Samples That Can and Cannot Be Measured

Particles to be measured on laser diffraction particle size analyzers are not limited to solids; they are sometimes liquids or gases. These particles exist in any medium (solid, liquid or gas). Table 1 shows the relationship between medium and particles that can be measured on a laser diffraction particle size analyzer.

Table 1 Relationship Between Medium and Particles That Can Be Measured on a Laser Diffraction Particle Size Analyzer

The most common method used for measurement is to disperse solid particles in a liquid. However, the dry measurement method whereby solid particles are injected and dispersed in air by compressed air and then measured also is rapidly gaining in popularity.
Measurement of liquid particles dispersed in a liquid is also commonly done, though it seems that there are not many people who know this. The measurement of emulsions is widely conducted, and laser diffraction particle size analyzers are used also in the development and quality control of new milk and fermented milk drink products.
Moreover, recently, these analyzers have come to be used for measuring gas particles, or bubbles, in liquids.
Well, then, the following five conditions at least must be satisfied even if combinations of particles and media can potentially be measured.

1) The refractive index of the particles and medium must not be matching.
2) The medium must allow a laser beam to penetrate. (Transmittance: 90 % or more)
3) The medium must not contain particles other than those to be measured.
4) A medium that does not contain particles must be prepared (since it is used for blank measurement).
5) The particle concentration in the medium must be appropriate or it must be able to be adjusted to an appropriate level.

Of course, even if the above conditions are satisfied, ultimately "samples that can and cannot be measured" will be determined by the specifications, features and options of the laser diffraction particle size analyzer that is actually used for measurement.
For example, in the case of a liquid medium, the specifications of the sampler must be checked since which kind of organic solvent can be used depends on the materials used in the circulation system.

The following describes the above conditions in more detail.

1) The refractive index of the particles and medium must not be matching.

A major premise in particle size measurement is that the refractive index of the particles and medium are not matching since the diffraction/scattering phenomenon itself will not occur if they are completely matching. Nevertheless, it is considered that there are almost no cases where the refractive index of the particles and medium are completely matching and the particles, in the strict sense, are transparent in the liquid.
I am often asked if transparent particles such as glass beads can be measured. They can, of course, be measured since the refractive index of, say, water (i.e. the medium) can never match that of glass.

2) The medium must allow a laser beam to penetrate. (Transmittance: 90 % or more)

The diffracted/scattered light emitted from particles, especially micro-particles, is faint and so a highly permeable medium is required.

3) The medium must not contain particles other than those to be measured.
Laser diffraction particle size analyzers irradiate many particles (i.e. a particle group) simultaneously with a laser beam, and detect overlaying diffracted/scattered light emitted from each individual particle to obtain the particle size distribution. Accordingly, all particles in a medium are targeted for measurement since it is impossible to selectively detect or exclude diffracted/scattered light that is emitted from specific particles.

4) A medium that does not contain particles must be prepared (since it is used for blank measurement).
The purpose of "blank measurement" is to ascertain the signal components of light emitted from all elements other than the particles to be measured, and subtract these components as the background level.
When solid particles are dispersed in a liquid for measurement, that liquid should be used for performing blank measurement, so the importance of this condition is hardly perceived. In the case of dry measurement, too, whereby solid particles are injected and dispersed in air by compressed air and then measured, this is no problem since blank measurement is possible with nothing injected in air. However, when a sample in a state where particles originally exist in a medium is to be measured, it is sometimes rather difficult to obtain just a "medium that does not contain particles." In the case of a liquid medium, a liquid must sometimes be prepared exclusively for blank measurement by using a filter to remove the particles.
And, when particles contained in film are to be measured, this means that measurement will be possible only if a film made of the same material not containing particles is available.

5) The particle concentration in the medium must be appropriate or it must be able to be adjusted to an appropriate level.
Particle concentration is an important measurement condition on a laser diffraction particle size analyzer.
When particles become smaller, the intensity of diffracted/scattered light per single particle rapidly weakens proportionately to the power of six of the particle size. However, if the particle amount is increased to obtain sufficient intensity and particle concentration becomes too high, then multiple scattering will occur. This will cause the relationship between particle size and the diffracted/scattered light intensity distribution pattern to break down and prevent measurement from being carried out accurately.
For this reason, the particle concentration must be adjusted to an appropriate range for accurate measurement. This range is several tens of ppm to about 100 ppm (% by weight) when a regular flow cell or batch cell is used.
When solid particles are dispersed in a liquid and measured, particles can be added or liquid (liquid medium) can be added to adjust the particle concentration. When solid particles are dispersed in gas (in air), the particle concentration can be also adjusted by altering the injection nozzle or compressed air pressure.
And, when measuring liquid particles in a liquid, the particles are already dispersed in liquid. If the particle concentration is higher than the appropriate range, it can be adjusted to an appropriate level by diluting the sample with liquid medium. However, measures need to be devised if the concentration is lower than the appropriate range. If the particle density is higher than the liquid medium density, separating the particles from the liquid by a centrifuge and discarding the supernatant liquid may be a viable solution.
Of the combinations shown in Table 1, it will often be fairly difficult to adjust the particle concentration of the "possible" items.
There is also another approach to shorten the optical path length to avoid the influence of multiple scattering instead of adjusting the particle concentration. This is the principle used for SALD-HC Series High-Concentration Sample Measurement System.