Merrill Seymour Goldenberg

Micrometer-Scale Particle Sizing by Laser Diffraction: Critical Impact of the Imaginary Component of Refractive Index

No HeadingPurpose.This study evaluated the effect of the imaginary component of the refractive index on laser diffraction particle size data for pharmaceutical samples.Methods.Excipient particles 1–5 μm in diameter (irregular morphology) were measured by laser diffraction. Optical parameters were obtained and verified based on comparison of calculated vs. actual particle volume fraction.Results.Inappropriate imaginary components of the refractive index can lead to inaccurate results, including false peaks in the size distribution. For laser diffraction measurements, obtaining appropriate or “effective” imaginary components of the refractive index was not always straightforward. When the recommended criteria such as the concentration match and the fit of the scattering data gave similar results for very different calculated size distributions, a supplemental technique, microscopy with image analysis, was used to decide between the alternatives. Use of effective optical parameters produced a good match between laser diffraction data and microscopy/image analysis data.Conclusions.The imaginary component of the refractive index can have a major impact on particle size results calculated from laser diffraction data. When performed properly, laser diffraction and microscopy with image analysis can yield comparable results.

Rotational Rheology of Bovine Serum Albumin Solutions: Confounding Effects of Impurities, Mechanistic Considerations and Potential Implications on Protein Formulation Development

PurposeTo show and rationalize the confounding effects on the rotational/oscillatory rheology of surface active impurities in commercial protein formulations such as bovine serum albumin, BSA.MethodsBulk and interfacial rotational/oscillatory rheology were used to study the viscosity, complex viscosity, storage/elastic modulus, G’ and loss/viscous modulus, G”, as a function of time of aqueous formulations of BSA and their purified components.ResultsViscosity/time profiles at steady shear for different commercial BSA products and lots showed viscosity increase, decrease and time-independent profiles at low shear rates. All lots showed shear thinning. BSA monomer and dimers/aggregates, in general, showed similar profiles. Addition of low levels of surfactant or high shear rates rendered all solutions to be Newtonian-like. Interfacial viscosity studies paralleled those on the rotational rheometer. G’ > G” with viscosity increase and G’ < G” with viscosity decrease over time.ConclusionsWe provide a rational explanation for the time-dependent and source-dependent rheological behavior of aqueous formulations of commercially available BSA proteins based on the migration of protein and surface active impurities to the air/water interface within the rheometer plates leading to the formation and breakdown of protein networks. Highly purified proteins is warranted in rheological studies of protein drug product candidates.