Patented Polarization Differential Scattering (PIDS) and particle sizing
Many samples have particle sizes that extend into the submicron range, creating a wider size-distribution range. As particle size decreases, the ratio of particle dimension to light wavelength decreases along with the interference effects to produce a smoother and less angular-dependent scattering pattern.
At this size range, the sensitivity of particle size to scattering intensity pattern is significantly decreased making it very challenging to obtain correct size values. Clearly, if light of a short wavelength is used, the ratio will be more pronounced, effectively extending the lower size limit. Combining the polarization effects of light scattering with wavelength dependence at high angles, it’s possible to extend the lower size limit to 40 nm—almost reaching the theoretical limit.
The origin of the polarization effect
If a small particle, much smaller than the light wavelength (d << l) is located in a light beam, the oscillating electric field induces an oscillating dipole moment in the particle (i.e., electrons in the atoms containing the particle move back and forth relative to the stationary particle). This induced electron motion will be in the direction of the oscillating electric field and perpendicular to the direction of propagation of the light beam.
Because of the transverse nature of light, the oscillating dipole radiates light in all directions except in the direction of oscillation. If a detector is facing the direction of oscillation, it will receive no scattering from single dipoles. When the light beam is either vertically or horizontally polarized, the detected scattering intensity at a given angle will be different—that difference is termed the PIDS signal. As the PIDS signal varies at different wavelengths, measurements at several wavelengths provides additional information to further refine the size determination.
Real data for real results
Particles below a few microns in diameter have very similar light scattering patterns in both shape and intensity making it difficult to distinguish the differences between such patterns. This can result in inaccurate sizing with low resolution and a high-degree of uncertainty when resolving the actual particle size.
Large particles clearly scatter light at low angles and with readily detectable maxima and minima in the scattering pattern allowing detectors placed at low angles relative to the optical path and with sufficient angular resolution to detect these maxima and minima. Small particles however, scatter light weakly and without any discernible maxima and minima until extremely high angles of measurement are reached making the detection and resolution of the scattering pattern difficult.
While compensating strategies such as back-scattered light help, they are not complete solutions. The PIDS system, offers a complete solution to submicron sizing.
The LS 13 320 MW instrument measures scattered light from samples over a range of angles. By measuring the differences between the vertically and horizontally polarized signals and not only the values of a given polarization, we gain important information about the particle size distribution of the sample. Intensity vs scattering angle data from the PIDS signals is incorporated into the standard algorithm from the laser light for a continuous size distribution. This simple interpretation of the raw data allows for quick confirmation of whether small particles are present or not as large particles don’t exhibit the differential signal shown by small particles.