Two of the World’s Best Measurement Labs Sized the Same Particles — and Got Different Numbers
In 2019, two national metrology institutes — Japan’s NMIJ and the United States’ NIST — ran what should have been the least surprising experiment in materials science. Both received particles from the same manufactured lot: polystyrene spheres, certified at 30, 50, 70 and 100 nanometres, about as well-behaved as particles get. Both used dynamic light scattering (DLS), a standard sizing technique. These are the laboratories other laboratories calibrate against. And as the study published in Metrologia reports, the apparent particle sizes were “invariably observed to disagree with each other.”
Nothing Was Broken
The uncomfortable finding was that both institutes were right. The instruments differed in laser wavelength, scattering angle, and sample concentration — and each of those quietly changes the number a DLS instrument reports. Only after the physicists corrected for every parameter did the two labs converge. Even then, the technique read the certified 29 nm particles as 33.2 nm — roughly 14% large — because DLS doesn’t measure the particle itself. It measures the particle plus the shell of water moving with it. The paper notes that international comparisons have generally found “a conspicuous difference” between DLS and methods that measure geometry directly, such as electron microscopy.
The Population One Instrument Couldn’t See
The stakes get clearer in a case documented by the EU and US nanomedicine characterisation laboratories. A sample measured by DLS showed a main peak at 87 nm — a tidy result. Laser diffraction on the same material found something else: a second population at 7.7 micrometres, nearly a hundred times larger, which DLS had missed entirely because large particles settle out of the laser’s path. One instrument said “uniform nano-suspension.” The other said “contains visible-scale aggregates.” Same sample.
What the Gatekeepers Concluded
Those two laboratories — the European Nanomedicine Characterisation Lab and the US National Cancer Institute’s characterisation lab, which screen materials before they reach regulators — drew a formal conclusion: “no single technique is capable to measure the particle size distribution” of every sample under every condition. Their standard practice is orthogonal measurement: multiple independent methods, cross-checked, with electron microscopy as the only direct witness to what crystals actually look like.
What This Means at the Purchasing Desk
Hydroxyapatite buyers live downstream of this physics. A “D50 of 5 µm” is not a property of the powder alone — it is a property of the powder plus the instrument, its settings, and the dispersion state of the sample. So the questions that matter on a datasheet are the ones metrologists ask each other: Which method produced this number? Dispersed or as-supplied? What are the D10 and D90, not just the median? Are there electron microscope images of this grade?
The Number That Actually Predicts Performance
If national laboratories need corrections to agree with each other, no buyer should compare two suppliers’ numbers at face value. What can be compared is consistency: whether a supplier’s distribution holds from lot to lot on the same declared method. That is a discipline of process control — the reason tightly controlled grades, such as Japanese eggshell-derived hydroxyapatite, are specified not for the figure on the sheet but because every delivery measures the same.
Sources: Takahashi et al. 2019, Metrologia (open access via PMC); Caputo et al. 2019, Journal of Controlled Release (open access).