Dynamic Light Scattering (DLS) particle size distribution testing is the internationally recognized method for measuring the hydrodynamic diameter and polydispersity of particles ranging from below one nanometer to several micrometers in liquid suspension. Performed in accredited third-party laboratories in accordance with ISO 22412, ASTM E2490, and GB/T 29022, DLS analysis provides rapid, non-destructive, and highly reproducible size characterization for nanomaterials, biopharmaceuticals, and colloidal systems across a wide range of industries.
Technical Overview
Dynamic Light Scattering (DLS), also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS), is a well-established analytical technique for determining the particle size distribution of particles, droplets, or macromolecules suspended in a liquid medium. The technique is particularly powerful in the nanometer size range, where traditional methods such as laser diffraction or sieve analysis lose resolution.
Measurement Principle
In a DLS measurement, a coherent laser beam illuminates the sample, and the light scattered by the suspended particles is detected at a fixed angle (commonly 90 degrees, or 173 degrees in backscatter configurations). Because the particles undergo Brownian motion, the scattered light intensity fluctuates over time. Smaller particles diffuse faster, producing rapid intensity fluctuations, while larger particles diffuse more slowly. These fluctuations are analyzed by a digital correlator to build an autocorrelation function, from which the translational diffusion coefficient is extracted. The hydrodynamic diameter is then calculated using the Stokes-Einstein equation, which relates the diffusion coefficient to temperature, the viscosity of the dispersant, and particle size.
Key Characteristics
- Measurable size range: typically from approximately 1 nm up to 6–10 micrometers, depending on particle density and instrument configuration
- Non-destructive measurement requiring only a small sample volume (often less than 1 mL)
- Rapid analysis, usually completed within a few minutes per sample
- Excellent repeatability and reproducibility for monodisperse and moderately polydisperse systems
- Statistically robust results, since millions of scattering events are averaged in every measurement
Testing Samples
DLS particle size distribution testing is applicable to any sample in which particles are stably dispersed in an optically transparent liquid. Typical sample categories include:
Nanomaterials
Metal nanoparticles (gold, silver, copper), metal oxide nanoparticles (titanium dioxide, zinc oxide, silica, alumina, iron oxide), quantum dots, upconversion nanoparticles, carbon-based materials such as graphene oxide dispersions, and functionalized nanocomposites.
Biological and Pharmaceutical Samples
Proteins, antibodies, enzymes, vaccines, viruses and virus-like particles, exosomes and extracellular vesicles, liposomes, lipid nanoparticles (LNPs), micelles, polymer-drug conjugates, and mRNA delivery systems used in modern biopharmaceutical development.
Polymer and Colloidal Systems
Polymer latices, emulsion polymers, waterborne dispersions, surfactant micelles, nanoemulsions, and microemulsions used in coatings, adhesives, and consumer products.
Industrial Suspensions and Dispersions
Chemical mechanical planarization (CMP) slurries for semiconductor manufacturing, printing inks, pigments, ceramic slurries, abrasive dispersions, cosmetic formulations, and food emulsions such as beverage bases and dairy protein systems.
Testing Items
A comprehensive DLS particle size distribution test typically reports the following parameters:
- Z-average hydrodynamic diameter: the intensity-weighted mean size calculated by cumulants analysis, the primary reported value under ISO 22412
- Polydispersity index (PDI): a dimensionless indicator of distribution width, where values below 0.1 generally indicate a highly monodisperse sample
- Particle size distribution curve: intensity-weighted distribution, optionally converted to volume-weighted and number-weighted distributions using Mie theory with known optical properties
- Representative size percentiles (for example, D10, D50, and D90) derived from the resolved distribution
- Zeta potential: optionally measured by electrophoretic light scattering (ELS) in accordance with ISO 13099 to evaluate colloidal stability
- Temperature-dependent or time-dependent sizing studies for formulation stability assessment
Testing Methods
Applicable Standards
- ISO 22412: Particle size analysis — Dynamic light scattering (DLS), the primary international standard governing the method
- ISO 13321: the predecessor standard for photon correlation spectroscopy, largely superseded by ISO 22412
- ASTM E2490: Standard Guide for Measurement of Particle Size Distribution of Nanomaterials in Suspension by Dynamic Light Scattering
- GB/T 29022: the Chinese national standard for particle size analysis by dynamic light scattering, technically aligned with ISO 22412
- ISO 13099 series: relevant when zeta potential is measured alongside particle size to characterize dispersion stability
Sample Preparation
Proper sample preparation is critical for reliable DLS results. Samples are diluted with a clean, dust-free dispersant (commonly filtered deionized water or an appropriate buffer) to achieve a suitable scattering intensity within the instrument's optimal detection range. Dust and large aggregates are removed by filtration through 0.22 or 0.45 micrometer membranes or by gentle centrifugation, since a single dust particle can dominate the scattering signal. The dispersant's viscosity and refractive index must be known or measured, as both directly enter the calculation of hydrodynamic diameter. Cuvettes are cleaned and handled in a dust-controlled environment, and samples are allowed to equilibrate at the measurement temperature, typically 25 degrees Celsius, before data acquisition.
