Sizing Non-Spherical, Submicron Particles with PIDS
Most laser-based particle-sizing devices make no allowance for the shape materials being tested, regardless of particle size. The mathematical models used to calculate distributions are based on spherical systems resulting in any reported distribution being essentially an equivalent spherical distribution of the material being analyzed. In most cases, this is sufficient as most particles approximate to a spherical system well enough.
LS Series multi-wavelength particle size analyzers use a complementary scattering technology for sizing non-spherical submicron particles using a system called PIDS or polarization intensity differential scattering.
The LS instrument with laser-based technology facilitates the analysis of particles without the risk of missing either the largest or the smallest particles in a sample.
PIDS vs Light Scattering for Sizing Small Particles
When illuminated by a laser beam, large particles scatter light at low angles with easily detectable maxima and minima in the scattering pattern. This means that detectors placed at low angles relative to the optical path and with sufficient angular resolution can detect these maxima and minima. Small particles scatter light weakly and can only measure maxima and minima at very high angles, impacting the detection and resolution of the scattering pattern. Different manufacturers use different solutions to address these limitations with varying degrees of success with most focusing on the measurement of back-scattered light.
PIDS technology is based on the Mie theory of light scattering and relies on the transverse nature of light. With a magnetic and electric vector (at 90°), if the electric vector is 'up-and-down' the light is considered to be vertically polarized. When a sample is illuminated with light of a given polarized wavelength, the oscillating electrical field creates a dipole (or oscillation) of the electronics in the sample. These oscillations are in the same polarization plane as the propagated light source, and oscillating particle dipoles will radiate light in all directions except that of the irradiating light source.
Three wavelengths (450 nm, 600 nm and 900 nm) successively illuminate the sample, with vertically- and then horizontally-polarized light. Scattered or radiated light from the samples is measured over a range of angles. By analyzing the differences for each wavelength, we gain information about the particle size distribution of the sample. What’s being measured is the difference between vertically- and horizontally-polarized signals not only the values at a given polarization.
Intensity and scattering-angle information from PIDS signals is incorporated into the standard algorithm from the intensity vs. scattering angle data from the laser light scattering for a continuous size distribution, which for the LS 13 320 particle size analyzer is 0.017 µm to 2,000.0 µm.