Liquid Chromatography Detectors - Liquid Light Scattering Detectors > Page 76

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.

 

In the LALLS detector, the scattered light is measured at a very small angle to the incident light (virtually 0o), and consequently the signal can be affected by scattering from contaminating particulate matter  present as impurities in the eluent. This can cause significant noise, and thus, reduce the detector sensitivity. Discussions on the subject have been given by Wyatt (48) and some early experimental results reported by D. T. Phillips(49).

The ratio of the intensity of the light scattered at an angle (f), (If) to the intensity of the incident light (Io), for Rayleigh light scattering is given by