Particle Size Distribution Overview
The particle-size distribution (PSD) of a substance such as a powder, granular material, or particles in a solution, consisting of values or a mathematical function (see graphs below) that defines the relative amount, typically by mass or by length of all of the particles in the sample according to their individual sizes.
In practice, there are very large variations in particle sizes and distributions within a very broad range of physical and chemical properties and morphological properties of particles. Such variations are defined approximately by particle size, for example, particles less than a certain size class are either called micronised or milli-sized, those of a larger size class are called nano-sized or milli-sized, and those of other size ranges may be called micro-sized or mega-sized, or the like. The composition of a mixture of all sizes is given by its particle size distribution.
The functional form of a particle size distribution is often specified in a combined plot, such as cumulative distribution graph (graph 1), which shows the percentage difference for the size of each particle in a sample and a frequency distribution graph (graph 2) which shows the percentage of a sample corresponds to a given diameter.
Graph 1: Cumulative Distribution (Process, 2020)
Graph 2: Frequency Distribution (Process, 2020)
The physical and biological properties of the particles within a sample can vary radically based on the sample’s particle size distribution. For instance, particle size analysis is used to characterize the nature and extent of the particulate matter or dust in the atmosphere. Particle size distributions can also develop into an important parameter in certain industries such as inorganic pigments, ceramics, minerals and pharmaceuticals.
Measuring Particle Size Distribution
There are numerous ways to measure a particulate sample’s size distribution, a common way is to use a light microscope. Normally, this technique uses sedimentation and laser diffraction methods to measure the particle size distribution. Utilising this method, the laser diffraction (Figure 1) will obtain a mass distribution of the sample, and the percentage of the particles at each size will be given.
Figure 1: Particle size distribution laser diffraction (Shimadzu, 2020)
Sedimentation tools, such as the Centrifuge Particle Size Analyzer (CPSA), can be applied to measure the composition and size distribution for particles, which are above ten microns in size and polydisperse (Figure 2). Mass determined by this method can be used to calculate the volume distribution statistic, and the volume distribution itself can be further used to calculate the true geometric mean of the particle size distribution.
Figure 2: CPS Disc Centrifuge (Instruments, 2020)
An alternative method to measure a sample’s size distribution can be the Optical Sizing Analyser (OSA). This method provides data based on the refractive index of the particle and the wavelength of the light used to observe it. These can then be used to determine the particle’s size distribution (Figure 3).
Figure 3: Optical Sizing Analyser (Interlab, 2020)
Why Measure PSD?
The ability to measure particle size distributions is critical to the many different industries and applications. Although there are many ways to adequately find this data, each involving different costs, some are more efficient than others. Usually, through a complex method involving laser diffraction, the particle size distribution can be got from one pellet. This pellet is usually made up of 2000 samples of 50 particles each. A simpler, but much more expensive, the method is to use the OSA. The sedimentation method is the quickest and most accurate way to obtain the size distribution of a sample.
As previously mentioned, particle size distribution is a very important aspect in almost every industry and is a representation of how one specific sample differs from others in terms of particle sizes. This is important in knowing how to select the correct particles for a specific function. For instance, in pharmaceuticals having the particles of a certain size is important when it comes to the performance and appropriateness of drug formulation.
Instruments, C. (2020). CPS Disc Centrifuge. Retrieved from CPS Instruments Europe: http://www.cpsinstruments.eu/pdf/General%20Brochure.pdf
Interlab. (2020). Optical Spectrum Analyzer. Retrieved from Interlab: https://www.interlab.pl/en/product/optical-spectrum-analyzer.html
Process, P. (2020). Particle Size Distribution (PSD). Retrieved from Powder Process: https://www.powderprocess.net/psd.html
Shimadzu. (2020). Particle Size Distribution Calculation Method. Retrieved from Shimadzu: https://www.shimadzu.com/an/powder/support/practice/p01/lesson22.html