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Liquid Light Scattering Detectors Light scattering detectors differ from evaporative light scattering detectors in that they respond to the light scattered by a polymer or large molecular weight substance present in the column eluent itself. The scattering is measured as it passes through an appropriate sensor cell while illuminated by a high intensity beam of light. The high intensity light source is achieved by the use of a laser (light amplification by the stimulated emission of radiation) that also generates the light at the appropriate wavelength for measurement. There are two forms of the detector: the low angle laser light scattering (LALLS) detector and the multiple angle laser light scattering (MALLS) detector. Both devices are commonly used but the multiple angle laser light scattering detector is more versatile as it provides molecular dimensions as well as the molecular weight of the eluted solute.  

The Evaporative Light Scattering Detector The evaporative light scattering detector, as its name implies, utilizes   a spray that continuously atomizes the column eluent into small droplets. These droplets are allowed to evaporate, leaving the solutes as fine particulate matter suspended in the atomizing gas.     Courtesy of Polymer Laboratories Inc.   Figure 48. A Diagram 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

Figure 52 Determination of Molecular Weight from Low Angle Light Scattering Measurements The detector sensitivity appears to be similar to that of the refractive index detector and with about the same linearity. However, the greatest advantage of this detector is that it can provide molecular weight data for extremely large molecules. The Multiple Angle Laser Light Scattering (MALLS) Detector The multiple angle laser light scattering detector differs from the low angle device, in that scattering measurements are made at a number of different angles, none of which are close to the incident light. This significantly reduces problems associated with light scattering from particulate contaminants. Data taken at different angles to the incident light allows the root-mean-square (rms) of the molecular radius  to be calculated in addition to the molecular weight of the substance. The relationship that is used is as

;                                            Now (c), (K), and (Rf) are either known or can all be calculated from known data and light scattering measurements; thus, by plotting  against (c) a straight line will be produced with  the intercept being . The Low Angle Laser Light Scattering Detector The optical system of the low angle laser light scattering detector produced by LDC Analytical of the Thermo Instruments Corporation is shown diagramatically in figure 51. To conserve space, a folding prism is used that allows the device to be contained to a reasonable size yet accommodate the length of the laser generator. Light from the laser passes through a diverging lens, through a chopper and then through the folding prism. On leaving the prism the beam passes

(14,15). Consider a cell packed with particulate material having the same refractive index as the mobile phase passing through. If a beam of light passes through the cell there will be little of no refraction or scattering. However, if the refractive index of the mobile phase changes, there will now be a refractive index difference between the mobile phase and that of the packing. As a consequence some light will be refracted away from the incident beam and the intensity of the transmitted light will be attenuated. Thus, if the transmitted light is focused onto an appropriate photocell, then any change in refractive index caused by the elution of a solute will produce scattering and a consequent change in electrical output. In practice, he optical dispersions of the media are likely to differ, and consequently the refractive index will only match at one particular wavelength. As a result the fully transmitted light will be largely monochromatic. Light of different wavelengths

for their different responses. The number of different angles of measurement differs with different instruments, and some measure the scattered light intensity at 16 different angles. In general, the more data points taken at different angles, the more precise the results will be. A diagram of a (MALLS) detector system which measures the light scattered at three different angles is shown in figure 53. Courtesy of Wyatt Technology Corporation   Figure 53 The Multiple Angle Laser Light Scattering Detector (miniDawn�)   This device (the miniDawn�), manufactured by Wyatt Technology Corporation, contains no mirrors, prisms or moving parts and the light paths are direct and not "folded". Light passes from the laser (wavelength 690 nm) directly through the sensor cell. Light scattered from the center of the cell passes through three narrow channels to three different photocells, set at 45o and 90o and 135o to the incident light. Thus scattered light is