ISO 13320:2020 Particle size analysis — Laser diffraction methods.
5.4 Sample inspection, preparation, dispersion and concentration
5.4.1 Sample inspection
Inspect the material to be analysed, visually or with the aid of a microscope, in order to: a) estimate the size range and particle shape; and b) check whether the particles have been dispersed adequately or will require further treatment.
The size distribution measured in a test sample is only valid for a batch of material if the sample is representative for that batch and has been dispersed adequately. See Annex C.
The inspection of the fully dispersed state of a suspension of particles is difficult to achieve with the naked eye. Any inspection using a microscope should be conducted from a pool of suspension without using a cover slip. Such inspections are very subjective, not definitive but are helpful in many cases.
Prepare a representative test sample of suitable volume for the measurement by using an adequate sample splitting technique, e.g. a rotating riffler (see ISO 14488).
Very small test samples can be taken from a well-mixed paste of particles in liquid. The consistency of the paste then minimizes segregation errors. The pastes are formed by adding dispersant to the sample drop by drop while mixing it with a spatula. A good consistency for the paste is one like honey or toothpaste. it; by mistake, the paste becomes too fluid, it shall not be used, and a new preparation shall be initiated.
If the maximum size exceeds the measuring range, remove the material that is too coarse, e.g. by presieving. In this case, determine and report the amount/percentage removed.
Sprays, aerosols and gas bubbles in liquid are usually measured directly, provided that their concentration is at an adequate level (see D.1 to D2). since sampling or dilution is generally very difficult without altering the PSD. If droplets are sprayed into still air, then the small droplets decelerate faster than the large ones, leading to a potential velocity bias. Therefore, it is preferable to spray into a suitable moving air stream matched to that of the spray. Consideration should also be given to the prospect of droplet evaporation, which may cause significant errors, especially for droplets in the sub-micrometer range. Rapid evaporation of such droplets reduces their size or even makes them disappear. Moreover, artefacts in the size distribution may appear due to a changing refractive index around the droplets, resulting from the evolving vapour and the temperature decrease during evaporation.
Dry powders may be dispersed either in air or in a liquid. The dispersion procedure should be adjusted to the purpose of the measurement, e.g. it shall be decided whether agglomerates should be detected or dispersed to primary particles. See Annex C.ISO 13320 pdf download.