Dynamic Light Scattering Instrument

Dynamic light scattering (DLS) is a non-invasive measurement technique used to measure the size of colloids or suspended particles in solution.

The basic components of a dynamic light scattering instrument are a laser or collimated light source, a sample holder, a detector and a correlator. The laser is directed into a sample containing particles. Light scatters off particles in solution and the intensity of this scattered light is measured over time. Using this dynamic scattering profile, DLS machines can estimate particle size and heterogeneity.

Dynamic light scattering is a fast, versatile technique which can measure a range of particle sizes. It is widely used across fields such as polymer science, analytical chemistry, and colloidal and quantum dot research.

How Does A DLS Instrument Work?

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1. Light enters the sample and scatters off any suspended particles
2. Scattered light is measured by the detector at a set angle. This creates a specific intensity pattern made up of constructive and destructive interferences.
3. As particles move, this intensity pattern changes. The rate of change of these patterns will depend on the size of the colloids.
4. The dynamic light scattering instrument turns this fluctuating intensity pattern into a correlation function.
5. From this correlation function, DLS software can calculate a diffusion coefficient and subsequently particle size.

The Stokes-Einstein equation states that the dynamic diffusion constant of a particle in solution, D_τ, relates to its hydrodynamic radius R_h.

Dynamic light scattering instruments use this principle to estimate particle size.

DLS instruments use the correlation function to work out particle size. This correlation function will drop off quicker for smaller particles. Larger particles move slower leading to a slower drop off of correlation function. By fitting an exponential fit to the correlation function, you can calculate particle size through the Stokes-Einstein equation.

By controlling parameters such as temperature (T), solvent viscosity (η), refractive index (n), light wavelength (λ), and the scattering angle (ϑ), DLS instruments can accurately measure the radius of monodisperse particles in solution.

DLS Measurement Range

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Most instruments are equipped to handle a broad range of particle sizes between 0.3 nm - 10 μm. To maximize your signal, you can vary solution concentration. The optimal solution concentration can be worked out empirically, with DLS capable of measuring signals from samples of a few ppm to 40 vol%.

DLS can also measure the distribution of size in your solution. Most DLS systems will provide this information with a polydispersity index, a measurement of how heterogenous your sample is. PDI takes into account many factors such as size, shape and molecular weight.

This is especially useful if you are trying to create or synthesise uniform particles of a specific size such as with gold nanoparticles. If your sample is too polydisperse your results might be unreliable. 

Why Use Dynamic Light Scattering?

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There are many reasons to use dynamic light scattering to characterize your solution.

The broad measurement range means DLS can be used to estimate sizes of many different materials in many different industries, including:

• Polymer Synthesis
• Characterizing protein sizes
• Quantum dot characterization
• Colloidal studies