Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

Measurement of Average Aggregate Density by Sedimentation and Brownian Motion Analysis



Richard E. Cavicchi, Jason King, Dean C. Ripple


The spatially averaged density of protein aggregates is an important parameter that can be used to relate size distributions measured by orthogonal methods, to characterize protein particles, and perhaps to estimate the amount of protein in aggregate form in a sample. We obtained a series of images of protein aggregates exhibiting Brownian diffusion while settling under the influence of gravity in a sealed capillary. The aggregates were formed by stir-stressing a monoclonal antibody (NISTmAb). Image processing yielded particle tracks, which were then examined to determine settling velocity and particle size based on mean square displacement (MSD) analysis. By using MSD analysis on tracks to determine particle size instead of imaged size—which is limited by diffraction and out-of-focus errors—we were able to accurately measure the hydrodynamic diameter down to 1µm. A relationship between size derived from the imaged particle area and MSD diameter was observed, suggesting a future method for correcting size distributions based on imaging. Stokes’ law was used to estimate the density of each particle. Polystyrene calibration microspheres ranging in size from 1µm to 5µm were used to verify the measurement method. It was found that the aggregates were highly porous with density decreasing from 1.079 g/cm3 to 1.030 g/cm3 as the size increased from 1.4µm to 5µm.
Journal of Pharmaceutical Sciences


Flow imaging, Image analysis, Microscopy, Particle Sizing, Physical Characterization, Protein aggregates


Cavicchi, R. , King, J. and Ripple, D. (2018), Measurement of Average Aggregate Density by Sedimentation and Brownian Motion Analysis, Journal of Pharmaceutical Sciences, [online], (Accessed June 19, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created May 1, 2018, Updated April 19, 2020