What is weighted?
Number-, volume- and intensity-weighted distributions can look very different, particularly when a small large-particle population is present.
Beyond the nominal value
Distribution width can determine whether a standard is usable.
A nominal 100 nm material is not one hundred percent 100 nm particles. The population has a distribution, and the reported width depends on the measurement method, weighting model, sample preparation and calculation.
CV (%) = (standard deviation ÷ mean) × 100Coefficient of variation is a useful normalized width metric when the mean and standard deviation refer to the same well-defined population and method. It should not be used to hide multimodal populations or compare fundamentally different weighting schemes.
Read the distribution correctly
Four questions before accepting “monodisperse.”
What is measured?
Hydrodynamic, projected and mobility diameters are different quantities. Their distribution widths need not match.
What was prepared?
Dilution, filtration, sonication, ionic strength and storage history can alter the measured population.
What was excluded?
Image segmentation, outlier rejection and fit models can narrow or broaden the reported result.
Certificate interpretation
Capture the complete statistical statement.
| Certificate field | Preferred interpretation | Weak interpretation |
|---|---|---|
| Mean or assigned value | Method-defined central value with units and conditions | The catalog number equals every particle |
| Standard deviation | Spread under the defined method and population | Universal spread across all methods |
| CV | Normalized width derived from the same mean and standard deviation | A universal quality score |
| Uncertainty | Uncertainty of the assigned measurement result | The particle population width |
| Range or percentiles | Population boundaries under a stated analysis | Guaranteed hard limits unless explicitly certified |
Questions engineers ask
Frequently asked questions
Does a low CV prove a sample is monodisperse?
It can support a narrow distribution under the stated method, but sampling, method, weighting and multimodality still matter.
Can DLS CV be compared directly with image-analysis CV?
Not automatically. The underlying weighting, measurand and population can differ.
Sources and verification
Technical basis
- NIST TEM sizing — Illustrates image-based nanoparticle size measurement and method definition.
- NIST DMA particle measurements — Illustrates mobility-diameter measurement for spherical particles.
- NIST measurement protocols — Supports explicit measurand, method and uncertainty statements.
Commercial specifications should be confirmed against the current Applied Physics quotation, certificate, lot documentation and product page before purchase or protocol use.
Continue the research
Related technical guidance
DLS vs. SEM/TEM/DMA
Compare how methods weight and define size.
NIST traceability
Separate distribution width from uncertainty.
Particle size standards
Audit whether the evidence supports calibration use.
Applied Physics technical pathway
Need a particle specification reviewed?
Send the target size, medium, concentration, application, certificate requirements and shipping destination. Applied Physics can evaluate the appropriate silica family or custom pathway.