Improving Diameter Accuracy for Dynamic Imaging Microscopy for Different Particle Types

Dynamic imaging analysis instruments are used for sizing particles of different types that might appear in a biopharmaceutical. These instruments are calibrated using polystyrene latex microspheres in water, which is a significantly different system than the typical particles imaged in a formulation. We show how the instruments, when reporting an equivalent diameter, set a threshold for image processing and then apply a built-in correction to account for fuzzy boundary effects. We investigate the degree to which the threshold value and built-in correction influences the size, and ultimately particle size distribution, that the instrument reports on other particle types. Size corrections for a dynamic imaging system in a typical optical configuration were determined by comparison of equivalent image diameters with diameters from Brownian motion tracking of particles. A variety of particles were characterized: aggregates made from a monoclonal antibody available as reference material RM 8671 from the National Institute of Standards and Technology, bovine serum albumin aggregates, silicone oil droplets, polystyrene microspheres, and ethylene tetrafluoroethylene particles, a protein aggregate simulant (National Institute of Standards and Technology reference material RM 8634). The results show that the protein aggregates and ethylene tetrafluoroethylene are very similar to one another but quite different from the polystyrene calibration spheres. This points the way to developing new correction factors and calibration procedures based on particle type.