Liquid Chromatography Detectors - Liquid Light Scattering Detectors > The Multiple Angle Laser Light Scattering (MALLS) Detector > Page 82

A continuous analog output is provided from the 90o sensor and all the sensors are sampled every 2 sec. The molecular weight range extends from 103 to 106 Daltons and the rms radii from 10 to 50 nm. The total cell volume is about 3 ml and the scattering volume is 0.02 ml. The detector has a sensitivity, defined in terms of the minimum detectable excess Rayleigh ratio of 5 x 10-8 cm-1. This is difficult to translate into normal concentration units but appears to be equivalent to a minimum detectable concentration of about 10-6 g/ml.

Figure 54. Calibration Curves

The relationship between the intensity of the scattered light, the scattering angle and the molecular properties are as follows:

                                     

where P(q) describes the dependence of the scattered light on the angle of scatter and the other symbols have the meanings previously attributed to them.

The relationship between the angle of scattering, (q), the molecular weight and the rms molecular radius of the solute is obtained using the appropriate equation. Employing suitable reference materials, graphs of the form shown in figure 54 can be constructed to evaluate constants (a) and (b) and thus permit the measurement of the molecular weight and molecular radius of unknown substances.