Even though there are many modern technologies used
to perform particle size analysis, these applications are mainly limited
to particles of micron in size or larger. Because of their small sizes
and strong particle-particle interaction, submicron and nanometer
particles in air are mostly in forms of aggregation and agglomeration.
Even though various electron microscopic techniques can be used to
image aggregated small particles and sometimes measure size of individual
particles, such measurements often lack statistical accuracy for samples
of broad distribution. For characterizing submicron and nanoparticles
in liquid, presently the most popular technology to measure properly
dispersed samples is dynamic light scattering1. Dynamic
light scattering detects time fluctuation of scattered light due to
the Brownian motion of particles. The detected scattering signals
can be processed using either photon correlation spectroscopy method
(PCS) or frequency spectrum analysis to determine diffusion rates
of particles and further obtain particle size and size distribution
information.
There are two types of analysis used in PCS. The so-called unimodal
analysis provides the average particle size and polydispersity index
of the sample using the cumulants method to fit the auto correlation
function (ACF) of scattered light for samples of narrow to medium
distribution. Polydispersity Index (PI) is an indication of distribution
broadness. In a more complex analysis, the Laplace inversion technique
is used to resolve the complete particle size distribution from the
ACF for samples of medium to broad distribution. The Stokes-Einstein
equation is applied in converting the diffusion coefficient to particle
size assuming spherical particles. For non-spherical particles, such
obtained values are only equivalent sizes.
1. XU, R. Particle Characterization: Light Scattering Methods, Chapter 5, Kluwer Academic Publishers, Dordrecht (2000)
