Light scattering particle size analysis is a fast, non-destructive and highly reproducible optical technique for measuring particle size distributions from the nanometer to the millimeter range. By combining laser diffraction (static light scattering) and dynamic light scattering (DLS), accredited third-party laboratories can deliver standards-compliant results in accordance with ISO 13320 and ISO 22412. This article explains the technical principles, sample types, measured parameters, test procedures, and typical applications of light scattering particle size analysis.
Technical Overview
Light scattering particle size analysis is a family of optical measurement techniques that determine particle size distributions by analyzing how dispersed particles scatter incident laser light. Large particles scatter light strongly at small forward angles and produce fine diffraction rings, while small particles scatter weakly over wider angles; nanoparticles additionally exhibit rapid intensity fluctuations due to Brownian motion. Because the method is fast, non-destructive, broadly applicable and easily automated, it has become the dominant technique for routine particle characterization in both industrial quality control and research.
Two complementary implementations are routinely used, both covered by international standards:
- Laser diffraction (static light scattering), governed by ISO 13320, measures the angular dependence of scattered light intensity and typically covers approximately 0.1 µm to 3,500 µm depending on instrument configuration.
- Dynamic light scattering (DLS, also known as photon correlation spectroscopy), governed by ISO 22412, measures time-dependent fluctuations of scattered light caused by Brownian motion and typically covers approximately 1 nm to 10 µm, making it ideal for nanoparticles and colloids.
Data interpretation relies on well-established physical models. In laser diffraction, the measured angular scattering pattern is inverted using Mie theory, which requires knowledge of the real and imaginary refractive indices of the particles and the dispersing medium, or the Fraunhofer approximation, which is valid for particles significantly larger than the wavelength and requires no optical property data. In DLS, the intensity autocorrelation function is analyzed using the cumulants or CONTIN methods, and the hydrodynamic diameter is calculated from the translational diffusion coefficient via the Stokes–Einstein equation. Results are reported in accordance with the data representation rules of the ISO 9276 series.
Testing Samples
Powder Samples
Ceramic and metal powders, battery electrode materials such as lithium compounds, graphite and silicon, abrasives, cement, mineral fillers, inorganic pigments, food powders, and pharmaceutical active ingredients and excipients. Powders can be analyzed either by wet dispersion in a suitable liquid medium or by dry dispersion with compressed air, depending on their flowability, friability and tendency to agglomerate.
Suspensions, Emulsions and Slurries
Paints, printing inks, industrial coatings, agrochemical emulsions, cosmetic formulations, dairy and beverage emulsions, chemical mechanical polishing slurries, drilling fluids and ceramic slurries. These liquid samples are usually measured directly after appropriate dilution to an acceptable concentration.
Nanomaterials and Colloids
Metal and metal oxide nanoparticles, colloidal silica, polymeric nanoparticles, liposomes, exosomes and extracellular vesicles, quantum dots, carbon-based nanomaterials and protein aggregates. These samples are typically characterized by DLS because of their sub-micrometer size and colloidal stability.
Polymer Latexes and Biomolecules
Polymer latexes and dispersions, proteins, polysaccharides, micellar systems and other macromolecules in dilute solution, for which DLS provides hydrodynamic size and polydispersity information.
Testing Items
Depending on the technique selected, light scattering analysis reports the following parameters:
- Characteristic percentile diameters, such as D10, D50 and D90, as well as other percentiles including D16, D84 and D99.
- Complete particle size distributions in volume, number or surface weighting, presented as both differential and cumulative curves.
- Distribution width indicators, including span, defined as (D90 − D10) / D50, standard deviation and modal diameter.
- Mean diameters, including the Sauter mean diameter D[3,2] and the volume moment mean diameter D[4,3].
- Hydrodynamic diameter and polydispersity index (PDI) obtained by DLS.
- Method quality data, including obscuration, fit residual, repeatability of replicate measurements and verification against certified reference materials.
