lunes, 26 de julio de 2010
Fraunhofer diffraction is the simplest method of determining particle size from light scattering measurements. It applies to particles larger than approximately one micron.
For particles larger than the wavelength of light, the light scatters from the edge of the particle at an angle which is dependent on the size of the particle. Larger particles scatter light at relatively smaller angles than light scattered from smaller particles. From observing the intensity of light scattered at different angles, we can determine the relative amounts of different size particles.
As the particles get close to or smaller than the wavelength of light, more of the light intensity is scattered to higher angles and back-scattered. The Mie Scattering Theory accounts for this different behavior and requires that we input information about the optical properties of the particles, such as refractive index. In order to make particle size measurements, the light intensity pattern must be measured over the full angular range. The Mie Theory applies to all sizes. When the particle size is larger that the wavelength of the incident light, the Mie equation reduces to the Fraunhofer equation. This allows one algorithm to cover the entire size range.
HORIBA incorporates the full Mie Scattering Theory over the entire size range of interest. An array of detectors. including high-angle and back-scatter detectors, and multiple light sources of different wavelengths are employed to provide an instrument that allows measurement of the full size range in one analysis. There is no need to combine results from two optical systems or analysis techniques, along with the problems that entails.
HORIBA offers a range of static light scattering particle size analyzers with different size ranges and with a range of capabilities.
Dynamic light scattering is a general term for the measurement of smaller particles (less than a few microns) by observing the Doppler shift of the incident light due to the Brownian motion of the suspended particulates. LB-550 Optics Layout Particles suspended in a fluid exhibit Brownian motion. This is random movement of the particles caused by the fluid molecules hitting the particles. Smaller particles with more more rapidly that larger particles due to their low inertia. When a coherent light source shines on these particles, light will be scattered from the particles, but the frequency will be shifted because the particles are in motion (Doppler shift). The speed of the particles determines how much the frequency is shifted.
By knowing the incident light frequency and measuring the scattered light frequency to determine the shift, we can calculate particle size.Dynamic Light Scattering Diagram 1 Early particle size analyzers using the PCS method, placed the detector at 90 degrees to the incident light. This required a very dilute suspension to prevent multiple scattering and over-attenuation of the light beam. This required a high powered laser and was sensitive to dust contamination of the fluid. The correlation technique limited the resolution capabilities of the algorithm, so it was often not capable of independently determining the presence of multiple modes in a sample.
More recent DLS instruments place the detector at or near 180 degrees to the incident light beam. This allows the measurement of higher concentration suspensions, eliminating the need for dilution and worries about dust contamination.
HORIBA's LB-550 places the detector at 180 degrees, allowing the measurement of samples up to 20% solids (by weight). The algorithm used to determine particle size from the light signal incorporates the Fourier-transformed power spectrum and iterative deconvolution of the relative contribution of various size particles in the mixture. This allows for and accurate accounting of not only median size, but also the distribution shape or multiple modes if they are present, without input from the operator.
HORIBA offers the LB-550 Dynamic Light Scattering Particle Size Analyzer, covering a range of 3nm to 6µm.
Arellano Wilson 1793016
Publicado por Tecnología en Telecomunicaciones - conocimientos.com.ve en 23:31
Etiquetas: 1III 2010-1 Arellano Wilson