What Is Dynamic Light Scattering (DLS)? Principle, Applications & Limits
Surface analysis revealing biological film contamination on OLED device layer structureWhat Is Dynamic Light Scattering?
Dynamic Light Scattering (DLS) — also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS) — is a non-invasive analytical technique that measures the size and size distribution of particles and macromolecules in solution or suspension by analysing the temporal fluctuations in scattered laser light intensity caused by Brownian motion. DLS is the dominant technique for measuring particle size in the 0.3 nm to 10 µm range — making it the method of choice for nanoparticles, colloidal dispersions, proteins, polymers, and emulsions.
Principle of DLS
Particles or macromolecules suspended in a liquid undergo Brownian motion — random thermal movement driven by collisions with solvent molecules. The rate of Brownian motion (diffusion coefficient D) is governed by particle size — smaller particles diffuse faster; larger particles diffuse more slowly. This relationship is quantified by the Stokes-Einstein equation:
D = kT / (3πηd)
where k is Boltzmann’s constant, T is absolute temperature, η is solvent viscosity, and d is the hydrodynamic diameter of the particle.
When a laser beam illuminates the suspension, suspended particles scatter light in all directions. Because particles are moving randomly, the scattered light from different particles alternately interferes constructively and destructively — causing the measured scattered intensity to fluctuate rapidly at rates proportional to particle diffusion speed.
A photon detector and digital autocorrelator measure the time-dependent correlation of intensity fluctuations — the autocorrelation function (ACF). The decay rate of the ACF is directly related to the diffusion coefficient, from which the hydrodynamic diameter is calculated via the Stokes-Einstein equation.
Key Output Parameters from DLS
Z-Average Diameter (Cumulants Mean)
The intensity-weighted mean hydrodynamic diameter — the primary result reported from DLS. It is most sensitive to larger particles (intensity ∝ d⁶ for Rayleigh scatterers) — making DLS particularly effective for detecting trace quantities of aggregates or large particles.
Polydispersity Index (PDI)
PDI characterises the breadth of the particle size distribution — ranging from 0 (perfectly monodisperse) to 1 (highly polydisperse). PDI < 0.1 indicates a narrow, monodisperse sample; PDI > 0.3 indicates significant heterogeneity. PDI is widely used for quality control of monoclonal antibody (mAb) formulations and nanoparticle preparations.
Size Distribution by Intensity, Volume, and Number
Full size distribution data (converted from the ACF by regularisation algorithms such as CONTIN or NNLS) provides intensity-weighted, volume-weighted, and number-weighted distributions — with different weighting revealing different aspects of polydisperse samples.
DLS vs. Other Particle Sizing Techniques
| Property | DLS | Laser Diffraction | NTA (Nanoparticle Tracking) | ESZ (Coulter) |
| Size range | 0.3 nm–10 µm | 20 nm–3.5 mm | 10–2000 nm | 0.4–1600 µm |
| Measurement basis | Diffusion coefficient | Light scattering angle | Individual tracking | Volume displacement |
| Concentration requirement | Low | Low–moderate | Low | Low |
| Sensitivity to aggregates | Very high | High | Moderate | Moderate |
| Primary applications | Nanoparticles, proteins | Emulsions, powders | Virus, EVs, exosomes | Cells, microparticles |
Industrial Applications
Biopharmaceuticals and Protein Analysis: DLS is the primary quality control tool for mAb formulation development — measuring hydrodynamic size and aggregation state. PDI monitoring verifies product consistency; aggregate detection (Z-average shift, PDI increase) identifies stability issues that could trigger immunogenicity. ICH Q5E references DLS for comparability of biopharmaceutical manufacturing processes.
Nanoparticle Characterisation: Polymer nanoparticles, liposomes, quantum dots, gold nanoparticles, silica nanoparticles, and carbon nanotubes are all characterised by DLS as part of formulation development, process scale-up, and quality release testing.
Colloidal Stability Assessment: DLS monitoring at elevated temperature or extreme pH tracks nanoparticle aggregation kinetics — providing colloid stability index and predicting long-term shelf stability.
Polymer Molecular Weight Characterisation: For linear polymers in dilute solution, DLS measures hydrodynamic radius, which is related to molecular weight through the Mark-Houwink relationship — providing a rapid molecular weight estimate complementary to GPC.
Why Choose Infinita Lab for DLS and Particle Size Analysis?
Infinita Lab provides DLS particle size and PDI analysis, zeta potential measurement, and comprehensive nanoparticle characterisation through our nationwide accredited analytical testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
What is the hydrodynamic diameter measured by DLS and how does it differ from geometric diameter? Hydrodynamic diameter is the diameter of a sphere that diffuses at the same rate as the measured particle in the given solvent. It includes the particle core, any surface coating or functionalisation, and the layer of solvent molecules that move with the particle (hydration shell). For hard spheres, hydrodynamic diameter ≈ geometric diameter; for soft or coated particles (proteins, polymer-coated nanoparticles), hydrodynamic diameter is larger than the geometric diameter.
What concentration range is optimal for DLS measurements? DLS requires dilute suspensions to prevent multiple scattering (photons scattered by more than one particle before detection) which distorts the autocorrelation function. Optimal sample concentrations depend on particle size and refractive index contrast, but typically range from 0.01–1 mg/mL for nanoparticles. For proteins (mAbs), concentrations of 0.1–10 mg/mL are typically used, often with dilution from formulation concentrations.
Can DLS measure particle size in polydisperse samples with multiple populations? DLS struggles to resolve multiple populations with less than a factor of 3 size difference — the technique inherently averages the diffusion coefficients of all populations. Multimodal size distributions with widely separated peaks can be partially resolved by regularisation algorithms, but populations within 2–3× of each other in size are not reliably separated. Techniques such as analytical ultracentrifugation (AUC) or nanoparticle tracking analysis (NTA) provide better resolution for polydisperse samples.
Why is DLS particularly sensitive to aggregates in protein formulations? DLS signal intensity is proportional to d⁶ for small particles in the Rayleigh scattering regime — meaning a 100 nm aggregate scatters one million times more intensely than a 10 nm monomer. Even trace quantities (sub-percent by mass) of large aggregates dominate the DLS autocorrelation function, causing a shift in Z-average and PDI. This extreme aggregate sensitivity makes DLS the most sensitive early warning tool for protein aggregation in biopharmaceutical development.
What is zeta potential and how does it relate to DLS measurement? Zeta potential is the electric potential at the hydrodynamic shear plane surrounding a charged particle in suspension — a measure of electrostatic repulsion between particles that governs colloidal stability. Particles with zeta potential > +30 mV or < −30 mV are generally considered stable against aggregation. Modern DLS instruments simultaneously measure hydrodynamic size and zeta potential (by electrophoretic light scattering, ELS) — providing comprehensive nanoparticle stability characterisation from a single measurement.
