Average Particle Size Determination by Light Scattering: Principles, Standards, and Testing Services

Average particle size determination by light scattering is a fast, high-precision analytical technique widely used to characterize powders, suspensions, emulsions, and nanomaterials in accordance with ISO 13320, ISO 22412, and related ASTM and GB standards. Accredited third-party laboratories rely on laser diffraction and dynamic light scattering (DLS) to report key metrics such as D10, D50, D90, D[4,3], and the intensity-weighted Z-average. This article explains the technical principles, applicable sample types, testing procedures, industry applications, and frequently asked questions for light scattering particle size measurement.

Technical Overview

Light scattering methods determine the average particle size of a sample by measuring how particles redirect, or scatter, incident light. Because scattering behavior depends systematically on particle dimensions relative to the light wavelength, mathematical inversion of the scattering pattern yields a particle size distribution, from which various average diameters are calculated. Two complementary techniques dominate routine industrial testing: static light scattering, commonly known as laser diffraction, and dynamic light scattering (DLS), also called photon correlation spectroscopy.

Laser Diffraction (Static Light Scattering)

Laser diffraction instruments illuminate a dispersed sample with a collimated laser beam and record the angular distribution of scattered light using an array of detectors. Large particles scatter light at small angles, while small particles scatter at large angles. The measured scattering pattern is deconvoluted using either the Mie theory, which requires knowledge of the real refractive index and imaginary (absorption) component of the particles, or the Fraunhofer approximation, which is acceptable for opaque particles larger than roughly 10 micrometers. Laser diffraction typically covers a broad measurement range from approximately 0.1 to 3500 micrometers and reports volume-based results such as D10, D50, D90, and the volume moment mean D[4,3]. The technique is standardized in ISO 13320 and GB/T 19077.

Dynamic Light Scattering (DLS)

Dynamic light scattering measures the time-dependent fluctuations of scattered light intensity caused by Brownian motion of particles suspended in a liquid. Smaller particles move faster, causing faster intensity fluctuations. An autocorrelation function is fitted to the fluctuation data, and the translational diffusion coefficient is converted to a hydrodynamic diameter through the Stokes-Einstein equation. DLS is ideal for nanoparticles and fine colloids in the approximate range of 1 nanometer to 10 micrometers, reporting the intensity-weighted Z-average and the polydispersity index (PDI). The method is standardized in ISO 22412 and GB/T 29022.

Testing Samples

Powder Samples

Granular and powder materials are suitable for both wet and dry laser diffraction analysis. Typical submissions include metal powders for additive manufacturing, ceramic and abrasive powders, pharmaceutical active ingredients and excipients, cement and mineral fillers, toner, and battery electrode materials. A representative sample of approximately 0.5 to 5 grams is generally sufficient, depending on particle size and bulk density.

Liquid Dispersions

Suspensions, emulsions, slurries, latex dispersions, inks, paints, and coatings can be analyzed directly after appropriate dilution to achieve the optimal scattering concentration. For DLS, the sample must be free of dust and large agglomerates, and the continuous phase should be optically transparent at the laser wavelength.

Nanomaterials

Nanoparticles, liposomes, polymeric colloids, protein aggregates, exosomes, and silica or gold nanodispersions are primarily characterized by DLS because of its sensitivity in the nanometer range. Where a number-weighted size distribution is required, DLS results may be complemented by other techniques in a comprehensive characterization workflow.

Testing Items

Light scattering particle size testing typically reports the following parameters:

• Average particle size: D50 median diameter, Z-average hydrodynamic diameter (DLS), volume mean D[4,3], and surface mean D[3,2]
• Particle size distribution: D10, D50, D90 percentile diameters, full distribution table, and cumulative or frequency curves
• Distribution width indicators: span value calculated as (D90 minus D10) divided by D50, and polydispersity index (PDI) for DLS
• Dispersion quality assessment: verification of complete deagglomeration and measurement repeatability
• Method validation data: instrument qualification, standard reference material checks, and repeatability statistics required by ISO 13320 and ISO 22412

Testing Methods

Laser Diffraction Workflow

The laser diffraction procedure begins with instrument qualification using a certified reference material. For wet measurement, the sample is dispersed in a suitable liquid, such as water, ethanol, or isopropanol, with an appropriate dispersing agent and, when necessary, ultrasonic energy to break up soft agglomerates. For dry measurement, the powder is fed through a venturi disperser using compressed air. Scattering data are collected in triplicate, background-corrected, and converted into a size distribution using Mie or Fraunhofer theory with the correct optical model. ISO 13320 sets acceptance criteria for repeatability, for example a coefficient of variation of no more than 3 percent on D50 for samples above 10 micrometers.

