Laser diffraction is a frequently requested technique in the particle characterization industry, but how do you know if it’s right for your material and process?
The experts at Particle Technology Labs have years of experience evaluating powders, semi-solid products, and suspensions and can help determine if this particle sizing technique is right for you.
What is Laser Diffraction?
Laser Diffraction is considered an ensemble technique. This means instead of determining the size distribution one particle at a time (such as with a particle counter), it does so by measuring the light scatter from a cloud or group of particles as it passes through a light source (a laser beam in this case) all at once. The angle of scatter is inversely proportional to the particle size (i.e., small particles scatter light at larger angles). This light scatter, referred to as a diffraction pattern, is then reported as an equivalent spherical diameter within the instrument software based on optical parameters input by the operator. Like many other particle sizing techniques, laser diffraction assumes that the particle is spherical to allow the reporting of a single value…the diameter of a sphere which would scatter the same angle of light detected.
Laser Diffraction Particle Size Analyzers
The most current laser diffractors on the market have a very wide particle size range capability, usually anywhere from tens of nanometers (tenths of a micrometer) to greater than 3000 micrometers, and can be used on a liquid or dry dispersion basis. To put this in perspective, the diameter of an average human hair is approximately 50 to 70 micrometers. This wide analytical range, plus the ease of use, speediness of the analysis, and being a technique supported by numerous quality standards (ISO, ASTM, USP, EP) has led the technique to be reliable, cost-effective, and a popular choice for general particle size determination.
Particle Technology Labs has several laser diffractors onsite by different manufacturers to ensure we find the correct instrumentation for your material. These include some of the top players in the industry, such as Malvern Panalytical, Beckman Coulter, and Sympatec.
What Data Will I Receive from a Laser Diffraction Test?
While each data output may cosmetically look a little different, the general content of the reports is all the same. A comprehensive summary including the general graphical particle size distribution on a volume-weighted basis will be provided, as well as the most commonly requested parameters (10th, 50th and 90th percentiles, etc.). Information on specific sizes can be reported on request. See a sample report.
Industries That Use Laser Diffraction Testing
Particle Technology Labs has used the laser diffraction technique for every industry imaginable. Our data has frequently been used to resolve manufacturing process issues, provide quality control criteria, evaluate the stability of products over time, and ensure compliance with regulatory agency guidelines such as those set by the EPA, FDA, European Union, just to name a few.
Because laser diffractors can analyze on a liquid (aqueous or organic carriers) and dry dispersion basis, the test is not limited to only powders. This technique is commonly used in the pharmaceutical industry to determine the particle size of Active Pharmaceutical Ingredients (APIs) in lotions, ointments, injectables, and oral medications. In addition, it also is frequently used in the environmental control and emissions industry to confirm emissions meet PM2.5 and PM10 requirements for respirable particles. Not to mention the industries making inks, paints, and other coating products and the food industry, all of which must control the particle size of materials and end products to ensure the desired performance, consumer satisfaction, and meet expectations, rely on laser diffraction.
When Not to Use the Laser Diffraction Technique
While this technique can be applicable to many products, there are circumstances where it may not be the best choice given certain properties of the material to be tested.
Laser diffraction will produce a general particle size distribution. It should not be used if the goal is to identify a low quantity outlier population within a sample. A good example of this is polishing slurries where a small amount of large particulate can cause abrasion on the polished components. Another is in the food industry, where a small number of large particles can impart a “gritty” texture when eaten, known as “mouth feel” in the industry. Laser diffraction is considered a low-resolution technique and thus will not be able to quantify minute differences in particle populations.
Laser diffraction may also not be suitable if the material is elongated or fiber-like. As noted, the particle size is reported on an equivalent spherical diameter. The farther the material’s shape is from this spherical assumption, the more likely the particle size generated will be biased depending on which dimension is captured as it passes through the laser. Another concern for dry dispersions is needle-like particles tend to crack under the applied delivery air pressure, essentially milling the particles and potentially producing results that are far from the natural state of the material collected in the field.
In addition, if the material to be analyzed is a suspension, the concentration of particles present is crucial. If the suspension is not concentrated enough, the amount of light scattered from the particles may not be sufficient to be accurately detected. If the sample is very concentrated and cannot be diluted without compromising its integrity, laser diffraction is not the most appropriate technique. Too concentrated samples violate the assumptions in the scattering theory of the technique and thus produce inaccurate results.
Lastly, if the sample to be analyzed is not homogenous (meaning there are multiple components present of similar size), an alternative higher resolution technique should be considered. As with several other particle sizing techniques, this technique cannot distinguish between an emulsion droplet and an API crystal during analysis. If quantification of different components is required, laser diffraction may not be suitable. These types of studies are better suited for image analysis and/or identification techniques also available at Particle Technology Labs.
How Particle Technology Labs Can Help
Particle Technology Labs is a fully independent, cGMP compliant and ISO 17025:2017 accredited laboratory. Our lab has nearly 30 years of experience in the particle characterization field. As an independent laboratory, we offer a variety of instruments manufactured by multiple vendors. Our experts can help evaluate if your sample is appropriate for laser diffraction. If you have any questions about the suitability of your sample for this technique, please do not hesitate to contact us. Particle Technology Labs provides consultations for testing inquiries free of charge, and we are always happy to discuss your testing needs.
By Jorie Kassel, Laboratory Division Manager