Dynamic Light Scattering Recap Part 3

In the last instalment of a three-part recap on how dynamic light scattering works, I would like to talk about the actual collection and processing of particle size data.

We have established that the essence of the dynamic light scattering method is irradiation of a sample with monochromatic light, generated by a laser, and then measurement of the resulting diffracted light. The particles under analysis, which are typically nanoparticles or polymer molecules, are held in suspension for this purpose.

Detection of the diffracted light is usually achieved using a photomultiplier, which is placed at right angles to the laser source. Laser light can be focused into the middle of the sample vessel, and saturation of the photomultiplier tube can be avoided, using collimating lenses.

In an ideal world there should be no contamination of the sample by particles that might affect the scattering of light. Filtration or purification of the dispersion is often carried out before measurement to address this issue. To avoid interaction between particles, which would disrupt the Brownian Motion on which the method depends, the sample is diluted to a low concentration.

Within the data gathered on intensity fluctuations there is a wide spectrum of Doppler-shifted frequencies. To make sense of it, we can use a digital correlator to compile the data for processing – rather than trying to measure directly. A correlator in a dynamic light scattering set-up compares the intensity of two signals, over a short time period, and works out how similar they are, using its correlation function.

I don’t propose to go into the mathematical formulae on which the calculations are based, but essentially they allow the correlator to assign values in relation to the speckled pattern that I talked about in part 1 of this recap.

If it compares the intensity signal at some point in the speckled pattern with itself after no time has elapsed, a perfect correlation will be observed and the correlator will assign a value of 1. Comparing the same intensity signal with another one, just a short time later, the correlator will find that there is less of a correlation and will assign a value below 1. In most of these speckled patterns the signal correlation falls to zero in about one to ten milliseconds. The timescale for the measurements therefore needs to be much faster – a matter of microseconds or even nanoseconds.

The fluctuations in intensity observed are directly related to the movement of particles in solution and, as discussed in part 2 of this series of articles, small particles move more rapidly than large ones – as predicted by the Stokes-Einstein equation. We therefore expect more rapid fluctuation in the intensity signal for small particles than for large ones, along with a faster rate in decrease of correlation. This, along with some further formulae, provides the basis for the correlator’s processing of data from dynamic light scattering.