Beckman Coulter LS 13 320, User Manual

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PN B05577AC 1-3
Theory
Polarization Intensity Differential Scattering (PIDS)
1
parallel, only Fraunhofer diffraction will take place. For these spheres, the Fraunhofer diffraction is
just a simplified form of the Mie theory with the limiting condition that d >> l.
Fraunhofer theory can only be used for particles that are:
•
Much larger than the wavelength of the light (typically > 30 mm) and non-transparent, i.e., the
particles have different refractive index values than that of the medium (typically with the
relative refractive index being larger than 1.2), or
•
Highly absorptive (typically with absorption coefficients higher than 0.5).
In Fraunhofer theory, the refractive index of the material is irrelevant because the scattering
intensity for large particles is concentrated in the forward direction (typically at angles smaller
than 10 degrees). For this reason, Fraunhofer diffraction is also known as forward scattering. The
angle for the first minimum of scattering intensity is simply related to the size by
EQ 1 . Most of the
scattering intensity is concentrated in a very sharp central lobe, which provides a much simpler
solution to apply to sizing large particles in a light scattering measurement.
EQ 1
Most of the scattering intensity is concentrated in a very sharp central lobe, which provides a much
simpler solution applicable to sizing large particles in a light scattering measurement.
As a final note on theoretical background, light scattering is an absolute measurement technology
only in the sense that once the experimental setup is correct, calibration or scaling are not
necessary in order to obtain the volume (or weight) percentage of each component. In addition,
choosing a correct optical model is often the key step in obtaining the correct results.
Polarization Intensity Differential Scattering (PIDS)
Many samples have particle sizes that extend into the submicron range creating a wider size
distribution range. However, as a particle size gets smaller, the ratio of particle dimension to light
wavelength (d/l) is reduced. Interference effects are thus reduced and the scattering pattern
becomes smoother and less angular dependent. Within the smaller range, the sensitivity of particle
size to scattering intensity pattern is greatly reduced causing it to be more and more difficult to
obtain correct size values. Obviously, if light of a short wavelength is used, the ratio will be great,
and so the lower size limit will be effectively extended. Combining the polarization effects of light
scattering with the wavelength dependence at high angles, Beckman Coulter can extend the lower
size limit to as low as 17 nm, almost reaching the theoretical limit. This approach is the Polarization
Intensity Differential Scattering (PIDS) technique..
The origin of the polarization effect can be understood in the following way. If a small particle, much
smaller than the light wavelength (d << l), is located in a light beam, the oscillating electric field of
the light induces an oscillating dipole moment in the particle, i.e., the electrons in the atoms
comprising the particle move back and forth relative to the stationary particle. The induced motion
of the electrons will be in the direction of oscillation of the electric field, and therefore
perpendicular to the direction of propagation of the light beam. As a result of the transverse nature
of light, the oscillating dipole radiates light in all directions except in the direction of oscillation as
θ
1.22
∗λ
d
-----------------
= sin