Particle size analysis is the measurement of the size distribution of particles in a sample. Particle size affects the physical and chemical properties of materials including dissolution rate, powder flow, reactivity, and appearance. Reliable particle size measurement is essential in pharmaceuticals, materials science, mining, food processing, and environmental monitoring.
Particle size is typically expressed as an equivalent spherical diameter, the diameter of a sphere that would behave similarly to the actual particle in the measurement technique. Different measurement principles produce different equivalent diameters for the same non-spherical particle. The most common descriptors include D10, D50 (median), and D90, representing the particle sizes below which 10%, 50%, and 90% of the sample volume falls.
Laser diffraction measures particle size by analyzing the angular pattern of scattered light. Large particles scatter at small angles, while small particles scatter at wide angles. The technique covers a size range of approximately 0.01–3000 µm using multiple detectors and light sources. Mie theory and Fraunhofer approximation convert the scattering pattern to a volume-based size distribution.
Dynamic light scattering (DLS) measures Brownian motion to determine hydrodynamic diameter in the 1 nm to 5 µm range. It is the primary technique for submicron particles, colloids, and macromolecules. DLS is highly sensitive to large particles and aggregates, which dominate the intensity-weighted size distribution.
Sieving separates particles into size fractions by passage through a stack of woven wire mesh sieves with decreasing aperture sizes. Sieving is applicable to dry or wet powders above approximately 20 µm and provides a mass-based size distribution. Air jet sieving improves fine particle separation.
Sedimentation methods measure the settling velocity of particles in a liquid under gravity or centrifugal force. The Stokes equation relates settling velocity to particle diameter. The X-ray sedimentation method detects changes in suspension density. Sedimentation provides a mass-based distribution from approximately 0.1–100 µm.
Image analysis directly measures particle dimensions from microscope or camera images. Static image analysis measures thousands of particles on a slide. Dynamic image analysis captures flowing particles. Image analysis provides shape parameters including aspect ratio, circularity, and convexity in addition to size.
Sample preparation significantly affects results. Dry dispersion uses compressed air for free-flowing powders. Wet dispersion suspends particles in a liquid with surfactant and sonication to break agglomerates. The dispersion conditions must be optimized and documented for reproducible results.
