Article updated on 8 September 2020.
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There are many industrial suppliers who specialize in, and sell, particle size analyzers to laboratories. Whilst we’re not going to focus on the individual companies (or their specific instrument), we’re going to look at the actual techniques that these particle size analyzers employ. For example, and to showcase why, the ZetaSizer, which is a common particle size analysis instrument in many laboratories is a dynamic light scattering method (DLS).
Particle size analysis methods are important in a range of scientific fields, from measuring the size of components in pharmaceuticals to particulate matter in foodstuffs to aerosols, sprays and pretty much any type of particulate suspension. Most particle analyzers are not just tools for measuring individual particle sizes, as they provide a distribution curve of all particles in a sample. Additionally, many can be used to determine other particle parameters, such as a particle’s shape, surface properties, mechanical properties, charge properties, and microstructure. But each method is different, and the properties that can be backed out are different.
Static Light Scattering (SLS)
Also known as laser diffraction spectroscopy, static light scattering (SLS) is a technique that measures the amount of light scattered by a sample and uses the molecule’s size and the intensity of light to deduce the molecular weight and size using Rayleigh theory (i.e. larger molecules scatter more light than smaller molecules and the intensity of the scattered light is proportional to the molecule’s molecular weight). Static light scattering (SLS) is a technique that measures the amplitude of scattering, regardless of the intensity fluctuations.
SLS can measure the amount of scattered light from a sample using two simple parameters/operational variables. Firstly, a range of angles can be used to scatter the light and a model of the different scattering angles backs out the size, shape and/or structure of the sample. The second way is through using a range of concentrations. By measuring the intensity of scattered light at a range of concentrations, the machine can deduce many properties of the sample, including the weight and second virial coefficient.
Dynamic Light Scattering (DLS)
Dynamic light scattering (DLS) is similar to SLS and can often be performed on the same instrument. However, there are some differences between DLS and SLS. First off, DLS measures the fluctuation/change with time, regardless of the amplitude, and records the fluctuation in the scattering intensity over a time range. This allows the machine to characterize the Brownian motion within the sample at various intervals. Measurement of the fluctuations can occur through correlation or spectral analysis and can be used to determine the hydrodynamic radius and diffusion coefficients of a sample. The recorded diffusion speed can then be converted into size, polydispersity and size distributions using the Stokes-Einstein relationship. DLS is often used as a tool for measuring the zeta potential of a molecule/particle and this enables the surface charges to be deduced.
Microscopy is a set of techniques that are also used to measure the size of individual particles by imaging them and extrapolating the image into imaging software (such as ImageJ)—which can be used to deduce the size of the particles in the image. There is a range of microscopy techniques that can be used, from optical microscopy to scanning electron microscopy (SEM) and transmission electron spectroscopy (TEM). The ability to image and measures each particle individually has led to microscopy techniques becoming a way of providing an absolute measurement of particle size—especially when single particles can be clearly distinguished from aggregates/flocculation (which is not always possible with other methods as the particles can’t physically be seen). The particle shape can also be easily measured, but in some microscopy techniques, only the 2D shape can be determined.
Sieving methods are not as high-tech as other methods (although more automated methods are available) and can suffer from reproducibility issues, but they are still used for some applications—namely in civil engineering and geological applications. Sieving analysis instruments are composed of a series of sieve meshes stacked on top of each other, in which the pores between the wires get smaller and smaller the further down the stack you go. So, the large particles get left behind at the top and the smaller particles travel to the bottom. The particles are either shaken, flowed through (if in a suspension), blown by air, or undergo ultrasonic agitation to move them through the stack. Because each sieve size is known, the relative distribution of particles which fall within defined particle size ranges can be produced, but it is not a technique for measuring individual particles. It also doesn’t account for particle shapes that are not spherical (or loosely spherical), such as rod-like particles, which can lead to inaccuracies in some results.
Particle counters are often used for measuring the size and number of individual particles as they pass by a detector/sensor. Particle counters come in two main forms, handheld particle counters for measuring particles in the air to ensure that the quality of the air is good (and free from various contaminants, such as dust and other large particulate matter, as well as toxic gases), and stationary particle counters which are often used for fluidic flows to measure the size of particulate matter within them. Liquid particle counters are used to ensure that a formulation, be it pharmaceutical or otherwise, is of high quality and are often used to enable a precise control in liquid suspensions and formulations.
Whilst there are many types of sensors/detectors, the most common employs a laser which is shone into the path of the area being analyzed. When the laser light is broken, the device records the number of times this happens and that backs out the number of particles. On the other hand, the degree of scattering on to a detector (opposite the laser beam source), or obscuration of the source light, relates to the size of each particle that passes by the particle analyzer.
Sources and Further Reading