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Both the supplier and user want each new lot to behave like the last, which they should, if we can make two assumptions: the particle size distribution analysis results for the new lot are within product specifications, and the analytical technique used to determine the size distribution is sensitive enough to detect changes. A number of methods can be employed to determine the particle size distribution of particulate materials, including microscopy and image analysis, sedimentation, laser light scattering (static and dynamic), the electrical sensing zone method, and others based on the interaction of particles with light and sound.
What is sensitivity as it relates to particle size distribution analysis? In the case of laser light scattering, clause 6.7 of the current ISO standard describing the general principles of the technique (ISO 13320-1:1999) provides a very brief definition. Essentially, sensitivity is the ability to detect small differences in the amount of material present at a given particle size.
The same clause also offers brief advice regarding how we can measure the sensitivity of a particular technique or instrument. The sensitivity of a specific analytical instrument can be determined by comparing analysis results for blends of known composition with those predicted for such a blend, given the particle size distribution of the individual components. This study will provide an example of such a sensitivity study performed for a laser particle size distribution analyzer using known mass blends of two abrasive powders, a medium garnet and a coarse garnet.
AnalysisTo ensure confidence in the data produced, each of the two garnet powders were analyzed six times using a high-definition digital laser particle size analyzer.* Overlays of the repeat analyses are shown in Figures 1 and 2 for the medium and coarse garnet, respectively. Repeatability must be demonstrated to ensure that the differences seen in succeeding distributions are due to differences in the sample and not random errors.
Once it was demonstrated that results for each of the two materials met or exceeded reproducibility expectations, nine different blends of the two powders were prepared, with the mass of each component determined using an analytical balance. The resulting mass percentages of each component in the nine blends are given in Table 1.
*Micromeritics Saturn DigiSizer 5200.
Sensitivity QuantificationThe difference between the measured and calculated distribution was determined for each size class in the distribution for all nine blends. The quality of agreement between the two distributions (measured and calculated) for each blend was quantified using the root mean square (RMS) of the differences between the distributions. The calculated RMS data, in units of volume percent of the distribution at a given particle diameter, are given in Table 2 for the nine analyzed blends of the two garnet powders. The maximum difference in volume percent of the distribution at a given particle diameter is included in the table.
For more information regarding particle size distribution analysis, contact Micromeritics Instrument Corp., One Micromeritics Dr., Norcross, GA 30093-1877; (770) 662-3633; fax (770) 662-3696; e-mail email@example.com; or visit www.micromeritics.com.