Nanoparticle tracking analysis (NTA)
Nanoparticle tracking analysis (NTA) is a powerful and frequently applied technique for the sizing of particles in liquid samples.
In NTA, the sample is illuminated by a laser beam and the light scattered by particles moving under Brownian motion is recorded by a microscope camera. Individual particles are tracked, and their hydrodynamic size is obtained based on a modified Stokes-Einstein equation. Particle concentrations can be calculated as well, but only with moderate accuracy. For NTA of protein particles, the measurable size range is between about 50-100 nm to about 600-1,000 nm (depending on the light scattering intensity of the analyzed particles). The optimal particle concentration depends on the respective NTA system and on the particle size but is generally between 10^6 and 10^10 particles/mL.
NTA is mainly a research tool to characterize particle-based APIs (e.g., VLPs, liposomes, polymeric nanoparticles), to detect aggregates within protein formulations, or for total virus particle detection, provided that the size is large enough. It delivers a better size resolution for polydisperse samples when compared to dynamic light scattering [DLS], however, protein monomers are generally too small to be detected. NTA is further employed to observe relative changes in particle concentration, e.g., during stability studies.
For monodisperse samples, NTA is straightforward and quick to perform. Analysis of polydisperse samples, however, might require a method development. The time required for a sample analysis and data processing is usually longer than for dynamic light scattering, but shorter than for resonant mass measurement [RMM].
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S. Zölls, R. Tantipolphan, M. Wiggenhorn, G. Winter, W. Jiskoot, W. Friess, A. Hawe, Particles in therapeutic protein formulations, Part 1: overview of analytical methods., J. Pharm. Sci. 101  914–35. doi:10.1002/jps.23001.