Enhancing Particle-Size Measurement Using Dry Laser-Diffraction Particle-Size Analysis - Pharmaceutical Technology

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Enhancing Particle-Size Measurement Using Dry Laser-Diffraction Particle-Size Analysis
The author examines dry dispersion and outlines the related analytical method development.


Pharmaceutical Technology
Volume 37, Issue 5, pp. 60-63

Analogous data for the high-energy venturi reflect the more aggressive nature of the dispersion mechanisms applied and show close agreement between the wet and dry results at an air pressure of approximately 1 bar. At higher pressures, there is evidence of primary particle breakdown with the reported particle size becoming smaller than that measured using the wet method.

These data suggest that either disperser could be chosen for analysis of the formulation provided that an appropriate air pressure was selected, but this conclusion raises a question: Are both dispersers equally suitable for this application or is one more appropriate than the other?

By examining how Dv50 (i.e., the median particle size based on a volumetric particle-size distribution) changes as a function of applied air pressure (see Figure 3), it is possible to identify the standard, less energetic venturi as the better choice. With the high-energy venturi, although the results match with wet measurement at 1 bar, any variation in pressure, to either side of that figure, produces a mismatch between dry and wet data. This mismatch suggests that the measurement result will be sensitive to slight variations in air pressure and that the method is not inherently robust.


Figure 3: Comparing pressure-titration data for the two venturis shows that the standard, less energetic design offers more robust measurement and a working pressure envelope that extends from 3 to 4 bar. DV50 is the median particle size based on a volumetric particle-size distribution.
In contrast, with the standard venturi, particle size is stable across a 1-bar pressure window, from 3 to 4 bar. This greater stability indicates that measurement with the standard venturi will be inherently more robust and that less aggressive dispersion is preferable for this relatively fragile powder.

Conclusion

Recent advances in laser-diffraction particle-sizing instrumentation have extended the measurement range of the technique, extending up to 3500 microns, and significantly improved the ease of use of these systems, a key determinant of general laboratory productivity. Equally importantly, however, recent instruments have brought enhanced dry- powder dispersion capability. Relative to wet measurement, dry-laser diffraction particle-size analysis is faster and has a lower environmental footprint because no dispersants are required. Developments in this area, therefore, offer significant practical benefit.

The latest laser-diffraction systems have dry-dispersion engines with a choice of disperser geometries, backed up with precise control, both of sample feed rate and the pressure of the compressed air used for dispersion. Such systems allow the user to control the mechanisms applied to disperse the sample, and most crucially, to efficiently disperse samples without impaction, where impaction must be avoided. As a result, modern laser-diffraction systems extend robust dry measurement to a wide range of sample types, including to materials that are both cohesive and relatively fragile. Such advances mark an important step forward that further enhances the suitability of laser diffraction for efficient particle-size measurement.

Carl Levoguer is a product marketing manager, laser particle-sizing and imaging, Malvern Instruments, Enigma Business Park, Grovewood Road, Malvern, Worcestershire, WR14 1XZ UK, tel. +44 (0) 1684 892456; fax +44 (0) 1684 892789,

Submitted Nov. 29. 2012; Accepted Jan. 30, 2013.


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