Particle size distribution (PSD) of solids or dispersed particles can be crucial for certain applications to ensure the functionality or safety of a product. Different powders and dispersions are utilized widely in the production of food, paints, cosmetics, pharmaceuticals, electronics, biorefinery products, and semiconductors, making PSD analysis an essential step in the quality control, research, and development procedures of these industries.
Several analytical methods exist for determining particle size distributions, and it’s not always straightforward to select the most appropriate one. To make the method selection easier, some of the most common particle size analysis techniques and the situations in which they are applicable are outlined below.
Which factors influence method selection?
The choice of the most appropriate particle analysis technique depends on several factors, including the following:
Particle shape. Some methods assume that the analyzed particles are ideally spherical, making others better suited for non-spherical particles.
Approximate particle size range. Some techniques can detect nanoparticles, while others only work with larger particle sizes.
Type of sample matrix. Some analyses are performed on powdered materials and others on liquid dispersions. If the sample has to be dispersed, the dispersion medium needs to be chosen carefully. The medium should not dissolve the particles or interact with them so that the shape or size of the particles changes.
The table below summarizes five particle analysis techniques based on the main factors that influence method selection. Additional information on each method’s principle, advantages, and disadvantages will be discussed later in the article.
Table 1: Comparison of particle size distribution analysis methods
Method | Suitable particle shapes | Size range of analyzed particles* | Analysis matrix | Method principle | Measured parameters |
|---|---|---|---|---|---|
Laser diffraction (LD) | Spherical | 0.010 µm to 2000 µm | Dry powders or dispersions | Scattering/diffraction pattern | Equivalent spherical diameter |
Dynamic light scattering (DLS) | Spherical | 0.3 nm to 10 μm | Dispersions** | Brownian motion | Hydrodynamic size |
Size and shape analyzer | All shapes | 2 to 3000 μm | Dispersions** | Image analysis | Equivalent spherical diameter, length, width, aspect ratio, etc. |
SEM | All shapes | > 10 nm | Dry powders*** | Image analysis | Diameter, width, length, aspect ratio, information about surface morphology |
Sieve analysis | All shapes | 30 µm to 120 mm | Dry powders | Gravimetric analysis | Weight retained on each sieve |
* These are estimations of the technical limitations since the size range can differ for instruments from different manufacturers.
** Analysis is performed on dispersions. Solid powders can be dispersed in a suitable dispersion medium before analysis.
*** Dispersion or wet samples can be dried to make them suitable for SEM.
When several analytical techniques are suitable, the selection can be based on “convenience factors” such as price and easy availability. It is also recommended to use the same analytical technique each time a particle size analysis is needed for a similar material or product, as this improves the comparability of results over time.
In some cases, it may be necessary to use two different methods, as the material may consist of differently-sized particles that cannot all be analyzed using the same method. If this is the case, the sample can be sieved to separate the larger particles from the smaller ones, and the sieved particles can be analyzed with separate techniques.
Laser diffraction in particle size analysis
Laser diffraction (LD) is a common method for analyzing the particle size distribution of powders and dispersions. Its principle is based on the angle and intensity of light that scatters from the particles: from larger particles, light scatters at a smaller angle and higher intensity than from smaller particles.
Typically for dispersions, particle sizes from 0.010 µm to 2 mm can be detected with laser diffraction, making it applicable for particles of a wide range of sizes. For samples in powder form, laser diffraction’s range of measurement lies between 0.4 µm and 2 mm. Suitable matrices are, for example, powdered chemicals, active pharmaceutical ingredients (APIs), cement, inks, paints, and coatings.
If the size of the particles in the sample is not known, laser diffraction is a good option to start. Laser diffraction is also applicable for larger particles than dynamic light scattering and it can be performed on solid particles (dry state) or dispersed particles (liquid state). Another advantage is its reasonable price range.
The most significant limitation of laser diffraction is the assumption that analyzed particles are spherical, as it measures the equivalent spherical diameter of the particles. Otherwise, the results will be approximate rather than accurate representations of PSD.
Dynamic light scattering in particle size analysis
Dynamic light scattering (DLS) is another common analytical technique for determining the particle size distribution of dispersions in a solvent. The technique is based on Brownian motion: smaller particles move and diffuse faster than larger particles.
The detectable size range is typically between 0.3 nm and 10 µm. Hence, DLS is applicable for smaller particles than laser diffraction, although the size range is narrower. Dynamic light scattering is suitable for samples including inks, pigments, microemulsions, and proteins. When DLS is used to analyze solids, they must be dispersed in suitable dispersion media (aqueous or non-aqueous) before analysis.
Similar to laser diffraction, significant disadvantages of DLS are its inability to detect particle shapes and the assumption that the analyzed particles are spherical. For this reason, DLS can determine the hydrodynamic diameter and size distribution of spherical particles only.
Particle size and shape analyzers
The principle of a particle shape and size analyzer is based on optical microscope imaging. This makes it possible to analyze non-spherical particles, for example, rod-like particles and fibers. Optical microscope images are processed with suitable software to allow determinations of different parameters, such as the particles' minimum and maximum Feret diameters and aspect ratio. Similar to laser diffraction and DLS, calculations are done automatically, making shape and size analyzers ideal for gathering data on a large number of non-spherical particles.
The detected size range is approximately 10 - 3,000 µm, which means that size and shape analyzers cannot provide information about nanoparticles.
Particle size and shape analysis with SEM
Scanning electron microscopy (SEM) is a powerful tool in particle analysis, as it provides information on the shape and size of small particles. Due to the higher resolution of electron microscopy compared to optical microscopy, SEM enables the detection of smaller particles. In addition to shape and size, SEM can provide information on the surface morphology of the particles.
Similar to shape and size analyzers, particle size is calculated from the images of the particles. If the analyzed material is not conductive – as is the case with most organic and biological samples – it needs to be coated with a thin layer of metal or carbon before SEM imaging. SEM can only be performed on dry samples. Dispersions and wet samples can be dried to make them suitable for SEM.
Sieve analysis
Sieve analysis is a straightforward and practical method for particle size analysis, most suitable for cohesionless solids: soils, aggregates, and similar materials. A set of sieves with gradually decreasing openings is assembled into a stack. Standard sieve sizes are usually utilized according to ASTM or ISO standards.
The sample is dried, weighed, and put on the top sieve, which has the largest openings. Afterward, the stack is mechanically shaken or agitated for a set time, which allows the particles to fall through the sieves. The material retained on each sieve is weighed and reported as a percentage, giving the particle size distribution.
Since sieve analysis is straightforward, standardized by using standard sieves, and suitable for a diverse range of materials, it is a widely used method for characterizing relatively large particles (from ~30 µm to ~120 mm).
Need help with method selection?
If you are unsure which particle size (and/or shape) analysis method to choose, do not hesitate to contact our testing experts at info@measurlabs.com. We help narrow down the options based on your sample type and the information you need to gather. You can also find more information about the different analyses and proceed to order them through the links below: