What is Dispersion? Method Development for Wet Dispersion Optimization-2

Laser Diffraction Particle Size Measurement for

Development of Wet or Liquid Dispersion Method 

 Measurement Conditions

It is also important to set appropriate measurement conditions to obtain robust results from laser diffraction measurement. The settings include:

 Obscuration range
 Measurement time
 Stirrer speed

The amount of sample to be measured

To make the sample concentration suitable for laser diffraction measurement, enough sample should be added to get a good signal-to-noise ratio or a representative sample of bulk material, without adding too much which might impact the measurement due to multiple scattering.

In a laser diffraction system, sample concentration is measured by a parameter called obscuration, which represents the proportion of laser light lost through passing the sample.

All measurement systems have some level of noise, and in the Mastersizer, this can be identified during the sample addition phase as random fluctuations in the data when the background signal has been removed (see Figure 6).

Therefore, sufficient sample needs to be added to obtain stable scattering signal data above these random fluctuation levels.

The system does not require much scattering above the noise level. For example, Figure 7 shows scattering data obtained from a 300nm sample providing stable, reproducible results at 3% obscuration. Thus, for fine particles, the obscuration lower limit is defined by the system’s noise level.

For coarse particles, the obscuration lower limit is defined by sample collection rather than signal-to-noise ratio. If high variability appears after measuring several representative samples of coarse material, try increasing the sample’s mass and the obscuration at which the measurements are carried out.

The upper obscuration limit in laser diffraction measurement is defined by an effect known as multiple scattering. The theory used to interpret scattering data in diffraction systems assumes that the laser light striking the detector has been scattered by a single particle.

If the concentration of particles in the cell is too high, it is likely that the laser light has been scattered by more than one particle before reaching the detector. This effect is depicted in the diagram below.

This multiple scattering phenomenon causes laser light to scatter at higher angles. Since scattering at higher angles is related to finer particles, multiple scattering results in an underestimation of particle size.

Figure 9 shows the particle size distribution measured on the same sample at obscurations between 5% and 18%. Because the size distributions measured at 5% and 7% obscuration are very similar, it can be seen that there is no multiple scattering in this obscuration range. As the obscuration increases beyond 9%, the distribution shape changes and finer particles appear.

From this, it can be determined that measurements beyond 9% obscuration are affected by multiple scattering, and the appropriate upper obscuration limit for this sample is 9%.

The extent to which the measurement can be affected by multiple scattering or sample collection depends on the particle size of the material being measured. Fine particle measurements are more affected by multiple scattering, while coarse particle measurements are more affected by sample collection. Table 3 below gives the recommended range of obscuration that changes depending on particle size.

Table 3: Recommended obscuration range based on particle size

Measurement Time

In wet laser diffraction measurement, the measurement time must be sufficiently long for a representative sample of particles from the dispersion device to circulate through the measurement cell. The required time depends on particle size and polydispersity of the sample.

Shorter measurements are only needed for fine, monodisperse samples, while coarser particles or those with broad distributions require longer measurements. If high variability appears in repeated measurements of the same sample for large particles or polydisperse samples, increasing the measurement time may enhance repeatability.

Figure 10 shows the particle size distribution of a sample containing material with a wide size distribution (from 1μm to 700μm). Repeat measurements on this sample were performed using a measurement time range from 1 second to 20 seconds.

Figure 11 shows the decrease in relative standard deviation in five repeat measurements as the measurement time increases. The variability does not exceed the acceptable range defined in the ISO standard [1] for measurement times exceeding 10 seconds.

Stirrer Speed

The stirrer in a wet dispersion system must ensure uniform dispersion and that the sample passing through the measurement cell is representative. For large or high-density materials, a stirrer speed assessment should be conducted to confirm that all particles are suspended in the sample. For emulsion samples, stirrer speed optimization can indicate at what speed the droplets start to break up as the stirrer operates.

Figure 12 shows the results of stirrer speed optimization for a copper powder sample. As the stirrer speed increases, larger particles are more frequently suspended in the sample, leading to an increase in measured particle size. For this sample, a stirrer speed exceeding 2500rpm is recommended as the particle size stabilizes in this range.

Conclusion

The ability to obtain reproducible results from wet laser diffraction measurement is determined by three key factors.

The first factor is collecting a representative sample of bulk material,

the second factor is achieving a stable state of dispersion, and

the third factor is setting appropriate measurement conditions.

This application note covered the tests that can be performed to assess how these factors affect the sample.

Performing some of the tests described here will improve the understanding of the material being measured and enhance the reproducibility of particle size results.

Also, developing a robust method assures that the method will continue to be used and maintained by ensuring the particle results are insensitive to small measurement condition changes experienced during the life cycle of the equipment and method.

References

[1] ISO13320(2009). Particle Size Analysis – Laser Diffraction Methods, Part 1: General Principles