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Particle size analysis techniques measure some property of
a particle and report results as the equivalent spherical
diameter based on this measured parameter
Different measurement techniques often give different
sizes for the same sample
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Dynamic light scattering is a non-invasive technique for
measuring the size of particles and molecules in
suspension
Brownian motion is the random movement of particles
due to collisions caused by bombardment by the solvent
molecules that surround them
The technique of dynamic light scattering measures the
speed at which particles are diffusing due toBrownian
motion
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According to the Stokes-Einstein theory the Brownian motion
depends on
o viscosity of the suspending fluid
o temperature
o size of the particles.
Brownian motion decreases with increase in particle size
Brownian motion increases with increase in temperature
The temperature needs to be stable during a measurement as
the convection currents in the sample cause non-random
movements which prevents accurate size interpretation
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Velocity of the Brownian motion is defined by the
translational diffusion coefficient (D)
The translational diffusion coefficient can be convertedinto a particle size using the Stokes-Einstein equation
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The diameter of a hard sphere that diffuses at the same
speed as the particle or molecule being measured is the
Hydrodynamic diameter dH The hydrodynamic diameter will depend not only on the
size of the particle core, but also on any surface structure,
as well as the type and concentration of any ions in the
medium
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A low concentration ionic medium will
produce an extended double layer of
ions around the particle, reducing the
diffusion speed and resulting in a larger,
apparent hydrodynamic diameter
Higher ionic concentration media will
compress the electrical double layer
and reduce the measured hydrodynamic
diameter
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LASERlight source
SAMPLEin the cuvette
DETECTORat 90O
to measure the scattered light
ATTENUATOR - The intensity of scattered light must be within a
specific range for the detector to successfully measure it. Hence
the intensity of incident light must be reduced/increased in
order to decrease or increase the intensity of scattered light
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CORRELATOR - The correlator compares the scattering
intensity at successive time intervals to derive the rate at
which the intensity is varying.
DATA ANALYSIS To analyse the correlated data
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LARGE PARTICLES SMALL PARTICLES
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Particle size is an important factor in determining the maximum
concentration at which a sample can be measured using DLS.
Particle-particle interactions which influence diffusion speed and therefore
the apparent particle size.
Multiple scattering, where the light scattered by one particle is then
scattered by another particle before reaching the detector, which reduces
the apparent particle size.
In a conventional 90 detection angle DLS instrument, the maximum
sample concentration is mainly limited by the effects of multiple scattering,
and generally samples must be significantly diluted to avoid this.