By Jorie Kassel, Laboratory Division Manger
Particle size specifications are designed to ensure raw ingredients and products perform as expected. Often, companies are required to verify that their material is meeting these specifications regardless of which supplier is used. Using a contract lab for this purpose is very common but not always straightforward. There are many techniques to determine particle size and the methodology used may be open to interpretation when reviewing a material specification sheet.
The experts at Particle Technology Labs have years of experience working as a third-party quality control laboratory with many diverse technologies in-house. We can help guide you in your quest to confirm your material is being produced to the desired specifications.
Three main questions to ask yourself when tasked with finding replicate testing
1. What technique was used to develop these specifications?
Often, a specification for particle size on a certificate of analysis will read as a single line: “D10 between 50-80 microns” or “NMT 35 particles greater than 25 microns,” or percent retained on “80 mesh: NMT 30%.” These phrases often times don’t come with many clues on how they were determined with the exception of explicit industry terminology that reveals perhaps the basic technique used.
If we are referring to a Dv10 or X10 value, most likely this could be assumed the work was done by a laser diffractor. The “v” in the label is critical in order to ensure you are comparing volume weighted results to volume weighted results (Many specifications omit this important designation and simply state “D10.” The knowledgeable customer is then left wondering what technique was used to determine the criteria and what data format should be used.)
In reference to the latter example specifications mentioned, if a mesh number is referenced, perhaps a sieving technique was utilized, and if number of particles are explicitly stated, this might be referring to image analysis or another particle counting methodology.
If specifications exist, it usually means a test method with sample preparation and equipment settings used for the test may be available from the corresponding manufacturer/supplier. Particle size determination is often defined on an “equivalent spherical diameter” and how this is determined varies based on the technique. For instance, a laser diffractor determines particle size of a material by evaluating the angle of light scattered from particles passing through a laser and comparing it to the scatter of a homogenous population of known equivalent spherical particles.
In comparison, a Single Particle Optical Sensing (SPOS) instrument determines particle size on the amount of light a particle obscures and compares this reduction in light intensity to a calibration curve created using known spherical particles which obscure the same amount of light. And of course, sieving determines the particle size based on a sphere passing through a known opening in a wire mesh or other material.
Since most particulate in the real world are not going to be spherical, it’s important to note that each technique applies specific assumptions making it near impossible to gather a single correct answer. And an incorrect assumption about the analytical technique that was used to set the specification could lead to unexpected and failing results. Unexpected and failing results cause significant production delays, in addition to wasted time and money. The best way to determine how a specification was established is to have a conversation with the vendor/manufacturer on what instrumentation was used and if a test method can be provided.
2. What instrumentation was used to develop these specifications?
Advancements in technology has led to the ongoing development of new instrumentation to improve particle characterization. Knowing what instrument is used is critical. Simply put, what makes a manufacturer different from another is probably what will contribute to variations in particle size results. Proprietary algorithms, optical bench design, disperser accessories, and different interpretation of software settings all can play a part in why the same sample can be analyzed using the same analytical technique but different equipment and different results are generated. Keep in mind that advancements from one generation to the next in the same technology can also lead to unexpected results, it isn’t always an easy transition.
3. What were the sample preparation steps?
Just as particle size is a subjective and approximate science, the preparation of a sample for particle size analysis can also be subjective. For example, the type of carrier fluids used to suspend particulate (gas, aqueous, organic), the type and duration of applied dispersion energy (air pressure, ultrasonication), instrument parameters and settings can all have an affect on the particle size results. It’s important to note that the variability in particle size determination can be compounded even further due to sampling bias, chemist-chemist or lab-lab variability. So ensuring your testing lab can reproduce what the manufacturer intended is paramount if an outsourced lab is to act as a quality check.
With the answers to these three questions, you’ll be in a good position to help your lab help you. Whether looking to confirm the particle size results of a current supplier, evaluating a new supplier, or simply spot checking your own processes, it will give you and the lab a jump start in meeting your specification.
How Particle Technology Labs can help
Particle Technology Labs is a fully independent, cGMP compliant and ISO accredited laboratory. Our lab has nearly 30 years of experience in the particle characterization field working with a multitude of sample types and techniques. As an independent laboratory, we offer a variety of instruments manufactured by multiple vendors. Our experts can help evaluate your vendor specifications and methodology to determine how best to meet them with your material. The experts at Particle Technology Labs can guide you through the entire process from evaluating your provided method, conducting the analysis, and assisting with data interpretation.
By Jorie Kassel, Laboratory Division Manager