Particle Sizing: Many size domains, many approaches

Approaches to particle sizing differ as widely, technologically, as the sample types, but in many instances the approaches are orthogonal or complementary

Lew Brown, marketing director at Fluid Imaging Technologies (Yarmouth, ME), says his company was one of the first vendors to introduce automated imaging-based particle sizing instrumentation. As the company name implies, Fluid Imaging specializes in characterizing particles suspended in fluid, in motion, in real time, through what is essentially an automated microscope.

“Most of the automated particle sizing techs are volumetric-based,” Brown explains. All such instruments measure a signal that is proportional to the particle’s volume. Coulter counters, laser diffraction, and light obscuration all work this way, but they work on the basis of a signal that is proportional to volume.

“Their main drawback is they’re indirect. They have to compare the signal they measure to a signal of known volume. This requires an enormous leap of faith,” Brown says. “It assumes all particles are spherical.” Delivering an “equivalent spherical diameter of x” is satisfactory for obtaining particle size distributions. But a particular equivalent diameter could represent particles of vastly different shapes and sizes, such as rods and spheres or irregularly shaped particles. “These particles are obviously quite different, but volumetric particle sizing says they’re the same.” Morphology and shape can have profound effects on the quality and physical characteristics of small particles, such as in paints or electronic materials.

Fluid Imaging, by contrast, takes an image of every particle and measures 30 or more parameters such as length, width, perimeter, and circularity.

But as every analyst knows, microscopes have their own shortcomings, specifically an analysis size domain restricted to about two microns and larger. Particles smaller than that require an advanced microscopy technique, such as electron microscopy (EM), but EM for particle characterization is extremely slow and laborintensive— too much so, Brown says, to be practical. Brown describes his instrument as a “hybrid between classical particle sizers and microscopy.” He claims the ability to make 30 measurements per particle, at a rate of up to 50,000 particles per minute. Once images and data are acquired, the data system performs statistical pattern recognition, such as looking for particles of similar size, shape, and properties. Brown mentioned its use for monitoring seawater for organisms that cause red tide. “We can train the system to look for that organism and monitor its appearance over time, and then we can proactively shut down shellfish beds to prevent illness.” Another use is distinguishing impurities and aggregates in mixtures for quality control.

An emerging trend

Unlike some particle size companies, Microtrac (Largo, FL) covers most of the bases in terms of instrumentation: diffraction, dynamic light scattering (DLS), image analysis, scanning electron microscopy, surface analysis, and particle charge systems. “We cover a lot of territory,” says Microtac’s advanced applications engineer Philip Plantz, PhD. DLS capabilities range from 0.8 nm to 3,000 nm (three micrometers). A water molecule’s diameter is about 0.3 nm, so DLS can easily characterize larger molecules and aggregates such as carbon nanotubes, buckyballs, and proteins.

Plantz calls image analysis one of the emerging trends in particle characterization. This technique is similar to imaging methods in biology, which capture real-time microscopic events. Data-rich images permit a range of calculations to determine a particle’s shape and morphology, and how shape and size change over time.

“If particle size distribution has shifted, process and quality control people will ask why.” Imaging helps with root cause analysis of what went wrong: the process, milling, raw materials, etc. “Imaging can give you an idea of what is actually happening,” Plantz adds.

Many quality and environmental labs use one sizing technique that provides a signature measurement or proxy for critical attributes. As one delves into product development and research applications, particle characterization becomes more multifactorial, less of a black-and-white exercise. Methods diverge, overlap, and complement each other, not only in size domain capabilities but in the information they provide about shape, surfaces, and other characteristics.

Dig deep

Particle Sizing Systems (Port Richey, FL) specializes in high-resolution instrument systems that characterize single particles, one at a time. According to company president Kerry Hasapidis, a few large particles can ruin the process, sample, or quality of the material. “We specialize in identifying those at part-per-trillion levels.”

Digging deeply into individual particles, viewing particle events individually is an emerging trend in particle characterization. This contrasts with “macro” methods that look for averages among very large collections of particles.

Hasapidis likens his approach to panoramic photography. “A panoramic lens will give you a lovely photo of a sunset behind a bridge. Techniques like laser diffraction provide this sort of data. But if you want to focus on small sections of the structure, to look for cracks or failures, you need a telephoto lens. Many companies that specialize in laser diffraction, even some of the larger players, are now selling more and more into specialized niches that involve higher resolution and single particle image analysis.”

Single particle sizing efficiently characterizes a mixture of particles not by their average properties but by some outlier such as abnormally large or small size, pointed or angled shape, etc. In fields like nanotechnology the outliers may be as significant, with respect to quality and performance, as “average diameter.”

Unlike other techniques explained in Lab Manager, particle sizing consists of dozens of different methods and hundreds of device choices. “Each instrument provides another tool,” says Plantz.

Moreover, the choice of approach is more heavily industry- and application-oriented. “Particle sizing touches so many industries,” says Gilbert Vial, product manager for physical measurement at Shimadzu Scientific Instruments (Columbia, MD). Each product or process operates within its own size domain, and each requires different types of particle characterization to get the job done. “Trends in particle size instrumentation tend to track the successes and needs of the industries they serve.”

For additional resources on particle sizing, including useful articles and a list of manufacturers, visit www.labmanager.com/particle-sizing