Particle size distribution (PSD) of solids or dispersed particles can be crucial for certain applications to ensure the functionality and safety of a product. Different powders and dispersions are utilized widely in the production of food, paints, cosmetics, pharmaceuticals, electronics, biorefinery products, semiconductors, etc., 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 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 PSD 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. Some techniques can detect tiny 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 solvent needs to be chosen carefully. The solvent should not dissolve the particles or interact with them so that their shape or size changes.
The table below summarizes four particle analysis techniques based on the main factors that should guide method selection. More information on each method’s principle, advantages, and disadvantages can be found later in the article.
Method | Suitable particle shapes | Size range of analysed particles* | Analysis matrix | Method principle | Measured parameters |
|---|---|---|---|---|---|
Laser diffraction (LD) | Spherical | 0.02 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 |
* These are estimations since the size range can be different for instruments from different manufacturers.
** Analysis is performed on dispersions. Solid powders can be dispersed in a suitable solvent before analysis.
*** Dispersion or wet samples can be dried to make them suitable for SEM.
When several analytical techniques are suitable, the selection may 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 techniques, 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 powdered samples and dispersions in a solvent. 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 ones.
Typically, particle sizes from 0.02 µm to 2 mm can be detected with laser diffraction, making it applicable for a wide range of sizes. Suitable matrices include, for example, sediment, cement, and silts. If the size range of the particles in the sample is not known, laser diffraction is a good option to start. Laser diffraction is also suitable for larger particles than dynamic light scattering and it can be performed on solid (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 their equivalent spherical diameter. If this is not the case, the results will be approximate rather than accurate representations of PSD. Even when the exact size of non-spherical particles cannot be determined, laser diffraction may be used to compare the size differences between different batches of the same material, for example, to analyze which batch includes larger or smaller particles.
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 ones.
The detectable size range is typically between 0.3 nm and 10 µm. Hence, DLS is applicable for smaller particles than laser diffraction, but 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 solvents (aqueous or non-aqueous) before analysis. Similar to laser diffraction, significant disadvantages of DLS include 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.
The price of particle size distribution analysis with DLS depends on the desired quality system; when the analysis is performed according to GMP (Good Manufacturing Practices) or GLP (Good Laboratory Practices), the price is considerably higher compared to a non-GMP or non-GLP measurement. In some cases, most notably in the pharmaceutical industry, analyses must be performed according to GMP or GLP.
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, including rod-like particles and fibers. Calculations are made from optical microscope images, which allow determinations of different parameters, such as length, width, and aspect ratio. Similar to laser diffraction and DLS, calculations are automatic, making shape and size analyzers ideal for gathering data on a large number of non-spherical particles.
The detected size range is approximately 2 - 3000 µm, which means that size and shape analyzers cannot provide information about nanoparticles. Solid samples should be dispersed in aqueous or non-aqueous solvents.
Particle size and shape analysis with SEM
Scanning electron microscopy (SEM) is a powerful tool in particle analysis, as its higher resolution compared to optical microscopy enables the detection of smaller particles. In addition to size, SEM can provide information on the shape and surface morphology of particles.
Similar to shape and size analyzers, particle size is calculated from images. Automatic image analyzer software exists, but it is not suitable for all samples and images. For this reason, particle size is usually calculated manually, which makes it time-consuming and expensive to obtain size distribution data on a large number of particles with SEM.
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.
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: