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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.