With the increasing importance of controlling particle sizes throughout industry, laser diffraction remains a proven, robust technique for particle characterization.
Particles are characterized for many different reasons, from the quality control of everyday products like food, toothpaste, and paint, to particle size control to ensure the consistency and efficiency of pharmaceuticals. Relevant, reproducible particle size data is essential. One of the most proven, robust techniques for particle size characterization is the laser diffraction technique, a method which can be used to non-destructively characterize wet or dry samples.
Scattering light
The laser diffraction technique, also referred to as low angle laser light scattering, allows the determination of the complete size distribution of a cloud of particles. Laser light is shone into a cloud, and the particles in the cloud scatter the light at angles that are directly proportional to their sizes. Scattering intensity is also dependent on particle size, diminishing with volume. Large particles therefore scatter light at small angles with high intensity, and small particles scatter light at large angles but with low intensity. Particle size distributions are then calculated by comparing a sample’s diffraction pattern with an appropriate optical model.
A Particle sizes are determined by laser light scattering from the particles in the laser diffraction technique, which is employed in the Mastersizer 2000 shown here.
Image: Malvern Instruments, Inc.
The most commonly used model is the Mie theory. It predicts scattering intensities for any particle, regardless of size or opacity. It allows for primary scattering from the surface of a particle as well as for secondary scattering caused by light refraction within the particle, which is especially important for particles less than 50 µm in diameter. In the Mie theory, all particles are assumed to be spherical, although most measured particles are not. Particle diameters are therefore calculated from the measured volume of the equivalent spherical particle.
Particle characterization instruments based on laser diffraction consist of a laser source and a series of photodetectors to measure the diffraction pattern of the scattered light. Since these instruments measure clouds of particles as opposed to individual ones, they have the advantage that, at particle sizes smaller than ~ 10 µm, the system is measuring literally millions of particles, which gives a high statistical significance to the measured results.
Particles, particles everywhere
“The laser diffraction method is so effective because of the wide dynamic range available in a single instrument, the different types of samples that can be measured, and the speed of the measurement, which also allows on-line applications,” explains Alan Rawle, division manager for applications support at Malvern Instruments, Inc., Southborough, Mass.
Laser diffraction can also characterize almost any type of particle. “All types of materials can be characterized with this technique—dry powders, suspensions, emulsions, and aerosols/sprays,” says Rawle. “The technique has evolved considerably since the mid-1970s with the advent of faster computations combined with laser and detector advances.”
In addition, the laser diffraction technique is able to acquire data rapidly, which allows many thousands of measurements to be averaged in a single result, providing repeatability.
“This is a widely accepted technique and it serves numerous industrial areas such as cements, foods, biotech, pharmaceuticals, and petrochemicals,” says Peter Bouza, center manager for commercial operations for particle characterization at Beckman Coulter, Inc., Miami, Fla.
“Benefits of laser diffraction include high resolution and ease of use,” he continues. “The only drawback is that you need to have enough sample to allow the laser to scatter light off of the particles and enable the system to make a reading. Therefore it is not good for very dilute samples, such as proteins, where researchers don’t have a highly concentrated sample.”
“There are few drawbacks of this technique,” agrees Rawle. “However, it provides no real shape information and thus a visualization technique is vital. Some work is needed to obtain the optical constants for smaller (< 25 µm typically) materials, but this is not onerous. The range of the technique is stated to be 0.1 – 3000 µm in ISO13320-1, but this can be extended in certain circumstances.”
Measuring micro particles
One circumstance in which the range of the laser diffraction technique can be extended is with the use of polarization intensity differential scattering, or PIDS. PIDS is an extension of laser diffraction that exploits the polarization of light to achieve high resolution sub-micron analysis. It is used by Beckman Coulter in its LST 100Q/200/230 Series and LST 13 320 Series of laser diffraction particle analyzers. PIDS uses three wavelengths of light, filtered for horizontal and vertical polarizations.
Six additional detectors are used to measure the differential intensity between scattered light of the two polarizations. The combination of the multiple wavelengths and polarizations provides information that differentiates between sub-micron particle sizes and dramatically increases resolution compared to laser diffraction alone. The resulting scattered light in PIDS is described by the same Mie theory as in laser scattering, so all scattering information is converted to particle size using the same algorithm in a single operation.
“Due to their ease of use and their ability to run samples from 40 nm to 2,000 µm in one pass, this makes the systems acceptable not only to research, but also to quality control situations in which companies need to test the size of their raw materials before moving on to further processing of their product,” says Bouza.
Future applications
Although laser diffraction is a mature technology, improvements in its implementation can still be made. “Ease of use is always something that is sought after,” says Bouza. “Fewer and fewer people are available to do more and more work. This requires that the systems that are offered to be less dependent on user interaction. End users like systems that are one-touch and go type of systems, but also demand high resolution, accuracy, and repeatability.”
“Improvements must come in the interface (software) between instrument and user, and these are the most visible,” says Rawle. “Ease of use is prime here. Also, the size of the instrument can be reduced significantly. Indeed, in principle, there is no reason why a (limited) instrument could not be incorporated onto a credit card.”
More industries will be implementing particle characterization systems in the future. “I foresee basically more of the same: more industries, more materials,” says Rawle. “Any use of particles and particulate systems benefit from the technique. In addition, I see more integration with LIMS (laboratory information management systems) and other systems to allow control.”
“Overall, laser diffraction has been around for many years, but manufacturers of such systems have been improving their ease of use, sample dispersion, and other technical capabilities,” concludes Bouza. “These systems are well accepted and I feel will continue to evolve over time.”
