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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

index is negative and consequently, the insertion of a liquid in the laser beam produces a concave lens that results in beam divergence. Buffet and Momis (27) used the thermal lens effect to develop a small volume detector, a diagram of which is shown in figure 20.   Figure 20 The Layout of a Thermal Lens Detector   The device consists of a heating laser, the light from which is passed directly through the sample via two lens and a half mirror. Another laser, the probe laser, passes light in the opposite direction through one lens, through the sample to the half mirror where the light is reflected onto a photocell. A filter and a pinhole screen is placed between the mirror and the photo-cell to remove the heating laser light. When an absorbing solute is eluted from the column through the cell, a thermal lens is produced causing the probe light to diverge, and the intensity of the light passing through the pinhole and on to the photocell is reduced

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 through some measuring attenuators and a calibrating attenuator shutter and then through the cell. An annular mask is situated between the cell and the relay lens and only allows light scattered in the cell at a low angle to pass to the relay lens. Between the annular shutter and the relay lens is a safety attenuator that ensures that

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

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

Courtesy of  LDC Analytical, Thermo Instruments Corporation.   Figure 51  Optical Diagram of a Low Angle Laser Light Scattering Detector   Between the forward detector lens and the rear detector lens is A filter holder and an analyzer/polarizer. Finally the light is focused through a sensor aperture to an opal diffuser that spreads the scattered light through a red filter  and onto the photo- multiplier. The device is frequently used with a refractive index detector in series to coincidentally measure the refractive index of the eluent. This is necessary to calculate (K) from the