Dynamic Light Scattering (DLS) Particle Size Analysis: Principles, Methods, and Applications

Dynamic Light Scattering (DLS), also known as Photon Correlation Spectroscopy (PCS) or Quasi-Elastic Light Scattering (QELS), is a fast, non-destructive, and highly sensitive technique for measuring the hydrodynamic diameter and size distribution of particles suspended in liquids, typically covering the range from below 1 nanometer to approximately 10 micrometers. As the reference method specified in ISO 22412, DLS particle size analysis has become the industry standard for characterizing nanoparticles, proteins, liposomes, lipid nanoparticles, emulsions, and polymer latexes in pharmaceutical development, nanomaterials research, and industrial quality control. This article explains the underlying principles, sample requirements, test items, measurement procedures, application fields, and frequently asked questions related to DLS analysis performed by accredited third-party laboratories.

Technical Overview

Dynamic Light Scattering (DLS) is an analytical technique that determines particle size by measuring the random fluctuations in the intensity of laser light scattered by particles undergoing Brownian motion in a suspension. Because smaller particles move faster than larger ones, the rate of intensity fluctuation carries direct information about the translational diffusion coefficient of the particles. The scattered intensity signal is processed by a digital correlator, which builds an autocorrelation function describing how quickly the signal decorrelates over time.

The core relationship of the technique is the Stokes–Einstein equation, which links the measured translational diffusion coefficient (D) to the hydrodynamic diameter (Dh):

• Dh = kBT / (3πηD), where kB is the Boltzmann constant, T is the absolute temperature, and η is the dynamic viscosity of the dispersing medium.
• The hydrodynamic diameter represents the diameter of a hypothetical hard sphere that diffuses at the same speed as the particle being measured, and it includes adsorbed solvent layers, surface coatings, and hydration shells.

Modern DLS instruments typically operate with a laser wavelength of 633 nm or 532 nm and offer multiple detection geometries. Non-invasive backscatter (NIBS) detection at approximately 173° suppresses multiple scattering effects and extends the usable concentration range, 90° side-scatter is the classical configuration described in ISO 13321 and ISO 22412, and low-angle forward scatter at approximately 13° is used for very large or weakly scattering particles. Data analysis is performed using the cumulants method, which yields the intensity-weighted mean size (Z-average) and the polydispersity index (PDI), or using inversion algorithms such as CONTIN and non-negative least squares (NNLS), which generate full intensity, volume, and number weighted size distributions.

Key performance characteristics of DLS include:

• Size range: approximately 0.3 nm to 10 µm, with optimal performance between 1 nm and 1 µm.
• Measurement time: typically 1 to 5 minutes per sample, including temperature equilibration.
• Sample volume: as little as 20 µL in micro-volume cuvettes, or 1 to 2 mL in standard cuvettes.
• Non-destructive measurement, allowing sample recovery.
• Excellent repeatability of typically better than 2% on the Z-average for monodisperse reference materials.

The internationally recognized standard governing the method is ISO 22412 (Particle size analysis — Dynamic light scattering, DLS), which supersedes ISO 13321 and defines requirements for instrument qualification, measurement procedures, and reporting of Z-average and PDI results. Electrophoretic light scattering (ELS) for zeta potential, governed by ISO 13099-2, is frequently performed on the same instrument platform.

Testing Samples

DLS analysis is applicable to any colloidal system, dispersion, or solution containing particles within the nanometer to low-micrometer size range that is optically transparent or only slightly turbid. Typical sample categories submitted to accredited third-party laboratories include the following.

Inorganic and Metallic Nanoparticle Suspensions

Gold and silver nanoparticles, silica and metal oxide nanoparticles (titanium dioxide, zinc oxide, iron oxide), quantum dots, carbon-based nanomaterials, and ceramic powders dispersed in aqueous or organic media. DLS verifies nominal particle size, agglomeration state, and batch-to-batch consistency of nanomaterials.

Proteins, Antibodies, and Biologics

Monoclonal antibodies, recombinant proteins, enzymes, and vaccine antigens. DLS rapidly screens for oligomerization, self-association, and subvisible aggregates, supporting developability assessment, formulation screening, and stability studies.

Liposomes, Lipid Nanoparticles, and Extracellular Vesicles

Liposome drug carriers, mRNA-loaded lipid nanoparticles (LNPs), exosomes and extracellular vesicles, virus-like particles, and bacteriophages. Hydrodynamic size and PDI are critical quality attributes for these systems and are routinely measured by DLS in accordance with ISO 22412.

Polymers, Latexes, Micelles, and Emulsions

Polymer latexes, block copolymer micelles, polystyrene and PMMA reference particles, nanoemulsions, microemulsions, lipophilic active ingredient dispersions, and self-assembled surfactant systems used in cosmetics, food, and coatings.