Dynamic Light Scattering Workflow

For DLS, a small aliquot of the dispersion is diluted with filtered, dust-free solvent to a suitable concentration, typically a dilute milky appearance. The sample is equilibrated at a controlled temperature, commonly 25 degrees Celsius, inside the instrument cell holder. The scattered photon count train is recorded, the intensity autocorrelation function is computed, and cumulants or distribution analysis is applied to extract the Z-average and PDI. ISO 22412 requires measurement repeatability verification and advises careful control of viscosity and temperature, both of which directly affect the accuracy of the calculated hydrodynamic diameter.

Sample Preparation Considerations

Correct dispersion is the most critical factor for meaningful results. Key considerations include selection of a wetting solvent that does not dissolve or swell the particles, adjustment of pH or ionic strength to prevent aggregation, application of adequate but not excessive sonication, and avoidance of contamination by airborne dust. Technicians document all preparation parameters in the test report to ensure reproducibility.

Applications

Light scattering average particle size determination supports quality control, research and development, and regulatory compliance across many industries:

• Pharmaceuticals: particle size of inhalation powders, suspensions, and injectable formulations, supporting pharmacopeial requirements and bioavailability studies
• Battery and new energy materials: grading of cathode and anode powders such as lithium compounds and graphite, where size distribution influences energy density and processing
• Additive manufacturing: verification of metal powder size cut and sphericity-related flow behavior for powder bed fusion processes
• Ceramics, abrasives, and mining: control of grinding efficiency and final product fineness
• Paints, inks, and coatings: pigment and filler sizing that determines gloss, opacity, and dispersion stability
• Food and cosmetics: emulsion droplet sizing, chocolate and milk particle fineness, and stability assessment of cosmetic dispersions
• Academic and materials research: characterization of nanomaterials, colloids, and polymer latexes for publications and grant deliverables

FAQ

What is the difference between D50 and the Z-average?

D50 is the median diameter from laser diffraction, meaning half of the particle volume lies below this size. The Z-average is the intensity-weighted mean hydrodynamic diameter from DLS calculated by cumulants analysis. The two values are not directly comparable because they are weighted differently, so the choice of technique should match the intended application.

Which standard applies to my sample?

ISO 13320 and GB/T 19077 cover laser diffraction for powders, sprays, and suspensions. ISO 22412 and GB/T 29022 govern DLS for colloidal dispersions. ASTM B822 is commonly cited for metal powders by laser diffraction, while ISO 9276 defines how particle size analysis results are represented.

How much sample is needed for testing?

For laser diffraction, typically 0.5 to 5 grams of powder or 10 to 50 milliliters of dispersion is adequate. DLS usually requires less than 5 milliliters of dilute suspension. In all cases, the submitted portion must be representative of the bulk lot.

What accuracy and repeatability can be expected?

Properly validated laser diffraction methods achieve repeatability within a few percent on D50, consistent with ISO 13320 limits. DLS repeatability on the Z-average for monodisperse reference materials is typically better than 2 to 5 percent. Accuracy depends strongly on correct optical properties, dispersion state, and instrument qualification.

Can water-soluble or moisture-sensitive powders be measured?

Yes. Water-soluble samples are dispersed in nonpolar solvents such as ethanol, isopropanol, or heptane, while moisture-sensitive powders are analyzed by dry dispersion under controlled conditions. Method development ensures the chosen medium does not alter the native particle size.

How long does a typical test take?

A routine single-condition measurement, including sample preparation, data acquisition, and report generation, is usually completed within one to three working days at accredited third-party laboratories, with express services available for urgent projects.