Static and Dynamic Light Scattering (SLS/DLS)

Light scattering is a well established technique to investigate properties of particles in solutions. Information such as size, molecular weight, diffusion and interaction strength are obtained. The basic principle is that a laser beam impinges on a sample and the scattered intensity is probed at a certain angle θ by a detector.

Basic Principles

From the intensity fluctuations in time (see Figure 1), information on the dynamics within the solution are obtained. The evaluation of the fluctuations is commonly named as dynamic light scattering (DLS) while the analysis of the absolute mean intensity is known as static light scattering (SLS). The intensity is very sensitive to variations in size of the solutes, so that it is advantageous to investiagte aggregation in solution.
The scattering vector Q is defined as difference between outgoing and incoming wave vector and its magnitude is

scattering vector
Q vector and Fluctuation

Figure 1: Scattering geometry and intensity fluctuations at detector

Static Light Scattering

The main quantities influencing the static light scattering intensity, are the molecular weight M, concentration and size of the particles in solution. Due to the long wavelength, particles of size of nanometers (typical for proteins) can be interpreted as (independent) scattering centers whose intensity interferes constructively. If the particles are big enough (>λ/20), an angle dependent change in intensity can be observed. The Rayleigh ratio R ∝ Is is introduced as quantity independent of the experimental setup. To have a direct estimation of the results, the scattering intensity is normalized by the solute concentration c and reduces for small concentration to

Where A2 is called second virial coefficient describing the interactions between the particles, Rg is the radius of gyration and K is an optical constant

If the particles are big in size or interacting strongly, further angular and concentration dependent corrections have to be considered, arising from the form and structure factor (e.g. see SAXS).

Dynamic Light Scattering

In DLS the motion of particles is probed by analyzing the correlation of intensity fluctuations.
The decay of the normalized intensity correlation function g(2)(τ) yields the diffusion coefficient D of the solutes by the so called Siegert relation

Experimental Setup

Figure 2: ALV experimental setup


The laser beam passes through a beam splitter in order to detect a reference intensity at the monitor diode. In transmission it impinges on a glass cuvette immerged in a toluene bath. This bath avoids reflexes and its temperature can be controlled accurately. The beam scattered by the sample is detected at an angle θ by an avalanche photo diode (APD) transferring the signal to a PC. The purpose of the attenuator is to protect the APD from a too high photon flux.

Technical specifications [1]:

  • Instrument : ALV-CGS-3
  • Angular Range : [17° - 150°]
  • Correlator : Multiple Tau 7004 /Fast Autocorrelator
  • Sampling Time Range : [100 ns - 3435.9 s]
  • Wavelength : λ= 632.8 nm

  • Example

    Figure 3: Example correlogram and CONTIN analysis of
    mixture gold colloids (d=30nm) and latex beads (d=600nm)

    DLS gives the possibility to separate particles which differ in their diffusive behavior. An example is displayed in Figure 3, where gold colloids and latex beads which differ by a factor of 20 in radius have been measured. From a CONTIN analysis or equivalently a double exponential fit, the diffusion constants and hydrodynamic radii can be extracted.

    SLS results of polystyrene particles, measured at multiple angles and varying concentration are shown in Figure 4 as a Zimm plot. This plot yields intuitively the values of the radius of gyration, second virial coefficient and molecular weight. For the sample studied in Figure 4 a molecular weight of 106 kDa is gained in good agreement with the nominal weight of 100 kDa.

    Figure 4: Zimm plot of polystyrene 100 kDa


    [1] R. Peters. ALV - Technical Documentation, 2009.