What is Dispersion? Developing Methods for Wet Dispersion Optimization – Part 2
Measurement Conditions
Obtaining robust and reproducible results from laser diffraction measurements requires the use of appropriate measurement conditions. Key parameters include:
- Obscuration range
- Measurement time
- Stirrer speed
Sample Quantity
To ensure that the sample concentration is suitable for laser diffraction measurements, enough sample must be added to achieve an adequate signal-to-noise ratio and provide a representative sample of the bulk material. However, excessive sample loading should be avoided, as it can lead to measurement errors caused by multiple scattering.
In laser diffraction systems, sample concentration is measured by a parameter known as obscuration, which represents the percentage of laser light lost as it passes through the sample. Every measurement system contains a degree of noise. In the Mastersizer system, this noise can be observed as random fluctuations in the data after the background signal has been removed during the sample addition stage (Figure 6).
To obtain stable scattering data above the noise level, sufficient sample must be added. However, only enough scattering to exceed the noise threshold is required. For example, Figure 7 shows scattering data from a 300 nm sample that delivers stable and reproducible results at 3% obscuration.
For fine particles, the lower obscuration limit is therefore determined by the system noise level. For coarse particles, the lower limit is governed by sampling considerations rather than signal-to-noise ratio. If multiple measurements of coarse materials show high variability, increasing the sample mass and obscuration level may improve repeatability.
The upper obscuration limit in laser diffraction measurements is determined by an effect known as multiple scattering.
The theories used to interpret laser diffraction data assume that light reaching the detector has been scattered by only one particle. If the particle concentration becomes too high, light may be scattered by multiple particles before reaching the detector.
Multiple scattering causes light to be detected at higher scattering angles. Because high-angle scattering is associated with smaller particles, multiple scattering results in particle sizes being underestimated.
Figure 9 shows particle size distributions measured from the same sample at obscuration values between 5% and 18%. At 5% and 7% obscuration, the measured distributions are very similar, indicating that multiple scattering is not occurring. Above 9% obscuration, the distribution shape changes and an increasing proportion of fine particles appears. This indicates that measurements above 9% obscuration are affected by multiple scattering and that 9% represents the appropriate upper obscuration limit for this sample.
The extent to which measurements are affected by sampling or multiple scattering depends on particle size. Fine particle measurements are generally more susceptible to multiple scattering, while coarse particle measurements are more heavily influenced by sampling effects.
| Particle Type | Recommended Obscuration Range |
|---|---|
| Fine particles | Approximately 5–10% (less than 5% may be required for particles smaller than 1 μm) |
| Coarse particles | 5–12% |
| Polydisperse samples | 15–20% |
Measurement Time
For wet laser diffraction measurements, the measurement duration must be long enough to ensure that a representative sample of particles circulates through the measurement cell.
The required measurement time depends on particle size and sample polydispersity. Only fine monodisperse samples require short measurement times. Coarse materials or samples with broad particle size distributions generally require longer measurements.
If repeated measurements of coarse or highly polydisperse samples show significant variability, increasing the measurement time may improve repeatability.
Figure 10 shows the particle size distribution of a sample containing particles ranging from 1 μm to 700 μm. Repeat measurements were performed using measurement times ranging from 1 to 20 seconds.
Figure 11 demonstrates the reduction in relative standard deviation observed across five repeat measurements as measurement time increases. The variability remains within the acceptable limits defined by the ISO standard for measurement times greater than 10 seconds.
Stirrer Speed
The stirrer in a wet dispersion unit must maintain a homogeneous dispersion and ensure that the sample passing through the measurement cell is representative of the bulk material.
For large or high-density materials, a stirrer speed titration should be performed to confirm that all particles remain suspended during measurement.
For emulsion samples, stirrer speed optimisation can help identify the point at which droplets begin to break apart due to excessive agitation.
Figure 12 shows the results of a stirrer speed titration performed on a copper powder sample. As stirrer speed increases, larger particles remain suspended in the dispersion, resulting in an increase in the measured particle size.
For this sample, stirrer speeds above 2500 rpm are recommended because the measured particle size remains stable within this operating region.
Conclusion
The ability to obtain reproducible results from wet laser diffraction measurements is determined by three key factors:
- Obtaining a representative sample of the bulk material
- Achieving a stable state of dispersion
- Establishing appropriate measurement conditions
This article has described a series of practical tests that can be used to evaluate how these factors influence measurement quality.
Conducting these tests improves understanding of the material being measured and can significantly enhance the reproducibility of particle size results.
Developing a robust measurement method also helps ensure that results remain insensitive to small variations in operating conditions throughout the lifetime of the instrument and analytical method, providing confidence that the method can be maintained and used reliably in the future.
References
[1] ISO 13320 (2009). Particle Size Analysis — Laser Diffraction Methods — Part 1: General Principles.
Further Reading
This article explored how measurement conditions such as obscuration, measurement duration, and stirrer speed affect the quality and reproducibility of particle size measurements.
To begin at the foundations of wet dispersion method development, read Part 1: What is Dispersion? Developing Methods for Wet Dispersion Optimization – Part 1.
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