Testing Methods
Laser Diffraction per ISO 13320
The test begins with representative sampling and sample splitting. For wet measurement, the powder is dispersed in a liquid selected for chemical compatibility, often with the aid of a surfactant and ultrasonic energy, following the dispersion guidance of ISO 14887. For dry measurement, the powder is fed into a compressed-air Venturi disperser that separates agglomerates without fracturing primary particles. The dispersed particles pass through a laser beam, and scattered light is collected by an array of detectors over a wide angular range. The measured scattering pattern is deconvoluted using the Mie or Fraunhofer model, and the resulting distribution is reported in accordance with ISO 9276. Obscuration is kept within recommended limits to ensure single-scattering conditions.
Dynamic Light Scattering per ISO 22412
The sample is diluted to an appropriate concentration in a cuvette and thermally equilibrated, typically at 25 °C. Intensity fluctuations of the scattered light, measured at 90 degrees or in backscatter geometry, are converted into an autocorrelation function. Cumulants analysis yields the intensity-weighted mean hydrodynamic diameter (Z-average) and the polydispersity index, while multi-angle or inversion algorithms such as CONTIN can resolve multimodal distributions. Careful filtration of the dispersant and dust-free handling are essential for reliable results.
Quality Control and Validation
Instrument performance is qualified with certified reference materials such as monodisperse polymer latex or glass bead standards. Typical acceptance criteria follow the indicative precision limits of ISO 13320, for example a relative standard deviation of no more than 3 percent on D50 and 5 percent on D10 and D90 for replicate measurements. Related method standards include ASTM B822 for metal powders by light scattering, GB/T 19077 for laser diffraction, and GB/T 29022 and GB/T 19627 for dynamic light scattering. Dispersion stability is monitored throughout the measurement to exclude dissolution, swelling or agglomeration artifacts.
Applications
Light scattering particle size analysis supports product development, process control and regulatory compliance across many industries:
- Pharmaceuticals: micronized active ingredients for inhalation and oral dosage forms, suspension stability, and bioavailability studies.
- Battery and new energy materials: cathode and anode active materials, conductive agents and separator coatings, where particle size influences energy density, rate capability and electrode processing.
- Ceramics and powder metallurgy: optimization of pressing, sintering behavior and final part density.
- Paints, coatings and inks: control of gloss, hiding power, tinting strength and long-term dispersion stability.
- Food and beverages: emulsion stability, mouthfeel and reconstitution behavior of powders.
- Environmental science and geology: characterization of sediments, soils and airborne particulate matter.
- Nanotechnology research and safety: size and polydispersity assessment of engineered nanomaterials.
FAQ
What size range can be measured by light scattering methods?
Laser diffraction typically covers approximately 0.1 µm to 3,500 µm, while DLS covers approximately 1 nm to 10 µm. Combining both techniques provides continuous coverage from the nanometer to the millimeter scale.
Should I choose laser diffraction or dynamic light scattering?
Laser diffraction is preferred for powders, granules and broad distributions above roughly 0.1 µm, offering rapid measurement and robust statistics. DLS is preferred for nanoparticles, colloids and biomolecules below about 1 µm, where it delivers hydrodynamic diameter and polydispersity information.
How much sample is required?
Laser diffraction usually requires tens of milligrams to a few grams of powder, or a few milliliters of liquid dispersion. DLS typically requires only a few drops to a few milliliters of dilute suspension.
Is wet or dry measurement better for powders?
Wet dispersion is recommended for cohesive, fine or agglomerated powders and for materials that must be measured in their application medium. Dry dispersion is faster and suitable for free-flowing, robust powders, and it avoids liquid-induced swelling or dissolution.
Which standards apply to light scattering particle size analysis?
The principal standards are ISO 13320 for laser diffraction, ISO 22412 for DLS, ISO 9276 for data representation, ISO 14887 for sample dispersion, ASTM B822 for metal powders, and the Chinese national standards GB/T 19077, GB/T 29022 and GB/T 19627.
How accurate and reproducible are the results?
With proper sampling, dispersion and validation, laser diffraction routinely achieves repeatability better than 3 percent RSD on D50 and 5 percent on D10 and D90, consistent with the indicative limits of ISO 13320. DLS results on monodisperse reference materials typically agree with certified values within a few percent.
