Dynamic Light Scattering Instrumentation for classical 90 degree scattering measurements

A conventional dynamic light scattering instrument is shown in figure 1. In most instruments, a monochromatic coherent He-Ne laser with a fixed wavelength of 633nm is used as the light source, which converges to a waist of focus in the sample, by use of a focusing lens. Light is scattered by the particles at all angles. However, a dynamic light scattering instrument with one detector only detects the scattered light at one angle and this, historically has usually been 90°. The intensity fluctuations of the scattered light are converted into electrical pulses, which are fed into a digital correlator. This generates the autocorrelation function, from which the appropriate data analysis is performed.

Why 90 degrees?

This arrangement is inherently lower sensitivity than the patented Malvern NIBS optics, and lower cost to implement. However for polydisperse samples, either because the materials have a broad distribution like many emulsions, or if they have a number of modes, such a main peak and aggregates, then the only way to produce the identical mean size for direct comparisons, is to use the same scattering angle. This can be important for project and QC continuity.

The standard APD and size optics in the Malvern Zetasizer Nano S90 and Nano ZS90 have been optimised to enable measurements of suitable samples as small as 1 nanometre and to exceed the capability of classical 90 degree measurement systems.