How dynamic light scatter works to accurately measure nano size particulates
The Controlled Reference Method
Particles suspended in a dispersing fluid are subject to random collisions with the thermally excited molecules of the dispersing fluid resulting in Brownian motion. The velocity and direction of the resulting motion are random but the velocity distribution of a large number of mono-sized particles averaged over a long period will approach a known functional form, in this case the size distribution of the particles.
In the Nanotrac, light from a laser diode is coupled to the sample through an optical beam splitter in the Nanotrac probe assembly. The interface between the sample and the probe is a sapphire window at the probe tip. The sapphire window has two functions! Firstly, it reflects the original laser back through the beam splitter to a photodetector. This signal which has the same frequency as the original laser acts as a reference signal for detection, offering Heterodyne detection.
Secondly, the laser passes through the sapphire window and is scattered by the particles which are in suspension but moving under Brownian motion. The laser is frequency shifted according to the Doppler effect relative to the velocity of the particle. Light is scattered in all directions including 180 degrees backwards. This scattered, frequency shifted light is transmitted through the sapphire window to the optical splitter in the probe to the photodetector. These signals of various frequencies combine with the reflected signal of un-shifted frequency (Controlled Reference) to generate a wide spectrum of heterodyne difference frequencies. The power spectrum of the interference signal is calculated. The power spectrum is then inverted to give the particle size distribution.
As particle size is determined from the velocity distribution of the particles moving under Brownian motion it is necessary to compensate for the physical parameters that directly affect the particle velocity. If the dispersing fluid molecules have a higher average thermal energy they will impart higher velocities to the particles with which they collide. Median particle velocity is directly proportional to the absolute temperature of the fluid. A viscous fluid slows the energized particles.
Particle velocity is inversely proportional to fluid viscosity. The Nanotrac incorporates a highly accurate temperature sensor in the sample cell. By describing the fluid temperature and viscosity characteristics in the Nanotrac algorithm, these parameters can be included in determining accurate particle size distributions. Also, because the laser light needs only to penetrate approximately 100 microns into the sample to generate a power spectrum, the Nanotrac can accurately determine particle size distributions at significantly higher concentrations than other methods.
Total Solutions in Particle Characterization