Particle size analyzer: which technique fits your sample

Choosing a technique for particle size testing

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, reference materials and what a particle size distribution analyzer claims

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.

What a viscosity analyzer measures: a condition as much as a number

Most materials of interest are not simple liquids: their apparent viscosity depends on shear rate, on time under shear and on temperature. A single viscosity figure is meaningless without the geometry, the shear rate and the temperature it was measured at.

A simple rotational instrument gives a figure at one condition, which is adequate for a specification on a consistent material. A rheometer sweeps shear rate and measures elastic behaviour, which is what formulation and processing questions actually need. Choose by whether the answer has to be a number or a curve.

Optical rotation, the automatic polarimeter and the saccharimeter

An optically active substance rotates polarised light in proportion to its concentration and path length, which makes rotation a concentration measurement for a known substance. An automated instrument removes the operator's judgement of the null point and controls temperature, which the measurement is sensitive to.

An instrument reading directly in a sugar scale is the same measurement expressed in the units a sugar industry standard defines. It is a specialised calibration rather than a different physics, and it is what a trade specification usually asks for.

Osmolality, and why it is measured rather than calculated

Osmolality is the total concentration of dissolved particles and it is what determines whether a solution is tolerated by cells or by a patient. It is measured by freezing point depression or vapour pressure rather than calculated, because dissociation and interactions make the calculation unreliable.

It is a release test for parenteral products and a routine check for culture media, where a medium of correct composition and wrong osmolality will not support cells. A small instrument on the bench is usually the right answer, calibrated against certified standards.

What a particle size analyzer price includes

The instrument is rarely the whole cost. A dispersion unit per sample type, the cell or cuvette set, the software modules that produce the report you need and the qualification on installation are quoted separately often enough that two quotations for the same technique are not comparable as they arrive.

The running cost is reference materials and the time to verify against them, and it recurs. Ask for a configuration against the sample types you actually have, and ask what it costs to add one more of them in two years.

Common questions

Which particle analyzer 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 does each particle size analyzer instrument give a different size?

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.

Why is a single viscosity figure not enough?

Because apparent viscosity depends on shear rate, time and temperature for most real materials. A figure needs its geometry, shear rate and temperature; where the answer is a curve, a rheometer rather than a simple viscometer is the instrument.

Why measure osmolality rather than calculate it?

Because dissociation and interactions make the calculation unreliable, and it is the measured total particle concentration that determines tolerance by cells or by a patient. It is measured by freezing point depression or vapour pressure.

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