Laser diffraction measurement for non-spherical particles

Many industrial particles, such as soils, sands and mineral powders are irregularly shaped and can have either a smooth or a rough surface. The analysis strategy for measuring randomly oriented particles employed by many commercial instruments is to use a spherical estimate and treat every particle as a sphere, regardless of the real shape.

This method produces a size distribution with one variable—diameter. This parameter is easily traceable and comparable to the results obtained from other technologies. This method sometimes works well for many irregularly shaped particles as the finite detector area integrates "evens out" the intensity fluctuation caused by any surface roughness. The rotation of particles also generates a smoothing effect allowing the scattering patters to approximate those created by spheres.

For many particles, because of their deviation 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 (see PIDS).

As covered in ISO 13320-1:1999 Particle Size Analysis, Part 1, particle shape from a sphere can be one of the many sources of bias.

  • Most particles, in reality, don’t fulfill the assumption of sphericity. Non-sphericity leads to different cross-sections in different orientations. Since particles are typically measured in all possible orientations, this leads to some broadening of the particle size distribution as compared to the equivalent volume distribution. The median and mean diameter may also shift, often to a larger size.
  • The particle surface may be rough instead of smooth causing diffuse light scattering at the boundary, which often has a similar influence as light absorption within the particle.
  • Particles may be optically heterogeneous (e.g., porous particles) leading to the apparent presence of significant amounts of very small particles, which aren’t really present.

Measuring up to ISO 13320

Named for the ISO 13320 standard and based on the Fraunhofer and Mie theories of light scattering, the LS 13 320 MW particle size analyzer can measure unknown sample distributions without having the analyst guess the type of distribution mode to pre-program the instrument.

Accuracy

  • Custom designed X-shaped detector array generates the most accurate measurement for characterizing the scattering pattern, resulting in the most accurate results possible.
  • Only the PIDS system delivers >>> level of accuracy by taking 36 detection measurements in the sub-micron region.

Resolution & Sensitivity

  • High log-spaced detector count for a clear difference in scattering pattern between size classes.
  • Continuous averaging amplification circuitry to increase signal-to-noise ratio
  • No pre-select curve fitting routines requirements mean no advance understanding distribution theory prior to analysis
  • Full invocation of Fraunhofer & Mie theories, including use of multi-wavelength modeling for maximum data analysis

Repeatability

  • Automatic alignment system for accurate angular calibration of the laser ensuring scattered light from particulates in the sample cell will fall on the correct detectors from an angular perspective and reproducible results.

The LS 13 320 MW from Beckman Coulter offers numerous advantages:

LS 13 320

  • High resolution, reproducible and accuracy (from 0.017 μm-2,000 μm)
  • Fast (fully automated sizing) (15-90 seconds to result)
  • One of highest submicron resolutions using PIDS technology
  • Non-invasive measurement
  • Performance certification
  • Compliance with regulatory standards ISO 13321, ISO 22412, 21 CFR Part 11

Related pages:

Patented Polarization Differential Scattering (PIDS) and particle sizing
ISO 13321-1 Standard for DLS – Particle Size Analysis