Analyzing Irregular Particles

The model used to analyse the scattering data acquired during a laser diffraction measurement assumes that the particles being measured are spherical. The particle size of irregular particles is therefore expressed in terms of a spherical equivalent diameter. In the case of laser diffraction, the diameter of the sphere that would produce an equivalent light scattering pattern to the measured particle is reported.

One assumption within the equivalent sphere approach is that measured particles scattering light in a way that approximates a sphere. For some irregular particles this is not the case and the scattering observed at wide angles can be significantly different to that seen for spherical particles. Analysis of the data produced by irregularly shaped particles can therefore lead to inaccurate results. For this reason, the Malvern Mastersizer 2000 system provides users with the ability to select an irregular-particle analysis mode, ensuring accurate results are obtained for products such as crystallites.

Differences in scattering between spherical and irregular particles having a similar volume-based size distributions. The differences observed, particularly at large angles (high detector numbers), are compensated for within the Mastersizer 2000 analysis to yield accurate results.

Another assumption made within the equivalent sphere approach is that the scattering pattern for irregular particles is measured for all possible particle orientations. This is a fair assumption within most applications. However, for high aspect ratio particles, such as plate-like particles or needles, flow alignment can occur during measurements causing one particle orientation to dominate. In this case, the relationship between the reported particle size and that of the particle measured may not obey a simple cross-sectional area dependence but may instead relate to the longest dimension or the cross-section of the largest face. This change in scattering can allow the particle aspect ratio to be calculated from the laser diffraction data.