Analytical technique measuring electrophoretic mobility,particle size, surface charges, zeta potential etc.,

1. ELECTROPHORETIC LIGHT
SCATTERING (ELS)
K.Lohitha

2.  It is an analytical method that measures the electrophoretic mobility
under an applied electric field.
Principle
 Based on electrophoresis & dynamic light scattering.
 the suspension is illuminated by a light source, generally a laser of a
visible wavelength (∼400–800 nm).
 The scattered light from the suspension is sampled by an optical
system connected to a photodetector and signal processing system
that allows the measurement of particle motion utilizing the Doppler
effect.

3. Instrumentation

4. ☻A laser beam passes through the electrophoresis cell, irradiates the
particles dispersed in it, and is scattered by the particles. Light scattered
by the particles is detected by a photomultiplier (PMT).
☻Use heterodyne scattering methods in which a
fraction of the laser beam is split away before reaching the
sample by a mirror and directed to the PMT detector where
it is combined with scattered light from particles diffusing in
the sample.
☻ The fraction of redirected light is referred to as the
“local oscillator” and unlike the light scattered by the sample does not
fluctuate.
☻It is used as a reference beam and must be much larger than the
average intensity of the scattered light produced by the diffusing particles.
By superposing the reference and scattered beams a frequency shift can
be obtained.
☻From the frequency (or Doppler) shift the particle velocity can be
calculated.

5.  The image shows a typical
ELS power spectrum
obtained for 262nnm latex
spheres under and an
electric field of 10V/cm. The
center peak is called the
reference peak with no
applied electric field.
 The sample peak is shifted -
19.3Hz which corresponds to
an electrophoretic mobility of
-3.61 M.U. and zeta potential
of -48.8 mV assuming the
Smoluchowski limit.
 The sample peak is broader
than the reference peak
indicating a range of zeta
potentials within the sample
rather than a single value.

6. Advantages Disadvantages
 Rapid measurement -
electrophoretic mobility of
hundreds of cells may be
measured simultaneously in a
matter of seconds.
 Highest resolution compared
to other methods
 Requires small sample volume
(0.1ml)
 Solution conditions of
physiological to very low ionic
strength can be used.
 Major disadvantage is that it is
strictly an analytical
technique,although the fine
differences in the cell mobilities
can be distinguished,the cells
cannot be physically separated
for further analysis.

7. Primarily used for characterizing the surface charges of colloidal
particles like macromolecules or synthetic polymers (ex. polystyrene)
in liquid media in an electric field.
In addition to information about surface charges, ELS can also
measure the particle size of proteins and determine the zeta potential
distribution.

8. Case studies