Industrial Dispersions and Process Liquids

Chemical mechanical planarization (CMP) slurries for semiconductor manufacturing, pigments and inks, coating dispersions, catalyst slurries, battery electrode materials, and colloidal silica. DLS supports incoming material inspection and in-process particle size monitoring.

Testing Items

A standard DLS characterization report from an accredited third-party laboratory typically includes the following measurable parameters.

• Z-average hydrodynamic diameter: the intensity-weighted mean harmonic size calculated by the cumulants method, the primary reported result under ISO 22412.
• Polydispersity index (PDI): a dimensionless indicator of distribution width derived from the second cumulant; PDI below 0.1 indicates a near-monodisperse sample, while values above 0.7 indicate a highly polydisperse distribution.
• Intensity-weighted size distribution: peak positions, peak widths, and relative peak areas, which are most sensitive to large particles and trace aggregates.
• Volume- and number-weighted size distributions: obtained through Mie-theory-based transformation using known or assumed refractive index and absorption values of the particles.
• Percentile diameters (d10, d50, d90) of each distribution, where applicable.
• Translational diffusion coefficient, reported at the measurement temperature.
• Derived count rate (scattered intensity), a useful indicator of concentration and scatterer cross-section.
• Correlogram quality parameters, including intercept and baseline values used to validate data quality.
• Optional complementary measurements on combined DLS/ELS platforms: zeta potential by electrophoretic light scattering per ISO 13099-2, and molecular weight estimation by static light scattering with a Debye plot.

Testing Methods

Accurate DLS results depend strongly on correct sample preparation, instrument setup, and data validation. Accredited third-party laboratories follow the workflow summarized below, aligned with ISO 22412.

Sample Preparation

• Assessment of the as-received sample: appearance, stability, and approximate concentration.
• Dilution with a dust-free, filtered medium matched to the original dispersant so that the attenuator reaches an optimal position and multiple scattering is minimized.
• Removal of dust and large contaminants by filtration through 0.22 µm or 0.45 µm syringe filters where the particle size permits, or by gentle centrifugation and decantation of the supernatant for larger particles.
• Selection of cuvette: disposable low-volume polystyrene or acrylic cuvettes for aqueous samples, and clean quartz/glass cuvettes for organic solvents and high-temperature measurements.
• Dust-free handling throughout preparation to avoid spurious large scatterers, which dominate the intensity signal.

Measurement Procedure

• Recording of dispersant parameters: viscosity, refractive index, and dielectric constant, corrected for temperature and any co-solutes.
• Temperature equilibration of the cuvette in the instrument, typically at 25.0 °C (other temperatures, for example 4 °C or 37 °C, can be selected to simulate storage or physiological conditions).
• Measurement using automatic or manual attenuator and cuvette position selection, with a minimum of three repeated measurements, each consisting of multiple runs of 10 seconds or longer as prescribed by ISO 22412.
• Selection of the analysis model: cumulants analysis for Z-average and PDI, and multi-modal distribution analysis (for example CONTIN or general-purpose algorithms) for polydisperse or multi-peak samples.
• Verification of repeatability: the coefficient of variation of the Z-average across repeated measurements should typically be below 2 to 5%.

Data Quality and Reporting

• Evaluation of the correlation function: high intercept, flat stable baseline, and smooth exponential decay are required for valid cumulants analysis.
• Assessment of count rate stability across runs to detect sedimentation, aggregation during measurement, or bubble formation.
• Reporting of measurement angle, temperature, concentration, dispersant properties, analysis model, Z-average, PDI, size distribution plot, and correlogram, together with a statement of measurement uncertainty where applicable.
• For wide or multi-modal distributions, results are interpreted with caution because the intensity weighting strongly emphasizes larger populations; complementary techniques such as laser diffraction (ISO 13320) or nanoparticle tracking analysis (NTA) may be recommended for confirmation.

Applications

DLS particle size analysis is applied across research, development, manufacturing, and regulatory submission contexts. Major application areas include the following.

• Pharmaceuticals and biopharmaceuticals: characterization of liposomes, lipid nanoparticles, polymeric nanoparticles, and nanoemulsions; aggregate and stability screening of protein therapeutics; support for formulation optimization, comparability studies, and regulatory documentation.
• Nanomedicine and drug delivery: verification of carrier size and PDI, which influence biodistribution, cellular uptake, release kinetics, and sterile filtration feasibility.
• Nanomaterials research and manufacturing: incoming inspection and lot release of nanoparticle raw materials, agglomeration studies, and surface functionalization verification.
• Semiconductors and electronics: particle size and stability control of CMP slurries, where large particles cause wafer defects.
• Paints, coatings, inks, and pigments: dispersion quality control, grinding endpoint determination, and shelf-life stability evaluation.
• Cosmetics and personal care: characterization of emulsions, liposomes, encapsulated actives, and sunscreen dispersions such as titanium dioxide and zinc oxide.
• Food and beverage: nanoemulsion and emulsion stability studies, beverage turbidity investigation, and encapsulated ingredient characterization.
• Polymers and chemicals: latex polymerization monitoring, micelle characterization, and colloidal silica quality control.
• Environmental and academic research: natural colloids, microplastic and nanoplastic fragment sizing, and fundamental colloid science studies.

