One goal in method development is to disperse aggregates to the single particle state prior to measurement. Ultrasound is frequently used to disperse aggregated particles, but it is critical to determine the exact amount of energy required. In the past it was required to design an experiment to assess the effect of ultrasound on the state of dispersion. This is now automated with the Method Expert software features, exclusive to the LA-960 particle size analyzer. A case study of how the Method Expert was used during a structured method development process is explained below.
The sample used in the case study is a pigment that arrived in the lab as a powder to be dispersed in water. First, it was determined that no surfactant was required to wet the powder. Next, an initial experiment generated a bimodal result, indicating aggregation. The internal ultrasonic probe in the LA-960 was turned on for 10 seconds and the larger peak diminished, suggesting that ultrasound aids dispersion. Then the Method Expert function was opened and the screen below shows how the effect of ultrasound experiment was easily setup in three steps for automatic execution.
Using these settings the system automatically took 5 measurements with 30, 60, 90, 120, and 150 seconds of ultrasound at the highest power level of 7. After all measurements are completed, the results are displayed in various formats so the user could select the optimum amount of energy (time).
The particle size distribution overlays and D10, 50, 90 vs. time plots can be viewed in greater detail, as seen below. Note that after 150 seconds a second larger peak (light blue) appears, indicating re-aggregation of the particles. This is also clearly seen by the sharp increase in D50 and 90 in the second plot.
The Method Expert provides Expert Advice information to guide the inexperienced user in selecting the best possible ultrasonic treatment.
It is always a good idea to document decisions made during method development. This is also automated with Method Expert, as seen below where the operator choose which results to save in various formats.
After the complete method is created, the next step is to test for repeatability. These calculations as defined in ISO 13320 are also automated calculations in the LA-960 software.
The user can then perform the other automated tests within the Method Expert software including structured tests to select the optimum:
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