The ability to distinguish and analyze the shape, size, and properties of particles, as both individual entities and as complete particulate systems, is of great use to many industries. Because particles can range from macroscopic particles that you can see with the naked eye to nanoparticles which require high powered microscopes to see them, there are a number of methods that can be used to analyze a particle sample—especially when they can also be present in a number of forms from powders to liquid suspensions. Here, we’re going to look at some of the most common particle analysis techniques and the key differences between them.
Techniques for Analyzing Particles Size of and Distribution
Light scattering techniques such as static light scattering (SLS) and dynamic light scattering (DLS) are two common techniques for analyzing the size of particles and the size distribution of particles in a sample (often in the micron to sub-micron size range). They are so similar that both modes can often be performed on the same instrument. Both techniques send light towards the particles and use how much light has been scattered as a way of determining the size and molecular weight of all the particles in the sample.
The main operational difference between the two techniques is that SLS measures the amplitude of scattering, regardless of intensity fluctuations, whereas DLS measures the fluctuation/change with time, regardless of the amplitude. In terms of determining the size, larger particles scatter the light at smaller angles, and vice versa. In terms of the analysis, while they can both be used to determine the particle size distribution and molecular weight of the particle, DLS can also deduce the zeta potential of the particles, which in turn provides information about the surface charges, the electrokinetic properties, and the stability of the particulate sample. However, they are the most expensive option of the main methods (and the ones documented here) but are still widely found in most analytical, characterization, and academic labs.
Dynamic Image Analysis
Another technique that is widely used is dynamic image analysis (DIA). DIA is different from other methods in that the approach is more qualitative in nature rather than quantitative. It is essentially an advanced imaging method that takes high quality and high-resolution images of a sample under a microscope, and then these images are analyzed using advanced software algorithms to accurately determine the size and shape of each of the particles in the sample. In some cases, particles can flow past the imaging system to take images of the particles in a larger and moving system, rather than just a few particles on a microscope slide.
DIA is advantageous over other methods as it provides highly accurate data on both the size and shape of the particles which are in the direct analysis area (or which pass through the analysis area). However, it can only be used with very small sample sizes, so it doesn’t always give the overall picture of a large particulate system—especially in terms of the particle size distribution—which also makes it harder to predict to macroscale properties which arise from micron-sized (and below) interactions between particles.
One technique that is lower-tech, but is still used within some industries, is the age-old art of sieving. It doesn’t produce as accurate results as other methods, as the quantitative results are an estimate based on the weight of each sieve layer. In terms of determining the shape of particles, it is done by the naked eye and not through advanced imaging software, so the shapes are a rough guide. Generally, the sieves (with a predetermined size) will be stacked on top of each other with the larger sieves at the top and the smaller sieves at the bottom. The sample is put in the top and the sieves are shaken, where the larger particles collect at the top and the smaller particles collect at the bottom.
Where many of the more advanced techniques are used for microparticles and nanoparticles, sieving is largely reserved to bulkier materials, such as those you can see with the naked eye. It is also slightly biased in terms of shape. Many of the holes will be a certain shape, so if a particle is an odd shape, then it may not fit through the sieves due to its shape and not its size. But it is a much cheaper technique to perform than the other particle analysis methods. So, where other methods may be used in pharmaceuticals or food research (to name a couple of the industries where they’re used), sieving is used more in environmental and mining applications to separate and analyze the larger matter.
Sources and Further Reading