Rayleigh interference method

Several methods based on Pick s second law are available to measure the diffusion coefficient. Among these are the following three classical methods the Schlieren method, the Gouy interference method, and the Rayleigh interference method. We describe here the Rayleigh interference method, for its application is also found in ultracentrifuge sedimentation. We use the classical Tiselius electrophoresis cells to illustrate (Figure 10.5). 

FIGURE 10.5 Rayleigh interference method for determination of diffusion coefficient. 

High-speed or meniscus-depletion equilibrium techniques have been attempted by employing scanning absorption optics, " and by using Rayleigh interference. Several values have been included for comparative purposes in Table III. In this method, the meniscus region is essentially depleted of sample (namely, zero concentration), and this simplifies the... 

Fluorescence is generally more sensitive to environmental factors than absorbance measurements. Signal intensity may be affected by pH, temperature, quenching, interfering substances, solvent, or interference from Rayleigh and Raman scattering. Many fluorescent species contain ionisable groups whose fluorescent properties are sensitive to pH. In some cases only one of the ionised species may be fluorescent. An example is the barbiturates which only fluoresce at elevated pH in the di-anionic form. The relationship of fluorescence intensity with pH should always be examined as part of the development of the method. 

The modified Rayleigh interferometer [54] is based on the same principle. Half of the beam (ordinary or extraordinary) is passed through the planar-oriented nematic cell and the other half transverses to the empty part of the cell. When the two beams are mixed an interference pattern results, from which the refractive indices n and n can be determined separately. In particular, changes in the refractive indices can be measured with high accuracy, using this method. 

Fluorimetry is essentially a trace method, since the emitted light is proportional to specie concentrations only at low concentration levels (10" -10 M). Concentration quenching usually occurs at higher levels, and linearity is destroyed. At the limits of detection, fluorescence can be drastically affected by the presence of fluorescing impurities, by any process which quenches fluorescence, or by interferences by Raman or Rayleigh scattering. 

---