ISO 13320 Particle Size Analysis - Laser Diffraction Methods

The standard “ISO 13320 Particle size analysis - Laser diffraction methods”, published by the International Organization for Standardization (ISO), provides guidance on instrument qualification and size distribution measurement of particles in many two-phase systems (e.g. powders, sprays, aerosols, suspensions, emulsions and gas bubbles in liquids) through the analysis of their light scattering properties.

This standard does not address the specific requirements for particle size measurement of specific materials. This standard is applicable to particle sizes between approximately 0,1 µm and 3 mm. With special instrumentation and conditions, the applicable size range can be extended to above 3 mm and below 0,1 µm. For spherical and non-spherical particles, a size distribution is reported in which the predicted scattering pattern for the volumetric aggregate of spherical particles matches the measured scattering pattern. This is because the technique's optical model assumes a spherical particle shape. The resulting particle size distribution for non-spherical particles differs from that obtained by methods based on other physical principles (e.g., sedimentation, sieving).

Laser diffraction or scattering techniques for determining particle size distributions are based principally on the fact that the angular distribution of light intensity scattered by a particle (its scattering pattern) depends on the particle size. When scattering occurs from a cloud or collection of particles, the scattered intensity for a given size class is related to the number of particles present in that size class and their optical properties.

A test sample dispersed at sufficient concentration in a suitable liquid or gas is passed through the beam of a monochromatic light source, usually a laser. The light scattered by the particles at various angles is measured by a series of photodetectors. Numerical values ​​from each detector are recorded for subsequent analysis. Within certain limits, such as the particle concentration in the measurement region, the scattering pattern of a collection of particles is identical to the sum of the scattering patterns of all individual particles. Theoretical scattering patterns of particle unit volumes in selected size classes are used to construct a matrix and, in conjunction with a mathematical procedure, to solve the inverse problem, resulting in an iterative volumetric particle size distribution to best fit the measured scattering pattern.

Briefly, this standard is typically used for particles in the size range of 0,1 µm to 3.000 µm using laser diffraction (also known as laser light scattering) and is applicable to liquid suspensions, emulsions, dry powders, and aerosols. It is suitable for particles that scatter light in a way that can be modeled using Mie theory or the Fraunhofer approach. In principle, particles are illuminated with a laser, the scattered light is measured from various angles, and the scattering pattern is analyzed to infer the particle size distribution.

Important factors covered by this standard include:

• Sample preparation (distribution techniques, avoiding clustering)
• Refractive index of particles and dispersant
• Data interpretation and reporting requirements
• Repeatability and reproducibility
• Device calibration and verification

This standard ensures consistency and accuracy in particle size analysis and is widely used in industries such as pharmaceutical, cosmetics, mining, cement and food processing.

According to this standard, laser diffraction works as follows:

• The basic principle is light scattering. When a laser beam passes through a sample containing particles, the particles scatter the light. Small particles scatter the light at wide angles. Large particles scatter the light at narrow angles.
• Detection of scattered light: Detectors placed at various angles collect the scattered light. The intensity pattern is recorded and used to calculate the particle size distribution.
• The mathematical models used include:
  • Fraunhofer approximation: Assumes large, opaque particles. Simpler, but less accurate, for small or transparent particles.
  • Mie theory: More complex, takes into account particle refractive index and absorption. Necessary for small or transparent particles.

The system components are:

• Laser source emits coherent light
• Sample disperser disperses particles in air or liquid
• The detector array measures scattered light at various angles
• The data processor uses models to infer particle size distribution

The types of distribution are:

• Volume-based (most common): Shows the volume percentage for each particle size.
• Number-based: Shows how many particles fall into each size range (may exaggerate fine particles).
• Surface area-based: Less common, based on total surface area.

With regard to repeatability and reproducibility, the ISO 13320 standard emphasizes verifying that measurements are consistent across runs and instruments.

Reporting should include information on the model used (Mie or Fraunhofer), refractive indices, dispersion medium, type of dispersion (volume, number), size range analyzed, and instrument settings (e.g., dimming, path length).

Best practices according to this standard include:

• Appropriate distribution: Prevents clumping that distorts dimension data
• Using the correct refractive index: Essential for accurate Mie-based results
• Instrument calibration: Maintains measurement accuracy
• Repeatability testing: Verifies consistent performance

Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the ISO 13320 Particle size analysis - Laser diffraction methods standard.