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Please see our Particle Size Analyzer (Particle Analyzers / Particle Sizer) section to find manufacturers that sell these products
A particle size analyzer, also known as a particle sizer, is used to measure the sizes of particles in a sample. It can also determine distributions of particle sizes.
Particle sizing systems are used for research (such as for proteins or other biomaterials), manufacturing (such as for cosmetics), and quality control applications (such as food safety and pharmaceuticals). Different types of particle size analyzers are designed for different purposes; for example, particles can be solids, such as in a powder form, but can also take the form of liquids or aerosols.
Range of instrumentation
Particle sizing systems come in several basic varieties, mainly depending on the median size and range of sizes of the particles and how they measure them. These include laser diffraction systems, dynamic light scattering systems, automated imaging systems, and electrophoretic light scattering systems. Particle size analyzers are available for measuring particles as small as subnanometer diameters, and as large as millimeter diameters.
Analysis and methods
Sample dispersion
Most particle size analyzers begin with sample dispersion, a necessary step that separates the particles so that they can be measured individually. Sample dispersion can be accomplished by wet or dry dispersion. In wet dispersion, the sample particles are suspended in a liquid dispersant, such as water, sometimes with an added surfactant that aids in particle dispersion.
Physical dispersion of the liquid sample is also used, i.e., stirring, shaking, or ultrasonic disruption. In dry dispersion, the sample (in the form of a dry powder) is dispersed by a stream of air. Dry dispersion is generally not suitable for small, submicron particles in powder form, since they stick together too strongly, or for fragile particles that might incur damage by the force of the air flow used in dispersal.
The different particle sizing systems measure particle diameters using a variety of methods. One of the main methods is laser diffraction. Laser diffraction can measure a wide range of particle sizes, is able to measure quickly and with high throughput, and is a well-established method that researchers can easily find help with if needed.
Laser diffraction systems work by measuring the angles of light scattered when a laser passes through a dispersed particulate sample. When laser light hits the particles, it bounces off larger particles at smaller scattering angles, and off smaller particles at larger scattering angles. From the intensity and angles of scattered light, the system calculates the sizes of the particles in the sample. Like many particle sizing systems, the laser diffraction method assumes that the particles are approximately spherical in shape. The scattered light is identified by detectors placed in different locations within the system. Usually, the detectors collect light scattered at different angles (such as a detector for light scattered at smaller angles, a detector for light scattered at larger angles, and perhaps additional detectors for angles in between these).
Another kind of particle sizing system is the dynamic light scattering type, normally used for measuring particles in solution. (This method is also referred to as photon correlation spectroscopy, or quasi-elastic light scattering.) When laser light is shone through the liquid sample, the Brownian motion of the particles in solution (as they repeatedly collide with solvent molecules) causes the light to scatter with different intensities. Smaller particles move farther and quicker, which scatters the laser light with different intensity fluctuations than the smaller and slower movements of larger particles. The system calculates particle sizes from fluctuations in these intensities of scattered light. Dynamic light scattering systems usually have one detector that collects the scattered light intensity fluctuations.
Automated imaging systems
Automated imaging measures particles directly. However, many measurements are required to build a statistically significant distribution of particle sizes (perhaps on the order of hundreds of thousands of measurements). Automated systems can be either static or dynamic (see below).
Electrophoretic light scattering systems
Electrophoretic light scattering measures the electrophoretic mobility of particles in liquid samples. While not a direct measurement of size, electrophoretic mobility is influenced by both particle size and electrical charge. The sample is added to a measurement cell that contains two electrodes, and then an electric field is applied to the electrodes. This causes electrically charged particles in the sample to move to the electrode with the opposite charge.
In electrophoretic light scattering, a particle’s velocity represents its electrophoretic mobility, which is converted to a quantity called zeta potential so that researchers can more easily compare it with measurements made in other types of conditions. The particle velocities are measured using the laser Doppler method, in either of two ways: namely, measuring a frequency shift, or locating the phase shift and measuring the light scattering there. The former gives a full distribution of zeta potentials, but is less sensitive. The latter, also known as phase analysis light scattering (PALS), is more sensitive, but gives only one average value of the zeta potential, rather than the full distribution of values.
Purchasing a particle sizer
One of the most important considerations for choosing a particle sizing system is the range of particle sizes that you expect to measure. Laser diffraction-type systems typically measure particles in the range of 10 nm to 3 mm. However, keep in mind that the absolute range of laser diffraction systems depends also on the number of detectors used and where the detectors are positioned. If time is a factor, bear in mind that laser diffraction systems can take anywhere from mere seconds up to 10 min to measure a sample (most measurements, however, are fairly rapid, e.g., less than a minute). If the limits of measurement are of concern, this method sensitivity is affected by the laser type used. For instance, for particles less than a micron in diameter, a laser that uses UV light, or visible light in the shorter-wavelength violet region of the spectrum, would be most effective. Dynamic light scattering-type systems usually measure particles in solution with diameters in the range of 0.6 nm to 6 μm.
Automated imaging systems are suitable for direct measurements of particles in the micron-to-millimeter diameter range, and can make either static or dynamic measurements using images collected by a digital camera equipped with magnification optics. This leads to another purchasing consideration: Do you want static or dynamic imaging? The static-type systems image dispersed particles that are not moving, such as a sample prepared on a slide. The dynamic-type systems image particles while they move in a stream past the camera. Dynamic imaging systems are often used along with another type of particle sizing system, such as laser diffraction. The ability to make both measurements allows researchers to gather more information about their samples, and provides an opportunity to verify one type of measurement with the other.
Another combination of measurement techniques also serves researchers well. For example, instruments that can perform both the dynamic light scattering and phase analysis light scattering techniques allow researchers to gather more information by measuring both particle size and zeta potential.
With the large number of particle sizing instruments available today—able to measure wide ranges of particle sizes, and combined with related and complementary techniques—you are likely to find the type of system that’s best for your lab.
A list of particle sizing system manufacturers is given in Table 1.
Table 1 – Manufacturers of particle sizing systems
Caitlin Smith is a freelance science writer who has a Ph.D. in Neuroscience from Yale University and postdoctoral work in Electrophysiology and Synaptic Plasticity; e-mail: [email protected].
Please see our Particle Size Analyzer (Particle Analyzers / Particle Sizer) section to find manufacturers that sell these products