Particle size analyzers: choosing a technique before a particle analyzer or a particle size analyzer instrument, and why two of them report different numbers for one sample

Choosing a technique

1. Match the technique to the size range. Dynamic light scattering covers the nanometre range and suits proteins, liposomes and colloids. Laser diffraction covers a broad range from sub-micron upward and suits powders, emulsions and suspensions. Imaging and sieving cover larger particles and give shape information. The range decides the technique.
2. Understand what is being weighted. Scattering techniques weight toward larger particles because scattered intensity rises steeply with size, so a small number of aggregates dominates a result. Number-weighted techniques such as imaging count each particle once. Converting between weightings introduces assumptions, so report what was measured.
3. Dispersion decides reproducibility. Whether particles are fully dispersed, and whether dispersion breaks them, is the largest source of variation in practice. Establish a dispersion method with its energy and duration, write it into the method, and keep it constant, because the sample preparation is the measurement.
4. Wet or dry measurement. Wet measurement disperses in a liquid and suits fine and cohesive powders; dry measurement is faster, uses no solvent and can attrit fragile particles. Running both on the same material and comparing is the quickest way to find out which one your sample tolerates.
5. Optical properties for laser diffraction. Diffraction analysis needs the refractive index and absorption of the material, and a wrong value shifts the reported distribution systematically. This input is frequently guessed, and it is worth establishing properly once for each material.

Why results differ between laboratories

Different techniques, different dispersion, different optical constants and different weightings all move the number, which is why a specification should name the technique and the method rather than a size alone. Comparisons between laboratories are only meaningful when the whole method is shared.

Where a result must be comparable across sites, agree the instrument type, the dispersion procedure and the reporting basis in writing, and verify with a common reference material.

Verification and reference materials

Certified reference materials of known distribution are the way to show an instrument is performing, and running one on a defined schedule catches drift and misconfiguration. The record belongs with the results.

Keep a retained sample of a stable in-house material as well. It costs nothing and it catches the class of problem a spherical certified standard never exercises.

Common questions

Which particle sizing technique should I use?

Choose from the size range and what you need to know: dynamic light scattering for the nanometre range, laser diffraction for a broad range of powders and suspensions, imaging where shape matters, sieving for coarse material.

Why do two instruments give different sizes?

Because they measure different physical properties and report different equivalent diameters with different weightings. A result is only interpretable alongside the technique and the dispersion conditions.

Wet or dry dispersion?

Wet for fine and cohesive powders that need help separating; dry for speed and where no solvent is wanted, provided the particles survive the energy. Running both and comparing tells you which your material tolerates.

Why does refractive index matter?

Laser diffraction analysis uses the material's optical constants to convert the scattering pattern into a distribution. A wrong value shifts the whole result systematically, and it is often guessed rather than established.

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