Beckman Coulter LS 13 320, User Manual

Page: 34 / 254

PN B05577AC 1-4
Theory
Polarization Intensity Differential Scattering (PIDS)
visualized in Figure 1.2 . Thus, if a detector is facing the direction of oscillation it will receive no
scattering from single dipoles. When the light beam is polarized in either the vertical or the
horizontal direction, the detected scattering intensity I
v
and I
h
at a given angle will be different. The
difference between I and I
h
(I-I
h
) is termed the PIDS signal. In addition, because the PIDS signal
varies at different wavelengths, measurements of the PIDS signal at several wavelengths will
provide additional information that can be used to further refine the size retrieval process.
Figure 1.2 Scattering From Different Polarizations
Since the PIDS signal is dependent on particle size relative to the light wavelength, valuable
information about the particle size distribution can be obtained by measuring the PIDS signal at a
variety of wavelengths.
3.1.4 Particle Non-Sphericity & Light Scattering
Laser Diffraction Measurement for Non-spherical Particles
Many industrial particles – such as soils, sands, and mineral powders – are irregularly shaped and
can have either a smooth or a rough surface. The analysis strategy that all commercial instruments
employ in the measurement of these particles, is to use a spherical approximation and treat every
particle as a sphere. This method produces a size distribution with diameter as the only variable,
which is easily traceable and comparable to the results obtained from other technologies. This
method sometimes works well for many irregularly shaped particles. Because the finite detector
area integrates and smoothes out the intensity fluctuation caused by any surface roughness, and
additionally because there is a smoothing effect from the rotation of the particles, the scattering
patterns approximate those created by spheres.
However, because many particles have a deviation from perfect sphericity, the size distribution
obtained is only apparent or nominal, and will be biased. In certain extreme cases, the results of
using a spherical model on non-spherical particles will be very different than from the truth. This
type of systematic bias often shows up when comparing laser diffraction results with those obtained
from other methods (see Particle Characterization: Light Scattering Methods, R. Xu, Kluwer