Table of contents 1 Why is particle size important?
WHEN TO CHOOSE LASER DIFFRACTION
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- Điều hướng trang này:
- WHEN TO CHOOSE DYNAMIC LIGHT SCATTERING
| WHEN TO CHOOSE LASER DIFFRACTION
Laser diffraction is the most popular particle size technique for reasons including speed, ease of use, and flexibility. The most basic laser diffraction system can measure solid particles in suspensions and emulsions. With the addition of a dry powder feeder the instrument can then also measure dry powders in air. This is a low concentration technique, so dilution is often required. The complex refractive index of the sample and diluent must be known for optimum accuracy, but this information is easier to obtain than is often indicated (more often by competitors than informed scientists). The HORIBA LA-960 has a wide dynamic range capable of measuring down to 30nm and up to 5000µm. This unique ability to measure particles < 100nm as well as agglomerates as large as hundreds of microns makes this a credible choice even for nanotechnology applications. Since this is such a powerful, flexible technique laser diffraction is often the best option for companies buying their first analyzer, or hoping to satisfy multiple needs and applications. 30 WHEN TO CHOOSE DYNAMIC LIGHT SCATTERING Dynamic Light Scattering (DLS) can measure suspensions and emulsions from 1nm to 1µm. Both the lower and upper limits are sample dependent. The lower limit is influenced by concentration and how strongly the particles scatter light. A low concentration sample of weakly scattering particles near 1nm can be extremely difficult or at least difficult to reproduce. The upper size limit is determined mainly by the density of the particles. DLS algorithms are based on all particle movement coming from Brownian motion. Motion due to settling is not interpreted correctly by DLS systems. In addition, particles settled on the bottom of the sample cuvette can not be inspected by the laser light source. Particles with a high density will settle more quickly than low density particles. The upper limit of DLS may be 8µm for emulsion samples where the two phases have similar density. The upper limit of uranium particles may be as small as 300nm. The upper limit of particles with a density of 1.7 may be around 1µm. Using DLS does not require any knowledge of the sample RI (it would be required to convert from intensity to volume distribution), or concentration. What is required is viscosity, especially for higher concentration samples. Although most modern DLS systems claim the ability to work at higher concentrations, this is again sample dependent. Serious DLS work could involve a dilution study to determine the nature of the particle-particle interactions and presence of multiple scattering. Easy samples are simply a matter of pipetting the sample into a cuvette and clicking one button. More sophisticated DLS systems can also measure other sample characteristics including zeta potential, molecular weight, and second virial coefficient. Generating this additional information may require a greater skill set of the operator. tải về 7.54 Mb. Chia sẻ với bạn bè của bạn:
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