34: Particle Size Determination
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In our coverage of high-performance liquid chromatography (see Chapter 28) we noted that a typical packed column used porous silica particles with a mean diameter between 3-10 µm in size. A column that a manufacturer advertises as using 5 µm particles, however, will contain particles that are smaller and particles that are larger. As the size of the particles has an effect on the pressure it takes to move the mobile phase through the column, knowledge of the distribution of particle sizes is of interest. In this chapter we consider different methods for determining particle size.
- 34.1: Overview
- Particles come in many forms. Some are very small, such as nanoparticles with dimensions of 1-100 nm and that might consist of just a few hundred atoms, and some are much larger. In this chapter we consider methods for determining the size of particles.
- 34.2: Measuring Particle Size Using Sieves
- The particulates in a solid matrix are separated by size using one or more sieves. Sieves are available in a variety of mesh sizes, ranging from approximately 25 mm to 40 μm. By stacking together sieves of different mesh size—with the largest mesh at the top and the smallest mesh at the bottom—we can isolate particulates into several narrow size ranges.
- 34.3: Measuring Particle Size by Sedimentation
- When a particle that is larger than 5 µm is placed in suspension it will slowly settle toward the bottom of its container due to the force of gravity, a process called sedimentation. The time it takes for a particle to move a fixed distance is inversely proportional to the difference in the density of the particle and the density of the fluid in which the particles are suspended, and inversely proportional to the square of the particle's diameter. Larger (and denser) particles, therefore settle
- 34.4: Measuring Particle Size Using Image Analysis
- A photograph that includes a scale can, in principle, be used to estimate the size of the resin's beads. An optical microscope or electron microscope with a digital camera can capture images of the microscope's field of view. Software is used to differentiate the particles from the background, to establish the particle's boundaries, and to determine the particle's size.
- 34.5: Measuring Particle Size Using Light Scattering
- The blue color of the sky during the day and the red color of the sun at sunset are the result of light scattered by small particles of dust, molecules of water, and other gases in the atmosphere. The scattering of radiation has been studied since the late 1800s, with applications beginning soon thereafter. The earliest quantitative applications of scattering, which date from the early 1900s, used the elastic scattering of light by colloidal suspensions to determine the concentration of colloida