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 and R&D processes 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 dry 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 selected 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 selected 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 |
Single particle optical sensing (SPOS) | Spherical | 0.5 µm to 400 µm | Dispersions** | Light obscuration and/or light scattering by individual particles | Equivalent spherical diameter, particle number concentration |
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 |
Mechanical sieve analysis | All shapes | 30 µm to 120 mm | Dry powders | Gravimetric analysis | Weight retained on each sieve |
Air jet sieve analysis | All shapes | > 20 µm | Dry powders | Gravimetric analysis (air-jet dispersion through a single sieve) | Weight retained on the 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 larger particles from 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 include, 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 suitable for smaller particles than laser diffraction, although the size range is narrower. Dynamic light scattering is often used to analyze 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.
Single particle optical sensing
Single-particle optical sensing (SPOS) sizes particles one at a time as the analyzed dispersion flows through a narrow sensing zone illuminated by a light source. Each particle either blocks part of the beam (light obscuration) or scatters it, and the signal magnitude corresponds to its size. Since particles are counted individually, SPOS gives a number-based distribution and the particle number concentration, rather than the overall average produced by laser diffraction or DLS.
The size range is typically 0.5 to 400 µm, depending on the sensor. The result is reported as an equivalent spherical diameter, so the method is best suited for spherical or near-spherical particles. A typical application is quantifying oversized particles or contaminants in otherwise homogeneous samples, such as pharmaceutical suspensions. Samples usually require dilution for particles to pass through the sensing zone one at a time.
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, such as 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 to 3,000 µm, which means that size and shape analyzers cannot provide information about nanoparticles.
Particle size and shape analysis with SEM and TEM
Scanning electron microscopy (SEM) is a powerful tool in particle analysis, as it provides information on the shape and size of smaller particles than analyzers based on optical microscopy. In addition to shape and size, SEM can provide information on surface morphology.
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, but dispersions and wet samples can be dried to make them suitable for SEM.
For the smallest nanoparticles (< 50 nm), transmission electron microscopy (TEM) can be used instead of SEM.
Sieve analysis
Sieve analysis is a straightforward and practical method for particle size analysis, most suitable for cohesionless solids such as soils and aggregates. A dried, weighed sample is placed on the top sieve in a stack of sieves with gradually decreasing openings. The stack is then mechanically shaken or agitated for a set time to let the particles fall through, and the material retained on each sieve is weighed and reported as a percentage, providing the mass-based particle size distribution.
Since sieve analysis is easy to perform, often standardized by using standard sieves (ASTM, ISO), 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).
For fine or cohesive powders that tend to clog conventional sieves, the air jet sieve method is a better option. A single sieve is used at a time, and a rotating jet of air disperses the sample and draws the fine particles through the mesh, while the material retained on the sieve is weighed. Repeating the measurement with different mesh sizes, available down to 20 µm, gives the particle size distribution.
All particle size analyses in one place
Measurlabs offers every major PSD analysis method under one roof, enabling you to get all the required information on your material without the need to consult multiple laboratories. You can find more information about the technical specifications and typical price ranges of different analyses through the links below:
Alternatively, you can always contact our experts using the form below. When you describe your sample type and project goals, we can help you choose the most suitable analysis method(s) and prepare a custom quote. Remember to include an estimate of expected sample volumes per month or year, as we offer discounted rates for large and recurring orders.