Liquid Chromatography Detectors - Liquid Light Scattering Detectors > The Multiple Angle Laser Light Scattering (MALLS) Detector > Page 81
Theory can provide explicit functions for (a) and (b) but these constants are obtained in practice by the use of calibrating standards of known molecular weights and molecular radii. In addition, a photocell will not have precisely the same response to low light intensities and calibration procedures are also necessary to account 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 (miniDawnŽ)
This device (the miniDawnŽ), 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 continuously sampled at three different angles during the passage of the solute through the cell.