU.156 The deflection-type DRI is relatively insensitive to the buildup of contaminants on the sample cell and is therefore of special utility in laboratories that process large numbers of samples, such as industrial laboratories. 

Fig. 18.4, designates the optical path for the upper prism in Abbe Refractometer. When a ray of light passes from the liquid medium and enters the upper prism, it gets refracted by an angle 0 between the lower face of the prism and the normal, an angle P between the emerging refracted ray at the upper face and the normal, and finally an angle a between the reflected ray at the upper face and the normal. Thus, we have ... 

A typical arrangement for an on-line process refractometer is shown in Fig. 6.45. Besides the prism, which is in contact with the process liquid, there is commonly a light source, a beam collimator and a photo-conductive sensor. As the nature or concentration of the process liquid varies, the critical angle changes, so moving the internally reflected collimated beam of light across the photo-detector. The latter is divided into two sections, one of which is arranged to receive a constant amount of... 

Refractometry can be used to determine the composition of a copolymer. In addition, differential refractometry has been used to study micellization in dilute block copolymer solutions (Tfizar and Kratochvfl 1972). The refractive index (n) is obtained in an Abbe refractometer via measurements of the critical angle for external reflection. The refractive index increment dn/dc, where c is the polymer concentration, can be related to the molecular weight of particles in solution. Further details of the method are provided by Pepper and Samuels (1989). 

The Waters Assoc. R401 differential refractometer is based on an optical-deflection design rather than on light reflection. This permits the use of a single cell throughout the refractive-index range of 1.00-1.75. The system also has a wider dynamic range of linearity for quantitation than the Fresnel-type refractometers. 

Reflective index was studied by abbe refractometer, and result is 1.53 at 589 nm, and its birefringence is 0.0001, which indicate that this film is not anisotropic. And the film is insulant, since its volume resisitivity is 1 x 10 Qcm, and its dielectric constant is 2.8. 

Most refractometers operate on the concept of the critical angle 4> it this is the angle for which (or 4> ) is exactly 90° (see Fig. 9b). A ray in the medium with any greater angle will be totally reflected at an equal angle as shown in Fig. 9c. The index of refraction is given in terms of the critical angle by... 

The most precise type of refractometer is the immersion refractometer. It contains a prism fixed at the end of an optical tube containing an objective lens, an engraved scale reticule, and an eyepiece. It also contains an Amici compensating prism (see below). In use, the instrument is dipped into a beaker of the liquid clamped in a water bath for temperature control. A mirror in the bath or below it reflects light into the bottom of... 

Differential refractometers operate by utilizing one of the following principles. In the first type, the measurement is based on an optical displacement of the beam (Figure 19-43). A mirror controls the reflection of the light beam. The light passes through the divided cell the first time without being reflected, and this then is followed by a reflected beam. If there is a difference in the solutions, a deflection will occur. The deflection is the sum of the deflection in the two cells. 

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