Reflection from liquid-cell

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. 

Part of the incident beam is reflected at each optical boundary (air-sample boundary in the case of a free-standing thin sample or air-window/window-sample boundaries in the case of a liquid cell), even at normal incidence. Of particular importance is that part of the radiation which undergoes multiple reflections at the two opposite surfaces of a plane-parallel sample. The double-reflected beam can interfere with the original beam, which results in sinusoidal type periodical features in the background of the spectrum (Fig. 6.5). Such features usually cause difficulties during evaluation of spectra. On the other hand, the interference provides access to the effective thickness of the sample as well as the optical quality of its boundaries (deviations from plane-parallelity cause reduced amplitudes of the interference fringes). The effective sample thickness can be calculated according to ... 

Ellipsometry is an optical technique that detects the change of the polarization state when light is reflected from a surface. For rather simple systems like transparent films on reflecting substrates, film thickness and refractive index can be determined with high accuracy. More complicated samples (e.g., multilayer structures or layers with a graded index of refraction on a reflective carrier) can be characterized with a sufficient set of independent experimental data obtained for multiple angles of incidence and/or multiple wavelengths (spectroscopic ellipsometry). With a liquid cell, ellipsometry can be performed also in aqueous environments. 

The second type of differential refractometer is based on Fresnel s law, which states that the amount of light reflected from a glass-liquid interface is dependent on the angle of incident light and the refractive index difference between the two phases. The two collimated light beams are focused on to the sample and reference cells on the face of the prism. The intensity of the reflected light is a function of the refractive index of the liquid in the cells and is measured by the twin element photodetector. Milton Roy/LDC manufacture a detector of this type. 

Solvents employed in paint manufacture can be qualitatively and quantitatively determined using liquid cells or from the spectra obtained from a few drops of the solvent, previously distilled, deposited between two crystals of an alkyl halide (in this case, for quantitative purposes an internal standard or the bands quotient approach must be employed). From these spectra the presence of several solvents in the formulation can be established and, if the mixture is not too complex, quantitative determination of all solvents can be carried out without chromatographic separation. Attenuated total reflectance (ATR) measurements offer a simple and fast alternative to the use of transmittance cells for qualitative and quantitative analysis. For this purpose, the solvent fraction obtained from the paint is placed over the surface of an ATR crystal (usually ZnSe). Figure 4 shows the spectra obtained for pure solvents and the solvent fraction of one polyester type paint. 

Cell thickness-dependence of the reflection of a cholesteric liquid crystal in the planar state. The pitch of the liquid crystal is P = 350 nm. The refractive indices of the liquid crystal are tig = 1 -7 and = 1.5. The liquid crystal is sandwiched between two glass plates with the refractive index = 1.6. The incident light is circularly polarized with the same helical handedness as the liquid crystal. Neglect the reflection from the glass-air interface. Use two methods to calculate the reflection spectrum of the liquid crystal with the following cell thicknesses P, 2P, 5P and lOP. The first method is the Berreman 4x4 method and the second method is using Equation (2.186). Compare the results from the two methods. 

Liquid NIRA differs from NIR applications for ground or powdered samples in a few key ways. Rather than utilizing tme reflectance measurements, liquid analysis involves transmission or a combination of transmission and reflectance sometimes referred to as transflectance [1]. Another way in which liquid measurement differs from the measurement of powders involves the introduction of the sample to the instrument. Preparation of a liquid sample may involve dilution, suspension, or homogenization prior to sample introduction. A different type of sample cell, usually a flow cell of some type, is then required. 

---