Dynamic light scattering
With the dynamic light scattering (DLS) is a method in which the scattered light from a laser to a dissolved or suspended sample is analyzed. It is most often applied to proteins such as polymers and biopolymers, for determining the hydrodynamic radius of the molecules. Dynamic light scattering is also known under the names, photon correlation spectroscopy "( PCS) or quasi-elastic light scattering ' ( QELS ) are known.
Description
When light is incident on small particles, it is scattered in all directions ( Rayleigh scattering). This also applies to the macromolecules in solution or suspension. The scattered light of different scattering centers thereafter interfere with one another. Laser light is used, which is a coherent and monochromatic, this interference leads to small fluctuations in the scattering intensity, since constantly change the distances of the scattering centers to each other by Brownian motion. If we analyze these fluctuations in terms of time scale on which they happen, you get so information about the rate at which the particles move in solution. This in turn allows a diffusion coefficient determined from the example, the hydrodynamic radius can be calculated according to the Stokes - Einstein relation.
Measuring apparatus
Traditionally, a goniometer is used for light -scattering experiments. It is on a fixed arm, the laser unit, and on a pivoting arm of the detector, usually a secondary electron multiplier ( SEM ) or an avalanche photodiode ( APD). In the center of the array is the measuring cell to work at any angle arrangement, usually a cylindrical quartz cell. In modern devices often is the possibility of being able to absorb the angular dependence, sacrificed for a compact arrangement. These devices measure at a fixed angle of eg 90 °, but can be used with simple cuboidal cells with very small volumes. This reduces the volume required for a measurement of sometimes more than 10 ml on some ul. Since large particles, and particles which are not spherical somewhat anisotropic scattering, is not possible with such a construction, a detailed analysis of the size of these particles.
Data Analysis
To determine the dynamic characteristics of the particles, an auto-correlation of the measurement signal is carried out. The autocorrelation function for a discrete time series can be calculated as follows:
Wherein the mean, the variance, the signal intensity, the number of data points and the value of the autocorrelation. is a count variable indicating the distance between start and end values . From the thus obtained curve is an exponential function can now be adjusted. The decay rate, which can be determined thereby directly correlated with the diffusion coefficient. With a known viscosity of the solvent can be from now on determine the Stokes -Einstein equation, the hydrodynamic radius of the particles measured. This information allows you to indirectly determine the molar mass. In order to determine not just a single size, but all distributions are adjusted sums of several exponential functions of the autocorrelation function, for example. Thus this noise is also not mitinterpretiert, it requires sophisticated process in order to obtain reliable results.