Raleigh Scattering If a strong beam of light is passed through a transparent, gaseous, or liquid medium containing solid or liquid particles, or even molecules of extremely high molecular weight, the light is scattered away from the direction of its incident path. The scattering is due to interference effects that arise from the density fluctuations in the scattering medium (i.e. the presence of particles or very high molecular weight molecules). There are two types of light scattering; one in which the wavelength of the scattered light differs from that of the incident light and that is called Raman Scattering; the other is when the scattered light has the same wavelength as the incident light and this scattering process is called Raleigh Scattering. In Raleigh scattering, the intensity of the scattered light is proportional to the product of the intensity of the incident light, an attenuation constant, a function of the refractive index and the Raleigh Constant. Now the Raleigh constant is a somewhat involved function of the molecular weight of the scattering substance, and so, a measurement of the intensity of the scattered light can give a value for the molecular weight. The scattering phenomena is used in a number of liquid chromatography detectors.

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Author: RPW Scott Book:Liquid Chromatography Detectors
Section:HPLC-Detectors   Evaporative-Light-Scattering

of the Evaporative Light Scattering Detector The atomizing gas can be air or, if necessary, an inert gas. The suspended particles pass through a light beam and the scattered light viewed at 45o to the incident light beam by means of a pair of optical fibers. The scattered light transmitted through the fibers is sensed by a photomultiplier and the output electronically processed and passed either to a computer data acquisition system or to a potentiometric recorder. A diagram of the light scattering detector is shown in figure 48. Theoretically, the detector responds to all solutes that are not volatile and as the light dispersion is largely Raleigh scattering, the response should be proportional to the mass of solute present; as a consequence, it is sometimes referred to as the mass detector. For a linear response, the droplet size must be carefully controlled as it also determines the particle size of the dried solutes

HPLC-Detectors   Evaporative-Light-Scattering