Instrumentation and Measurement Procedure
A standard DLS instrument consists of a stable laser light source, a temperature-controlled sample cell holder, a highly sensitive detector positioned at a defined scattering angle, and a digital autocorrelator. During measurement, the laser attenuation is automatically adjusted to ensure the detected count rate falls within the optimal range. Each sample is typically measured in multiple runs (for example, three measurements of ten to fifteen runs each) to verify repeatability. Instrument performance is routinely verified using certified reference materials such as traceable polystyrene latex sphere standards.
Data Analysis and Reporting
The cumulants method defined in ISO 22412 is used to derive the Z-average diameter and the polydispersity index. For polydisperse samples, inversion algorithms such as CONTIN, non-negative least squares (NNLS), or general-purpose multi-modal deconvolution are applied to resolve the intensity-weighted size distribution. Conversion to volume or number distributions requires accurate refractive index and absorption values for the particle material. The final test report documents the measurement conditions, raw correlation data, calculated size parameters, distribution plots, and any deviations or observations relevant to data interpretation.
Applications
DLS particle size distribution testing supports quality control, regulatory compliance, and research and development across many sectors:
- Pharmaceuticals and biotechnology: characterization of protein aggregation, liposome and LNP size control, formulation screening, stability studies, and batch release testing for injectable and ophthalmic products
- Nanomaterial regulation: generating nano-specific data required for notified substance registrations, safety assessments, and nanomaterial definitions applied by regulatory frameworks worldwide
- Cosmetics and personal care: verifying the size of nanoparticle UV filters and emulsion droplets to support ingredient labeling and product claims
- Semiconductors and electronics: quality control of CMP slurries, where particle size directly affects polishing performance and defect levels
- Inks, coatings, and pigments: optimizing dispersion fineness, hiding power, and storage stability
- Food and beverage: characterizing emulsion droplet size and protein suspensions to control texture, mouthfeel, and shelf life
- Academic and industrial research: routine nanoparticle characterization for publications, patents, and product development programs
FAQ
What size range can DLS measure?
Most DLS instruments reliably measure hydrodynamic diameters from approximately 1 nm to about 6–10 micrometers. The practical upper limit depends on particle density, since dense large particles may sediment during measurement, and the lower limit depends on the scattering contrast between the particles and the dispersant.
What is the difference between DLS and laser diffraction?
DLS is optimized for the nanometer to sub-micrometer range and measures diffusion behavior, making it highly sensitive to small particles and subtle changes such as early aggregation. Laser diffraction covers a broader range (roughly 0.1 to several thousand micrometers) and is better suited to larger powders and sprays. The two techniques are complementary, and accredited third-party laboratories often apply both for full-range characterization.
What does the polydispersity index (PDI) mean?
The PDI is a dimensionless measure of the width of the size distribution derived from cumulants analysis. Values below 0.1 indicate a nearly monodisperse sample, values between 0.1 and 0.4 indicate a moderately polydisperse distribution, and values above roughly 0.7 indicate a broad or multi-modal distribution that may require more careful interpretation or complementary techniques.
What sample concentration is required for testing?
The optimal concentration depends on the particle size and material. As a general guideline, samples should be diluted until they appear nearly transparent or lightly opalescent, so that multiple scattering effects are minimized. The instrument automatically adjusts laser attenuation, and experienced operators confirm the count rate is within the valid range before accepting data.
Can DLS analyze polydisperse or agglomerated samples?
Yes, but with limitations. Because scattering intensity scales strongly with particle size (approximately to the sixth power for small particles), the intensity-weighted result is dominated by larger particles or aggregates. Multi-modal distributions can be resolved if the size modes differ by roughly a factor of three or more. For heavily agglomerated or very broad distributions, complementary techniques such as electron microscopy or nanoparticle tracking analysis are recommended for cross-validation.
How should samples be submitted for DLS testing?
Samples should be supplied as stable liquid dispersions of sufficient concentration, clearly labeled with the dispersant composition, approximate solids content, and any known particle material properties (refractive index and absorption). Information on the expected size range and the purpose of the test helps the laboratory select appropriate preparation and analysis protocols. Typical sample volumes of a few milliliters per specimen are sufficient.
Why is the hydrodynamic diameter larger than the dry particle size?
The hydrodynamic diameter measured by DLS includes not only the solid particle core but also any adsorbed surface layers, surfactants, stabilizing polymers, and the solvation shell that moves with the particle during Brownian motion. Consequently, DLS results for the same material are often slightly larger than dry-state sizes obtained by electron microscopy. Both values are correct; they simply describe the particle in different states.