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Please check out our Particle Size Analysis section for more information or to find manufacturers that sell these products.
Have you ever dug into an old pint of ice cream from the back of the freezer, only to find that your mint chocolate chip has morphed from smooth and creamy to icy and granular? Both are ice and cream, yet nobody would call the latter preferable. It's a matter of size—specifically, of the ice crystals in the cream.
How do researchers measure size? Using particle size analysis, of course.
From the food industry to cosmetics, biopharmaceuticals to nanotechnology, particle size analyzers play key roles in R&D, manufacturing, and quality assurance. Everything from the quality of printer toner, to the palatability of chocolate, to the bioavailability of protein biopharmaceuticals depends on precise control of particle size. Particle size analyzer manufacturers have devised several approaches to the problem, each with its own pros and cons. The key point to remember: know precisely what your device is measuring, because no two approaches are the same.
"It is really important to understand how the device makes its measurement and see what it measures to determine whether one instrument works better than another," says Lew Brown, director of marketing at Fluid Imaging Technologies, Inc.
For instance, sedimentation analysis, in which larger particles sediment in a liquid faster than smaller ones, works well if all particles are spheres, but can provide misleading results if some particles are short but wide cylinders (like a platter); such particles will sediment more slowly, and thus will appear to be smaller than they are. Sieving, another older technique, uses a series of stacked filters with decreasing pore sizes to determine the mass distribution of particle sizes. Yet this approach, too, will tend to underestimate a particle's dimensions if it is much longer than it is wide.
Electrozone Sensing Particle Size Analyzers
One more modern approach to particle sizing is electrozone sensing (the operating principle behind Beckman Coulter's popular Coulter counters), in which changes in an electrical field (as particles pass through an electrified pore) are used to approximate size.
"You pass particles through a small orifice and measure the change in the electrical field as the particles goes by, which is proportional to volume," says Brown. "So you get an electrical signal, and based on calibration, say that corresponds to a particle of this volume."
According to Brown, the technique is especially popular for counting blood cells ("Coulter counting"), but there are other applications as well, including quality controlling aluminum powders used to make rocket fuel. "They had a real tight spec for that," he says.
Several analytical methods are based on a particle's optical properties. Laser diffraction (or laser scattering), for instance, estimates particle diameter by the degree of light scattering the particles induce—a phenomenon akin to what happens to your car's headlights in a snowstorm.
"The primary dataset is the change in intensity of light scattered as a function of angle," explains Paul Kippax, product manager for diffraction products at Malvern Instruments Ltd. "As particle size gets smaller, the reflected angle moves to wider and wider angles, all the way back to back-scatter."
Laser diffraction particle size analysis works best for particles between 100 nm and 2-to-3 mm in diameter. For smaller particles, between 0.6 nm and 6 um, dynamic light scattering (DLS, also called photon correlation spectroscopy) works better. The technique measures the intensity of reflected light at a single specific angle as a function of time, says Kippax.
"Dynamic light scattering looks at Brownian motion," Kippax says. "Small particles move very, very rapidly; for large particles, you may have to measure for five minutes to get enough movement." As a result, this technique works best for relatively small particles (for whom Brownian motion differences are most pronounced), and has become especially popular, he says, in nanotechnology and biopharmaceutical development (where, for instance, molecular aggregates must be avoided).
Despite its emerging popularity, however, DLS has the drawback that signal degrades with decreasing particle size, says Yanyin Yang, product specialist at Shimadzu Scientific Instruments, Inc. "According to Raleigh Scattering, intensity is proportional to the sixth power of particle diameter," Yang says. "Therefore, the signal becomes very weak in the lower detection range."
Shimadzu's new IG-1000 particle size analyzer, also intended for nanoscale particles, overcomes this particular shortcoming. The system uses the diffraction of light to estimate particle size.
"When AC voltage is impressed on the electrode array, due to dielectrophoresis, particles are drawn towards the electrodes, thus forming a density grating," explains Yang. "When a laser comes through this grating, it gets diffracted and the resulting light intensity is then recorded on the detector side. When AC voltage is removed away, particles will gradually release from the electrode array. As a result, the intensity of diffracted light changes. In general, smaller particles will travel faster so that the decay of diffracted light intensity will be more drastic."
Optical methods (laser diffraction, dynamic light scattering), electrozone sensing, sieving, and sedimentation, all work by assuming a spherical shape for all particles. That's fine if that assumption is correct, but oftentimes, it is not. In such cases, it helps to actually see the particles being analyzed. Microscopy, of course, is one solution. But several companies now offer imaging-based particle size analyzers, including both Malvern Instruments and Fluid Imaging Technologies.
"With us, the image is everything," says Fluid Imaging's Brown. Instead of assuming all particles are spherical—" a horrible assumption to have to make," says Brown—the company's FlowCAM system actually images and stores each particle individually, computing 26 separate parameters for each one.
Operating like a combination microscope and flow cytometer, the instrument can either image continuously or when triggered by either scatter or fluorescence—for instance, when analyzing drinking water.
Following data collection and analysis, users can then pull up a size distribution plot, just as with any optical technique. But they can also distinguish between particles that have the same "equivalent spherical diameter," but different aspect ratio (ratio of width/length).
According to Kippax, when purchasing particle size analyzers, key questions to consider are: anticipated particle size range (e.g., imaging systems cannot process particles smaller than about 0.5 um in diameter), concentration (not all systems handle dilute samples well), and usage environment (that is, will the device be used in a lab, manufacturing plant, in the field, etc.).
Please check out our Particle Size Analysis section for more information or to find manufacturers that sell these products