Dynamic Light Scattering Recap

Over the last couple of years we have looked deeply and widely into the subject of dynamic light scattering. I think it’s worth stopping, from time to time, to look again at the basic principles and restate them – perhaps in different words – to make sure they are understood.

By now you will be aware that dynamic light scattering – or photon correlation spectroscopy, as it is sometimes known – can be used to determine the size – or, more precisely, the hydrodynamic radius – of polymer, protein and colloidal particles in a solution. It can also be applied to determination of nanoparticle size distribution in suspensions and to detection of small quantities of high-mass materials within samples of protein.

Essentially, a solution or a suspension of the substance being analysed is subjected to irradiation by monochromatic laser light. Fluctuations in the diffracted light’s intensity with time are recorded. With the aid of an autocorrelator, the intensity data collected can be used to give information on the size distribution of the sample’s molecules or other particles.

How it works is that the particles (as long as they are much smaller than the wavelength of light) diffract the light in every direction, creating an effect known as Rayleigh Scattering. Projecting the diffracted light onto a screen produces a characteristic speckled image.

Within the speckled pattern, dark areas are produced when light diffracted by the particles arrives out of phase – and so interferes destructively. Bright areas result when light diffracted by the particles arrives in phase – thus interfering constructively.

Importantly, the particles in the sample are not usually stationary, as collisions with solvent molecules cause them to move about randomly. This is Brownian Motion, which is crucial to analysis by dynamic light scattering because it allows application of the Stokes-Einstein equation. The formula relates the particle’s velocity, when in solution, to its hydrodynamic radius.

I will leave it there for now, but if you take a look through the other articles on the Dynamic Light Scattering website you will find out much more about how the technique is applied, how it has been advanced through instruments such as the latest DelsaMax range of analysers from Beckman Coulter and how it relates to the topic of zeta potential.

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