Particle size analysis is used to characterize the size distribution of particles in a sample. This analysis can also be used to measure the particle size of solid materials, suspensions, emulsions, and aerosols.
Particle Size Analysis
Particle size analysis is used to control quality and determine the efficiency of a manufacturing process or the performance of a final product in many industries. All particle size analysis techniques report the particle size as a single number which can be an issue when using shapes that are not a sphere. To overcome this issue, all particle size analysis techniques relate a one-dimensional property of a particle to the size of an “equivalent sphere”. This is done using measurements of surface area or volume, based on the technique being used.
Measuring the size of the particles and drops of the spray with Malvern spraytec. Image Credit: Cergios / Shutterstock
Different Particle Size Analysis Techniques
The particle size analysis techniques available to measure the size distribution of a particle include the following:
- Laser Diffraction
- Dynamic Light Scattering
- Image Particle Analysis
- Acoustic Spectroscopy
Laser diffraction was developed in the 1970s as a much faster and accurate particle size analysis technique compared to sedimentation. This method,widely used as the standard technique in many industries, measures the light scattering when a particle passes through a laser beam. The angle of scattered light is directly related to the particle size where the larger particles scatter the light as narrow angles with high intensity and smaller particles scatter the light at wider angles with low intensity. This phenomenon is known as the “Fraunhofer diffraction theory”. Laser diffraction has a wide detection range (0.2–2000 μm) and is fast and reliable.
The Mastersizer 3000 laser diffraction particle size analyser from Malvern
Dynamic Light Scattering
Dynamic light scattering (DLS) is a non-invasive and sensitive technique to measure the size of particles in suspension and molecules smaller than a micron in a solution. During DLS, a laser is shot through a polarizer and into a sample and the light is scattered in all directions. The scattered light enters another polarizer where it is collected by a photomultiplier and the resulting image is projected onto a screen, known as a speckle pattern. This process is repeated, and the set of speckle patterns produced are analyzed by an autocorrelator that compares the light intensity of each spot over time. Varying mathematical approaches are used then to infer the autocorrelation data.
Image Particle Analysis
Image Particle Analysis (IPA) generates data by capturing images of each particle, providing the ultimate sensitivity and resolution. The measurements include particle size, particle shape, and population distributions. The main imaging methods are optical microscopy and transmission electron microscopy (TEM); however, TEM images are challenging as nanoparticles accumulate together on the TEM grid. This can be solved by manually measuring the size or using processing algorithms, but these methods are limited. After obtaining the images, analysis is performed digitally.
Acoustic Spectroscopy uses ultrasound for collecting information of particles dispersed in a fluid. The dispersed particles absorb and scatter sound waves, similar to light. This technique measures the attenuation coefficient vs the ultrasound frequency, which gives raw data that can be used to calculate the particle size distribution of any particle that is in a fluid system.
In conclusion, there are many different particle size analysis techniques, and the nature of the particle determines which technique is used. Each technique has a history following its conception and they all have evolved to become more efficient. With more research into the particle size analysis techniques, methods to improve and their uses will become available.