![]() 4b includes additionally the contribution from the frequency changes of light scattered by PSS molecules under an electrical field of 40 V/cm.įigure 5 shows heterodyne power spectra obtained by Fourier transform of the autocorrelation functions shown in Fig. 4a is a result of interference between the scattered light and the modulated reference light. The oscillating correlation function shown by Fig. The frequency of light scattered by particles undergoing electrophoresis is shifted by the amount of the Doppler effect, υ D the dynamic viscosity of the surrounding fluid.įigure 4 shows two examples of heterodyne autocorrelation functions of scattered light from sodium polystyrene sulfate solution (NaPSS MW 400,000 4 mg/mL in 10 mM NaCl). This results in faster measurements, higher reproducibility even at low applied electric fields as well as higher sensitivity of the measurement. Thus, its beat frequency is solely the modulation frequency and is therefore independent of the electrophoretic motion of the particles. An extra modulator detects the interference between the modulated and unmodulated laser light. But any non-linearity of the modulator or any change in the characteristics of the modulator with time will mean that the generated sine wave will no longer reflect the real conditions, and the resulting zeta-potential measurements become less reliable.Ī further development of the PALS technique is the so-called "continuously monitored PALS" (cmPALS) technique, which addresses the non-linearity of the modulators. The application of large fields, which can lead to sample heating and breakdown of the colloids is no longer required. This rate is compared with a mathematically generated sine wave predetermined by the modulator frequency. Phase-analysis light scattering (PALS) is a method for evaluating zeta potential, in which the rate of phase change of the interference between light scattered by the sample and the modulated reference beam is analyzed. This modulation scheme is employed by the heterodyne light scattering method, too. Modulators used in ELS may include piezo-actuated mirrors or acousto-optic modulators. Such shifting may be referred to as frequency modulation or, more colloquially, just modulation. This limitation can be overcome by shifting the frequency of one of the beams relative to the other. Hence, the sign of the zeta potential cannot be ascertained. Instead, only the magnitude of the velocity (i.e., the speed) can be determined. If the frequencies of the intersecting laser beams are the same then it is not possible to resolve the direction of the motion of the migrating particles. developed an ELS instrument of heterodyne-type optics that is now available commercially. The observed spectra resemble each other. ![]() The frequency shifts from both types of optics obey the same equations. The scattered light from the particles, which migrates inside the fringe, is intensity-modulated. Those cross inside the electrophresis cell at a fixed angle to produce a fringe pattern. In a fringe optics ELS instrument, a laser beam is divided into two beams. Ware and Flygare developed a heterodyne-type ELS instrument, that was the first instrument of this type. There are two types of optical systems: heterodyne and fringe. The scattered light is detected by a photo-multiplier after passing through two pinholes. 11).Ī laser beam passes through the electrophoresis cell, irradiates the particles dispersed in it, and is scattered by the particles. Commercial heterodyne optical system of an electrophoretic light scattering instrument with a modulator (from Ref. Sample dilution is often necessary to eliminate multiple scattering of the incident laser beam and/or particle interactions.įig.3. The last set of calculations requires information on viscosity and dielectric permittivity of the dispersion medium appropriate electrophoresis theory is also required. Instruments for applying the method are commercially available from several manufacturers. The method is used for measuring electrophoretic mobility, from which zeta potential can then be calculated. With electrophoretic light scattering, oscillating electric field performs this function. With dynamic light scattering, Brownian motion causes particle motion. The frequency shift or phase shift of an incident laser beam depends on the dispersed particles mobility. JSTOR ( September 2017) ( Learn how and when to remove this template message)Įlectrophoretic light scattering (also known as laser Doppler electrophoresis and phase analysis light scattering ) is based on dynamic light scattering.Unsourced material may be challenged and removed.įind sources: "Electrophoretic light scattering" – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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