1. Choose the right dispersion medium

DLS does not directly image a particle’s outline. It infers hydrodynamic size from diffusion behavior, using information such as medium viscosity and temperature. When testing the same type of particle in different media, make sure the medium settings are updated along with the sample name.

Bettersize recommends first checking whether the medium maintains the intended dispersion state. If switching to a lower-viscosity solvent would alter the particles or formulation, measurement convenience alone is not a good reason to make the change. For systems containing salts, buffers, or other additives, consider whether dilution will change the medium composition.

Before measurement, verify the test temperature, the medium viscosity at that temperature, and the medium refractive index. Allow the sample to equilibrate to the set temperature before starting. Recording these conditions provides a clearer basis for subsequent formulation comparisons and repeat experiments.

2. Keep the dispersion stable and define what you want to measure

Suspensions and emulsions should remain as uniform as possible during measurement. Visible settling, creaming, or phase separation changes the particle population in the detection region, making results collected at different times harder to compare directly.

What counts as “well dispersed” depends on the measurement objective. If you want to characterize particles after a defined dispersion treatment, evaluate a suitable preparation method. If you are studying aggregation within a formulation, the aggregates themselves are part of the information you need. Additional sonication or dispersant may then alter the state you intended to investigate.

Start by defining whether the measurement should represent the original formulation or a dispersion after a specified treatment. Then standardize the mixing procedure, treatment conditions, and time between preparation and measurement. This makes it easier to relate changes in particle size to differences between samples.

3. Control dust and impurities, and assess filtration before using it

DLS is particularly sensitive to larger scatterers. For particles much smaller than the wavelength of the incident light, where the Rayleigh approximation applies, the scattered intensity from a single particle is approximately proportional to the sixth power of its diameter (I ∝ d⁶), provided the material and optical conditions are the same. Even small amounts of dust or large particles can therefore contribute disproportionately to the signal and interfere with analysis of the target particles.

Filtration can help reduce certain contaminants. Common membrane pore sizes include 0.22 μm and 0.45 μm, but selection should account for the target particle size, membrane material, and compatibility with the medium. Verify that filtration does not retain the target particles or alter their state. If the membrane also removes aggregates that are genuinely present in the sample, the resulting distribution describes a selectively filtered sample. This is especially relevant when studying aggregation.

Begin with contamination sources you can control. Use a compatible membrane to prefilter the medium used for dilution, and check its background scattering. Check the cleanliness of the sample cell and sampling tools, and minimize sample exposure. Then decide whether the sample itself should be filtered based on the measurement objective. When using filtration or sonication, document the conditions and evaluate any changes caused by the treatment.

4. Find a concentration that provides sufficient, interpretable signal

At very low concentrations, particle scattering may be too weak. At high concentrations, multiple scattering, particle interactions and other effects may influence the measurement. The appropriate concentration depends on particle size, scattering properties, the medium, and the instrument’s detection configuration, so a single concentration cannot suit every sample.

A slightly opalescent appearance that remains transparent can be a starting point for an initial concentration check. However, particle size and the refractive index contrast between the particles and medium can make samples at the same concentration look different. A concentration series is therefore needed to establish the applicable range. Using an appropriate series of dilutions, check for an interval over which the measured size shows no clear systematic change, and assess the correlation function, signal quality, and repeat measurements together.

For concentrated or highly turbid samples, suitable BeNano series configurations offer backscatter detection and an adjustable scattering volume. Optimizing the detection position and shortening the optical path through the sample can reduce multiple scattering and help limit unnecessary dilution. Confirm the applicable range using representative samples, a concentration series, and data quality checks.

When diluting, maintain medium conditions compatible with the original system as far as possible. If particle size changes with dilution, consider both measurement effects and changes in the sample, such as altered medium composition or particle interactions. Do not automatically treat the most dilute result as the correct answer.

5. Interpret the size distribution alongside the average

Samples with narrow size distributions are generally easier to analyze by DLS. Broad or multimodal distributions require more careful interpretation. Larger particles may carry greater weight in the scattering signal, so a single average size may not adequately describe the entire sample.

When reviewing a report, consider the Z-average hydrodynamic diameter obtained by cumulants analysis, the polydispersity index (PDI), the intensity-weighted size distribution, and the correlation function together. Compare the consistency of repeat measurements as well. PDI provides information related to distribution breadth, but its meaning should be assessed in the context of the sample and analysis method.

If the sample has an inherently broad size distribution, preserve that characteristic and assess whether DLS can answer the question at hand. Narrowing a distribution through filtration or dispersion treatment is meaningful only when the treatment fits the measurement objective and its effects have been evaluated.