Reliable and stable results with laser diffraction are achieved by understanding trends through repeat testing. At PTL, several measurements are typically done in each liquid dispersion analysis with a delay between each measurement to observe if any trending occurs. Obscuration is the measurement of laser-light that is blocked by particles during an analysis. Obscuration and particle size are key variables to help distinguish trends in the data. These data points can increase or decrease during analysis with consecutive measurements and can identify how particles behave in the recirculator.
Laser diffraction is an analysis technique that can detect particle size by measuring the angle at which particles scatter light. In a liquid analysis, the sample is mixed into a carrier fluid to create a dispersion and this is typically added dropwise to a recirculator which circulates the mixture in the instrument throughout an analysis. Understanding the relationship between particle size and obscuration can help determine the path toward a stable analysis.
Examples of Trending During Analysis
For the purposes of this discussion, agglomerates are clusters of primary particles that are slightly stuck to one another. Introducing a dispersion force, such as sonication or vortexing, can aid in dispersing agglomerates.
Figure 1. Distribution of decreasing particle size and increasing obscuration.
During the analysis illustrated by the distribution in Figure 1, particle size between each measurement decreased while obscuration slightly increased. It is likely that the high stir speed in the recirculator further dispersed the agglomerates in the sample. As agglomerates disperse, smaller particles are analyzed by the instrument. The obscuration increased as a result of finer particles obscuring the laser. To achieve a more stable analysis, additional sonication or vortexing is appropriate to fully disperse the sample.
Figure 2. Distribution of increasing particle size and decreasing obscuration.
In the above analysis, the particle size increased slightly while obscuration steadily decreased. This trending may suggest particle dissolution in the carrier fluid. If the material dissolves slowly, particle size may increase moderately as smaller particles dissolve at a faster rate than larger ones, and, if the sample dissolves quickly, both obscuration and particle size will decrease.
The distribution can differ depending on the dissolution rate. So, it is important to observe the recirculator before and after analysis for signs of the solution turning from cloudy to clear. Dissolution causes bias in the testing, as the entire size-range of the sample is not analyzed throughout the measurements. When sample dissolution is encountered, the carrier fluid should be reevaluated.
Figure 3. Distribution of increasing particle size and decreasing obscuration.
Similar to the previous example, the analysis above demonstrates an increase in particle size alongside decreasing obscuration. While this trending is similar to dissolution, it can be confirmed by visual observations of the sample preparation under a microscope or of the dispersion in the recirculator.
The particles may be flocking or settling in the recirculator. As particles group together, obscuration of the laser-light lessens compared to the onset of analysis. The result is decreased obscuration and increased particle size as the instrument recognizes these groups as one singular particle.
Carrier fluid and material interactions can cause flocking; creating bias in the testing. The addition of a surfactant, a material that is miscible with the designated solvent and reduces tension between the sample and solvent, may be evaluated to lessen this effect. If the particles are visually settling in the recirculator due to the size and density of the material, increasing the stir speed may be necessary to maintain a well-dispersed sample throughout the analysis. Note that increasing the internal stir speed can cause sheering at higher rotations per minute depending on the sample type. Adjusting the stir speed multiple times might be necessary to determine an ideal speed that creates a good dispersion while not destroying the true particle size of the material.
Each new sample that is tested at PTL via a liquid laser diffraction analysis is evaluated in order to reduce these kinds of trends. While these examples and possible solutions are definite, in reality, there are many variables and interactions between materials and carrier fluids where it may be impossible to provide completely stable results. At PTL we aim to reduce trending as much as possible and cater to your specific particle-sizing needs. Contact us today to get started!
By Amanda Redman, Particle Characterization Chemist II.