Beckman Coulter LS 13 320, User Manual

Page: 35 / 254

PN B05577AC 1-5
Theory
PIDS Sizing
1
Academic Publishers, 2000, and Comparison of sizing small particles using different technologies, R.
Xu, O. Diguida, Powder Technology, 2003).
As it has been summarized in the international standard ISO 13320-1:1999 Particle Size Analysis –
Laser Diffraction Methods- Part 1: General Principles the deviation of particle shape from a sphere
can be one of the many sources of bias:
“6.6.3. Another main source for bias arises from the departure from the theoretical assumption for the
particulate material. Again, the errors can come from different sources.
• Firstly, most particles in real life do not fulfill the assumption of sphericity. Non-sphericity of particles
leads to different cross-sections in different orientations. Since particles are generally measured in all
possible orientations, this leads to some broadening of the particle size distribution as compared to the
equivalent volume distribution. Moreover, the median and mean diameter may be shifted, often to a
larger size.
•
Secondly, the particle surface may be rough instead of smooth. This causes diffuse light scattering at
the boundary, which often has a similar influence as absorption of light within the particle.
•
Thirdly, the particles may be optically heterogeneous, as is the case for porous particles. This may lead
to an apparent presence of significant amounts of very small particles, which are non-existent.”
PIDS Sizing
As mentioned above, with its patented PIDS assembly the LS 13 320 can provide information about
particles within the range of 0.017 microns. Particles in this range offer limited information in
diffraction patterns, so the PIDS technology is required to accurately measure these small particles.
Components
The PIDS assembly consists of:
•
An incandescent light source and polarizing and bandpass filters
• PIDS sample cell
•
An additional seven photodiode detectors, six to measure scattered light plus one to monitor
the beam strength
Functions
The PIDS assembly functions are:
• To illuminate sample particles sequentially with beams of light centered at three different
wavelengths polarized vertically and then horizontally (relative to the plane containing the
light source, scattering region, and detectors) at each wavelength.
•
To determine the difference in scattered intensity between the vertical and horizontal
polarizations as a function of angle for the three wavelengths of light.