Essential for Many Industries, Particle Sizing Continues to Undergo Innovation

Particle size measurement has become a critical application for chemicals, foods, paints, cosmetics, coatings, materials, and many other industries. Particle size, shape, density, and distribution affect the physical properties and chemical behaviors of all products comprised of particles or that use them as ingredients: The size of stationary phase particles affects chromatography retention time, pigment particles dictate hue and finish in paints, and physical dimension imparts mechanical, optical, and electronic properties to nanomaterials. Within critical size domains from nanometers to about 10 microns, the physical state can be as important as chemical composition.

Numerous technologies have emerged for measuring particle size. Sieving and sedimentation, among the oldest methods, provide quantitative sizing from millimeters upward. Optical sizing under a microscope, where particles are visualized and counted against the backdrop of a graticule (grid of evenly-spaced horizontal and vertical lines) and counted manually, is still used for many applications. Microscope-based sizing has been semi-automated through software that counts particles either directly or from photomicrographs.

The most sophisticated particle sizing techniques exploit the interaction between light, sound, or electricity and particle analytes. Electroresistive methods rely on the fact that non-electrically-conductive particles reduce the flow of electricity through a conductive fluid. The most common electroresistive particle sizing instrument is the Coulter counter, which quantifies suspended cells. Light- or laserbased techniques measure dimensions and distributions of suspended or, in some cases, dissolved species or particles suspended in air.

Tried and true

Although one of the oldest methods for particle sizing— and still the least expensive—sieving still serves many industries for particles in the size range up to about 4 mm. Sieving uses various techniques to get particles through the sizing mesh, including oscillation/shaking and sound. Several instrument makers offer sieve-based particle sizing. Most, including Retsch (Haan, Germany) and W.S. Tyler (St. Catharines, ON) and Hosokawa Micron (Summit, NJ), also specialize in particle-generating machines.

Hosokawa’s particle-sizing devices employ pneumatic sieving, most notably in the Micron Air Jet Sieve product line. The benchtop device, just slightly larger than a test tube agitator, measures particle sizes from 20 to 4,750 microns and serves primarily the food, pharmaceutical, mineral, cement, powder coatings, and chemical industries. “Laser techniques work best below about 20 microns,” says Tim Calvo, Lab Equipment Product Manager.

Micron Air Jet particle size instruments use a large air volume, up to 97 cfm, to pull a negative pressure on the system, which generates an air jet emitted through a rotating nozzle located below the sieving screen. The air flow disperses material atop the sieve and carries the finer particles through the screen and into the collection apparatus. Where systems that rely on oscillating or sound operate through a stack of up to six screens, Micron Air Jet systems use a single screen and performs the analysis in anywhere from 15 seconds to 5 minutes.

Seeing the light

Particle sizers that rely on dynamic light scattering (DLS) serve a sweet spot for particle analysis, between 0.6 nm and up to about 6 microns, while laser diffraction operate optimally in the 1-10 micron range.

Noting a maxim of measurement science, Jeff Bodycome, Ph.D. of Brookhaven Instruments (Holtsville, NY) observes that “life gets more difficult at the extremes. DLS has a broad range for very small particles but once particles get too large, it’s hopeless. If all your particles are larger than a few microns, you’re better off with diffraction and, larger than that, with sieving.”

Interest in DLS began in the fine chemical industry, particularly for latex materials used in paint. Today its principal markets are biotech, where it characterizes protein and protein aggregate solutions, and nanomaterials.

It may surprise that DLS uses light at 637 or 660 nm to measure characteristics of species that are much smaller than the wavelength. That is impossible to do with conventional microscopy, for example, whose limit is objects roughly half a wavelength in size. DLS works because it does not “detect” the molecule or particle, but calculates its hydrodynamic radius as a function of its mobility through the solution. “It measures how far the particle moves under Brownian motion,” notes Dr. Bodycome. Because this effect is a function of the sixth power of the hydrodynamic radius, DLS picks up species in very low abundance provided they are much larger than the analyte.

Purchase considerations

Purchase decisions for particle size analyzers are based on matching the analyte particle with instrument capabilities. Users with low- or sub-micron particles will require a lightor laser-based system, while those with larger particles can usually get by with a much less expensive “sieve shaker.” Quality control labs that analyze samples from large vats of material should consider purchasing separate sample prep equipment, known as a riffler, to improve the likelihood that analysis samples will be representative of the batch. Price is of course a consideration, but users should weigh the consequences of regrinding against instrument acquisition costs, cautions Tim Calvo of Hosokawa.