Limitations of Laser Diffraction Particle Analysers
As with any scientific method, there are a number of limitations, caveats, and difficulties associated with this method.
To begin, a number of steps must be completed prior to analysis in order to obtain an accurate and reliable measurement using laser diffraction as a method of PSD analysis. Before the optical model can be established, the optical properties
of the sample to be analyzed as well as the surrounding medium must be known. This can be a problem for new or composite materials, resulting in the establishment of the optical model throughout the process.
However, knowing the refractive index may preclude the use of laser diffraction because the models can only return accurate predictions of particle size at specific refractive indices for both the sample and the medium. As a result,
determining the appropriate refractive indices required for accurate measurement is frequently a trial and error process.
Finally, laser diffraction particle size analysis is based on two assumptions. The first is that the particles are perfect spheres, and the second is that the material is homogeneous, resulting in a uniform refractive index. As the
eccentricity of the sample changes, the assumption that all particles are spherical has an effect on the accuracy of particle measurement. The individual particle volume is calculated by assuming a perfect sphere when constructing
the volume distribution of the sample; however, it has been demonstrated that the size of a non-spherical particle is larger than the equivalent sphere of the same volume calculated by laser diffraction (Jonasz, 1991). As a result,
laser diffraction underestimates the volume of particles with high eccentricity.
In conclusion, laser diffraction as a method of particle size distribution analysis has the following drawbacks:
- Incorrect measurements can occur if the refractive indices used to build the optical model are incorrect.
- Assuming that all particles are perfect spheres can reduce measurement accuracy, as samples with high eccentricity reduce measurement accuracy.
- Non-homogeneous sample analysis can result in incorrect measurements due to changing refractive index values.
- The measurement time can range between 2 and 10 minutes.
- Testing must be done offline, which limits the use of laser diffraction as an in-process control method.