Malvern Zetasizer Nano Series, Руководство пользователя

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The light scattered at an angle of 17° is combined with the reference beam. This
produces a fluctuating intensity signal where the rate of fluctuation is
proportional to the speed of the particles. A digital signal processor is used to
extract the characteristic frequencies in the scattered light.
Optical Modulator
A refinement of the system involves modulating one of the laser beams with an
oscillating mirror. This gives an unequivocal measure of the
sign of the Zeta
potential .
A second benefit of the modulator is that low or zero mobility particles give an
equally good signal, so measurement is as accurate as for particles with a high
mobility.
This technique ensures an accurate result in a matter of seconds, with possibly
millions of particles observed.
The Electroosmosis effect
The walls of the capillary cell carry a surface charge so the application of the
electric field needed to observe electrophoresis causes the liquid adjacent to the
walls to undergo electroosmotic flow. Colloidal particles will be subject to this
flow superimposed on their electrophoretic mobility. However, in a closed system
the flow along the walls must be compensated for by a reverse flow down the
centre of the capillary.
There is a point in the cell at which the electroosmotic flow is zero - where the
two fluid flows cancel. If the measurement is then performed at this point, the
particle velocity measured will be the
true electrophoretic velocity . This point
C H A P T E R 1 5 Z e t a s i z e r N a n o S e r i e s
P a g e 1 5 . 6
M A N 0 3 1 7
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Time
Intensity of
scattered
light
Cell
Incident
beam
Scattering
beam
17
°
Detector
ILL
6777