Polarization Intensity Differential Scattering (PIDS) technology, combined with laser diffraction, enables direct detection of particles as small as 10 nm.
Particles below a few microns in diameter have similar light scattering patterns that are alike in shape and intensity. These physical properties make it difficult to distinguish the differences between such patterns, which means inaccurate sizing with low resolution, resulting in a high degree of uncertainty when resolving actual particle size.
Vertically polarized scattered light has different scattering patterns and fine structures from that of horizontally polarized light for small particles. The main characteristic of the horizontal scattering intensity (Ih) for small particles is that there is a minimum of about 90°. This minimum shifts to larger angles for larger particles.
Thus, although both vertical scattering intensity (Iv) and (Ih) contrast only slightly in the case of small particles, the difference between them can reveal a more distinguished fine structure, thereby making the sizing of small particles possible. Combining polarization effects with wavelength dependence at large angles, we can extend the sizing limit to as low as 10 nm.
This combined approach is known as the Polarization Intensity Differential Scattering (PIDS) technique patented by Beckman Coulter.
Large particles scatter light strongly at low angles and with readily 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.
Conversely, small particles scatter light weakly and without any discernible maxima and minima until extremely high angles of measurement are reached. This makes detection and resolution of the scattering pattern difficult. Manufacturers have adopted different solutions to overcome these limitations with varying degrees of success.
Most efforts have focused on measuring back-scattered light. While such strategies help, they are not complete solutions. For this reason Beckman Coulter developed the PIDS system, creating for the first time a complete solution to the problem of sub-micron sizing. The technology employed in PIDS is elegant yet simple and takes advantage of the Mie theory of light scattering.
PIDS relies on the transverse nature of light, i.e., that it consists of a magnetic vector and an electric vector at 90° to it. If, for example, the electric vector is “up and down” the light is said to be vertically polarised. When a sample is illuminated with a light of a given polarized wavelength, the oscillating electric field establishes a dipole (oscillation) of the electrons in the sample. These oscillations will be in the same plane of polarization as the propagated light source.
The oscillating dipoles in the particles radiate light in all directions except that of the irradiating light source. PIDS takes advantage of this phenomenon. Light at three wavelengths (475, 613 and 900 nm) sequentially irradiates the sample, with first vertically and then horizontally polarised light. The LS 13 320 XR measures the scattered light from the samples over a range of angles. By analysing the differences between the horizontally and vertically radiated light for each wavelength, we gain information about the particle size distribution of the sample. We are measuring the differences between vertically and horizontally polarised signals, and not simply the values of a given polarisation.
The intensity vs. scattering angle information from the PIDS signals is then incorporated into the standard algorithm from the intensity vs. scattering angle data from the laser light scattering to give a continuous size distribution.
Another major benefit of acquiring PIDS data is that by simple interpretation of the raw data we can quickly confirm if small particles are actually present, as large particles do not exhibit the differential signal shown by small particles.