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 evenlyspaced 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 laser-based techniques measure dimensions and distributions of suspended or dissolved species or particles suspended in air.
Sieving is one of the oldest and least expensive particle sizing methods 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.
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 operates optimally in the 1-10 micron range. “Life gets more difficult at the extremes,” notes Jeff Bodycome, Ph.D. of Brookhaven Instruments (Holtsville, NY). “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.”
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 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 light- or laser-based system, while those with larger particles can usually get by with a much less expensive “sieve shaker.” Quality control labs analyzing 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 might want to weigh the consequences of regrinding against instrument acquisition costs.
• Incorporates three lasers for highest accuracy from 0.04 to 2,500 microns
• Switch between dispersion modes without having to switch hardware or realign the system
•Optical components are permanently mounted on a cast iron base plate to ensure system is always aligned
• Fully compliant with ISO 13320 and 21 CFR Part 11 standard
Cilas Particle Size
LS 13 320
• Now allows high-resolution, reproducible measurement of samples from .017 to 2000 µm
• Adds Rosin-Rammler and Folk & Ward Phi methods to its analytical capabilities
• Tornado Dry Powder Dispersing System keeps samples intact
• Reproducibility is typically better than one percent
• Features a fast CCD camera and an ergonomic sipper design
• Features automatic adjustment for fluid darkness; sees through black diesellubri cating oils
• Able to handle fluid viscosities up to 320 ISO grade without dilution
An optional Automatic Sample Processor is available that automatically runs 24 samples
• Provides reference method for measuring porous HPLC column materials
• Uses automated image analysis to characterize silica particles
• Features an electrical sensing zone (ESZ) to measure porous particles
• ESZ is the reliable method, provided the device is mass calibrated to compensate for particle porosity