Every characterisation method measures its own quantity

How it works

Imaging counts individual objects and shows shape, but it samples a tiny population and requires a dried, often coated, specimen — a state that may not exist in the medium of use. Dynamic light scattering reports a hydrodynamic size in suspension, weighted so that a small number of large aggregates can dominate the result. Gas adsorption reports accessible surface area of a dry powder, including roughness and porosity that an idealised geometric estimate ignores. None of these is wrong; each answers its own question. Maha’s proposed reporting rule is simple: name the method beside every number, and never let a value measured one way silently satisfy a specification written for another.

A concrete case

A dispersion is described as “20 nm”. Microscopy of the dried sample supports 20 nm primary particles; light scattering in the buffer reports 140 nm. Both can be true at once if the particles aggregate in that buffer, and the second number is the one that matters for what the dispersion will do.

Questions worth asking

• For every reported number, ask: which method, which specimen state, which weighting, how many objects.
• Ask whether the value was measured in the medium where the material will be used.
• When two methods disagree, treat the disagreement as information about the sample rather than as an error to be averaged away.

Sources

• NIST Technical Note 1297 — Guidelines for Evaluating and Expressing Uncertainty ↗

  Uncertainty components are classified by how they are evaluated — Type A by statistical analysis of repeated observations, Type B by other means — and a reported result is expected to carry the basis of its uncertainty rather than a bare number.

  Boundary: A general metrology guideline. It supplies no particle-size uncertainty budget and certifies no instrument or laboratory.

  Locator, anchor and reuse basis

  Read at: §2 Classification of components of uncertainty; §3 Type A; §4 Type B; §7 Reporting uncertainty. Inspected 2026-09-19. Original paraphrase and link only.

  Verify by searching the source for: the statistical analysis of series of observations is termed a Type A evaluation. If that phrase is not there, or does not carry the meaning stated above, this citation is wrong and we want to know.

Continue

• Report a nanomaterial size the way a measurement is reported
• One batch is one batch
• Why the same material behaves differently when it is small