FAQ

What size range can DLS measure?

Typical instruments cover approximately 0.3 nm to 10 µm hydrodynamic diameter. The most reliable range is roughly 1 nm to 1 µm; particles larger than several micrometers sediment rapidly and scatter in a manner that makes correlation analysis less accurate, while extremely small particles scatter weakly and require higher concentration or laser power.

What sample concentration is required?

The usable concentration depends on the particle material and size. As a general guideline, aqueous samples between approximately 0.01 mg/mL and 1 mg/mL often work well for many nanomaterials, while concentrated samples can be diluted. Very high concentrations cause multiple scattering and particle interactions that bias results, whereas very low concentrations produce insufficient scattering. Backscatter optics with automatic attenuation tolerate a wider concentration window than classical 90° systems.

How much sample volume is needed?

Standard measurements use approximately 1 mL in a 10 mm path cuvette, but low-volume cells allow measurements with as little as 20 to 40 µL. Submitting 1 to 5 mL of sample gives the laboratory flexibility for optimization and repeat measurements.

What is the difference between Z-average, intensity, volume, and number distributions?

The Z-average is the cumulants-derived intensity-weighted mean and is the standard reported value in ISO 22412. Intensity distributions weight scattering intensity, which scales with roughly the sixth power of diameter, so large particles and trace aggregates are strongly emphasized. Volume and number distributions are mathematically transformed from intensity data using particle optical properties; they are model-dependent but better represent the true proportion of small particles.

What is considered a good polydispersity index (PDI)?

A PDI below 0.1 indicates a near-monodisperse system, 0.1 to 0.25 indicates a moderately narrow distribution, and values below 0.3 are generally acceptable for many drug delivery applications such as lipid nanoparticles. PDI values above 0.5 indicate broad or multi-modal distributions for which cumulants analysis becomes approximate and distribution analysis is recommended.

How does DLS differ from laser diffraction?

Laser diffraction (ISO 13320) is optimized for particles from roughly 0.1 µm to several millimeters and is the preferred method for powders and coarse suspensions, whereas DLS excels in the nanometer range and requires only very small sample volumes. The two techniques are complementary, and laboratories often recommend laser diffraction for confirmation when DLS indicates a significant coarse fraction.

How does DLS differ from nanoparticle tracking analysis (NTA)?

NTA tracks individual particles under a microscope and reports number-based concentrations and size distributions, typically for particles from about 30 nm to 1 µm. DLS measures an ensemble average far more quickly and with better precision for monodisperse samples, but NTA resolves polydisperse mixtures and provides particle concentration information that DLS cannot.

Does DLS measure the same size as electron microscopy?

No. DLS reports the hydrodynamic diameter, which includes solvent layers, adsorbed surfactants, and polymer coatings, and it assumes spherical shape. Electron microscopy measures the dry core diameter of individual particles. Hydrodynamic size is therefore usually larger than the TEM or SEM size, and the difference itself carries useful information about coating thickness and hydration.

Can DLS be performed in organic solvents or at different temperatures?

Yes. Glass cuvettes and solvent-resistant seals allow measurements in ethanol, toluene, hexane, and other media, provided the correct viscosity and refractive index of the solvent are entered. Temperature-controlled measurements, for example from 2 °C to 90 °C, support aggregation-on-heating studies, melting point determination of liposomes, and temperature-dependent stability assessment.

How accurate and reproducible are DLS results?

Instruments are qualified with NIST-traceable polystyrene latex standards, on which Z-average repeatability is typically better than 2% and reproducibility better than 5%. Accuracy for real samples depends on sample preparation quality, correct dispersant properties, and freedom from dust; strict adherence to ISO 22412 and experienced laboratory practice ensure reliable, comparable results.

Can DLS also measure zeta potential?

Yes. Most modern DLS platforms combine dynamic light scattering with electrophoretic light scattering (ELS), allowing zeta potential measurement per ISO 13099-2 on the same sample. Zeta potential complements size data by indicating colloidal stability and surface charge characteristics.

What are common sources of error in DLS measurements?

Frequent issues include dust contamination, bubbles in the cuvette, sedimentation of dense large particles, inappropriate concentration causing multiple scattering, incorrect viscosity or temperature inputs, and strong fluorescence from the sample. Accredited third-party laboratories control these factors through filtered reagents, validated preparation protocols, correlogram quality checks, and replicate measurements.