Particle size and distribution (PSD) of a powder or particles dispersed in a fluid, is a list of values that defines the relative amount, by mass or by number of particles according to size. The PSD of a material is important in understanding its physical and chemical properties. Controlling PSD is very important for various industries such as paints and pigments (it has influences on the aspect and coverage), cosmetics (it has effect on the appearance and efficacy), catalysts (it influences the active surface area), drugs (influence on dissolution, adsorption), and nanoparticles.
EMSL Analytical offers a very diverse range of techniques for measuring the particle size and distribution and associated properties.
Mechanical Sieving with or without Hydrometry(ASTM D422; Minimum particle size 75 microns by sieving, smaller size by sedimentation)
This test method covers the quantitative determination of the distribution of particle sizes in soils. The distribution of particle sizes larger than 75 microns is determined by mechanical sieving, while the distribution of particle sizes smaller than 75 microns is determined by sedimentation process using a hydrometer.
ASTM D422 with hydrometry analysis of a soil sample.
Sonic Sieving (Minimum particle size 10 microns-sample dependent)
In this sieving method the particles are lifted and forcibly dropped in a column of oscillating air at a frequency of thousands of cycles per minute. Due to this motion, sonic sieving is able to sieve fractions much finer than the regular mechanical sieving.
Image Processing/Light or Electron Microscopy
The microscopy techniques have the ability measures the size (equivalent diameter), shape, distribution, agglomerations, and connectivity between particles.
300 nm limit for Polarized Light Microscopy (PLM)
100 nm limit or lower for Scanning Electron Microscopy (SEM)
sub-nanometer range limit for Transmission Electron Microscopy (TEM)
SEM image of encapsulated drug particles.
TEM image of 30 nm gold particles.
Dynamic Light Scattering (DLS)(Minimum particle size 0.6 nm)
DLS measures the Brownian motion of particles in a liquid medium that relates to the particle size. This technique measures the hydrodynamic diameter (the diameter of a hypothetical sphere that diffuses in the same fashion in the liquid medium as the particle being measured).
DLS analysis of 30 nm gold particles.
Zeta Potential and Isoelectric Point
Zeta potential is the charge on a particle at the shear plane. This value of surface charge is useful for understanding and predicting interactions between particles; therefore it is an indication for the agglomeration tendency of the colloidal particles. It is usually requested in conjunction with DLS measurements.
Isoelectric point (IEP) refers to the conditions (especially pH) at which the particle surface charge is zero. At pH values lower than the IEP, the particle surface charge is positive and at pH values higher than the IEP, the particle surface charge is negative. Values of IEP are obtained by measuring the zeta potential as a function of pH and identifying the pH at which the zeta potential value crosses zero. IEP gives indication about the stability of a colloidal suspension in regards to the agglomeration tendency.
Single Particle Mode Ion Coupled Plasma-Mass Spectroscopy (SP-ICP-MS)
Time resolved ICP-MS mode offers the possibility to collect the intensity for a single particle as it is vaporized and atomized in the plasma. Each measured data point can be correlated to the size and mass fraction of the particle.
SP-ICP-MS analysis of 50 nm gold particles
Small Angle X-Ray Scattering (SAXS)
This technique measures the diffuse X-ray scattering at very low angle caused by electron density fluctuations in the material under study. The scattering angle is inverse proportional to the size of the particle.
SAXS analysis of 25 nm TiO2