Nanoparticle Tracking Analysis (NTA)
The analysis of nanoparticles is a ubiquitous requirement in a broad range of industry sectors. Product performance and stability frequently depend on the ability to manufacture particle suspensions to fine tolerances without the presence of contaminants or aggregates. Foremost in such analyses is particle size and size distribution measurement for which a number of techniques are well established and commonly employed in routine quality control as well as in a research and development environment.
Nanoparticle Tracking Analysis (NTA) is a unique method of visualising and analysing particles in liquids that relates the rate of Brownian motion to particle size. The rate of movement is related only to the viscosity of the liquid, the temperature and size of the particle and is not influenced by particle density or refractive index.
In the LM10 and LM20 instruments, the sample is injected into a viewing chamber approximately 0.3 ml in volume. The particles contained in the sample are visualised by virtue of the light they scatter when illuminated by laser light. The technique samples from a volume of 120x80x20 microns. Sample concentration is adjusted to between 107 and 109 particles/ml such that a statistically significant number of particles are present in the beam path.
Higher concentrations (e.g. 109/ml) allow for shorter analysis times as does the analysis of smaller particles (due to the increased speed of Brownian movement). Typical analysis time is approximately 30s (900 sequential images at 30 fps) however longer analysis times improve accuracy and statistics.
The light scattered by the particles is captured using a scientific digital camera and the motion of each particle is tracked from frame to frame. This rate of particle movement is related to a sphere equivalent hydrodynamic radius as calculated through the Stokes-Einstein equation. The technique calculates particle size on a particle-by particle basis overcoming inherent weaknesses in ensemble techniques. Also, since video clips form the basis of the analysis, accurate characterisation of real time events such as aggregation and dissolution is possible.
NTA currently operates for particles from 10 to 1,000 nm, depending on particle type. Analysis of 10 nm particles is only possible for particles with a high refractive index, such as gold and silver. The upper size limit is restricted by the limited Brownian motion. At 1,000 nm (1 micron), the particle moves very slowly and accuracy starts to diminish. The viscosity of the solvent also influences the movement of particles and it too plays a part in determining the upper size limit for a specific system.