Laser diffraction vs. PIDS to measure small & non-spherical particles

Small particles, big challenge

Smaller particles pose a real challenge for laser diffraction technology.

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 detectors placed at low angles relative to the optical path and with sufficient angular resolution can detect these maxima and minima.

As particles become smaller, the ratio of particle dimension to light wavelength (d/λ) is reduced, resulting in a smoother and less angular scattering pattern, making measurement difficult. In addition, small particles scatter light weakly and maxima and minima can be measured only at very high angles, which affects detection and resolution of the scattering pattern.

Different manufacturers use different solutions to address these limitations with varying degrees of success. Most focus on the measurement of back-scattered light.

Bias in sizing of non-spherical particles

Most laser-based particle-sizing devices make no allowance for the shape of materials being tested, regardless of particle size.

Mathematical models used to calculate distributions are based on spherical systems, so any reported distribution is essentially equivalent to a spherical distribution of the material being analyzed. In most cases, this is sufficient, as many particles emulate a spherical system closely enough.

But for many particles that deviate from perfect sphericity, the size distribution obtained is only apparent or nominal, and will be biased. In some extreme cases, results using a spherical model on non-spherical particles will be very different from reality. This bias emerges when comparing laser diffraction results with others, such as polarization intensity differential scattering (PIDS).

PIDS vs laser diffraction for sizing small & non-spherical particles

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.

LS 13 320 Particle Size Analyzer & the combined power of PIDS

LS 13 320

The LS 13 320 is a versatile laser diffraction particle size analyzer that uses PIDS technology for sizing non-spherical submicron particles.

Because the LS 13 320 combines three approaches, particle sizes ≥ 40 nm can be accurately measured instead of being extrapolated:

  1. Increasing the angular detection range to expand the lower sizing limit.
    If the angular location of the first minimum in the scattering pattern is the criteria to size a sphere, the maximum detecting angle must be greater than 90° to accurately measure a sphere with a diameter < 0.5 μm. [?] To size a submicron particle, the detection angular range must cover angles at least as large as 90° with a maximum detecting angle of 175°.
  2. Shortening the wavelength.
    Scattering patterns are a function of light wavelength and particle size. Variations are related to the ratio between particle dimension and wavelength (d/λ). Interference effects that create the fine structure in a scattering profile are significantly reduced when d/λ is < 0.5 μm. The shorter the light wavelength, the greater the ratio, resulting in an extended lower sizing limit.
  3. Using the polarization effects of scattered light.
    By combining polarization effects with wavelength dependence at large angles, the lower sizing limit can be extended as low as 40 nm, almost reaching the theoretical limit.

This combined approach—the PIDS technique—was patented by Beckman Coulter. With PIDS, there’s no mixing of technologies. All signals are from the same scattering phenomenon and treated integrally in a single data retrieval process, just as with a typical laser diffraction measurement.

This enables the LS 13 320 analyzer to deliver unsurpassed accuracy and reproducibility, as well as the highest resolution for sizing particles without the risk of missing the largest or smallest particles in the sample.