How Does Dynamic Light Scattering Work?
Dynamic light scattering or photon correlation spectroscopy is a biophysical technique to determine the size of particles in a solution. It involves the same basic principles as classical light scattering experiments, but it also offers information about the motion of the particles in the solution; moreover, this technique makes it possible to analyze samples where many different particle sizes are present. Does this Spark an idea?
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Basic Principles
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When light encounters molecules much smaller than its wavelength, it undergoes a form of scattering called Rayleigh scattering. The electric fields of the light polarize the molecule, shifting electrons around so that the molecule now has a positive side and a negative side; the charge difference between the two sides is the dipole moment. The oscillating dipole moment induced by the light causes the molecule to re-radiate the light in all directions, and the intensity of scattered light depends on the wavelength and how easily the molecule can be polarized, which in turn is related to the molecule's size. Larger particles cause a similar but more complicated type of scattering called Mie scattering, which does not depend on wavelength.
Time-Dependent Fluctuations
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The particles in solution have thermal energy and are constantly jostled by water molecules, so they move more or less randomly--a type of motion called Brownian motion. When they scatter light, the scattered light may interfere constructively, in which case its intensity will increase, or destructively, in which case its intensity will decrease. Since they are constantly moving, the intensity fluctuates. This fluctuation in intensity can be analyzed on a computer once it has been measured.
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Basic Setup
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In a dynamic light scattering experiment, a laser beam passes through a lens that narrows the beam and shines through a solution or suspension containing particles, molecules or droplets. Scattered light leaving the solution passes through another lens and strikes a detector that measures its intensity. The fluctuations in intensity make it possible to calculate the diffusion coefficient, which in turn can be used to calculate the the diameter of the particles if the viscosity of the fluid is known.
Assumptions
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Several assumptions are involved. First, the particles or molecules must be much smaller than the wavelength of the laser light in order for Rayleigh scattering to apply; this will typically be true for proteins in solution. Second, the particles and molecules are assumed to be moving more or less randomly in the solution. Given this second assumption, the particles scattering the light are moving as they do so, which results in a Doppler shift--a change in the wavelength of the light.
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References
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