Nanoparticle Particle-Size Testing by DLS — Dispersion & Reporting
Dynamic light scattering characterizes motion-derived hydrodynamic size in a dispersion. The result depends on the medium, concentration, preparation and distribution analysis; it should not be presented as a direct image-based measurement of dry primary-particle diameter.
State the property and dispersion conditions
Provide the medium, dispersant, concentration, temperature and viscosity information relevant to the analysis. DLS is useful for comparing appropriately prepared dispersions, but agglomerates, dust or changing particle populations can complicate interpretation. Specify whether the objective is lot comparison, process troubleshooting or behaviour in a use medium. A number without conditions cannot meaningfully serve as a supplier specification.
Agree preparation before the comparison
Dilution, sonication and filtration can alter the sample that reaches the instrument. Discuss whether these steps are necessary and whether an as-received state can also be evaluated. Use appropriate medium blanks and reference checks and record elapsed time after preparation. The NIST batch-mode aqueous DLS protocol is a technical reference, not a claim that every complex matrix can be measured by the same settings.
Ask what the output distribution means
NIST's work on DLS distributions describes interpretation problems with weighting, mean values and commercial output choices. Request the cumulant result and relevant quality measures alongside any derived distributions, with their basis identified. Conversion between intensity-, volume- and number-weighted results depends on assumptions and should not be treated as an independent measurement. A large-particle tail warrants investigation rather than being removed only to improve a mean.
Use an orthogonal method for a different question
If the decision needs particle morphology or primary size, discuss electron microscopy and its own preparation/counting limitations. If a broad or multimodal population exceeds the useful DLS interpretation, agree an alternative technique. Compare methods by the property they measure, not by forcing their means to match. The RFQ should ask for a suitability assessment and interpretable report, with uncertainty and exclusions stated.
Sample considerations
- Provide aqueous/non-aqueous medium, concentration, surface treatment, stability history and hazards.
- State intended property and reporting basis; agree dilution, sonication, filtration and measurement timing.
- Supply medium blanks and representative lots; define quality criteria and any orthogonal analysis.
Questions to ask a laboratory
- Are concentration and medium suitable for interpretable DLS in this sample?
- Which reported distributions are measured or model-derived, and what assumptions are used?
- What evidence triggers an alternative method for a broad or unstable population?
Laboratories with relevant capabilities
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Sources and scope references
Related testing briefs
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