Particle size measurement light diffraction

Photon correlation spectroscopy (PCS), also referred to as dynamic light scattering, is a technique that is used to measure particles in the size range of 1 -0.001 p,m. Unlike particle sizing by laser diffraction, the sample, dispersed in a diluent, is not circulated, stirred, or sonicated during the measurement. The technique is dependent upon a stable suspension of particles that are in constant random motion due to collisions with molecules of the suspending liquid. 

A convenient description of light scatter, or diffraction as it relates to particle size measurement (4, 5.), is shown in Figure 1. 

Particle agglomerate sizes can be determined using a laser diffraction particle size analyzer, such as a Brinkmann PSA-2010. With this instrument, the principle of particle size measurement is based on various light scattering angles generated by paiTicles of... 

Laser has been widely used for the measurement of particle size through light scattering [30]. Laser scattering measurements are very accurate and fast. The techniques based on the Fraunhofer diffraction theory can measure the size of particles in the range of 2-100 pm. The Mie theory can extend the measurable size range to 0.1-1000 pm, if special light collection systems are used. 

Spectroscopy provides the laboratory analyst, and the production engineer, the ability to perform particle size measurement over a very broad range of sizes in a timely manner. Spectroscopy in the form of diffraction and dynamic light-scattering technology have been demonstrated to provide information that can be applied to research and process studies as well as to the routine measurement demands of the plant. Examples of particle size measurement capability for the plastics industry from 0.003 to 3000 pm were presented, as well as a short discussion of means to convert the data from the newer technology to the classical sieve measurements. A final comment on the advancing science of spectroscopic particle size measurements was also presented. 

The surface mean diameter is the diameter of a sphere of the same surface area-to-volume ratio as the actual particle, which is usually not a perfect sphere. The surface mean diameter, which is sometimes referred to as the Sauter mean diameter, is the most useful particle size correlation, because hydrodynamic forces in the fluid bed act on the outside surface of the particle. The surface mean diameter is directly obtained from automated laser light diffraction devices, which are commonly used to measure particle sizes from 0.5 to 600 p.m. X-ray diffraction is commonly used to measure smaller particles (see Size TffiASURETffiNT OF PARTICLES). 

The particle size analyzer, based on laser light diffraction, consists of a laser source, beam expander, collector lens, and detector (Fig. ] 3.45). The detector contains light diodes arranged to form a radial diode-array detector. The particle sample to be measured can be blown across the laser beam (dry sample), or it can be circulated via a measurement cell in a liquid suspension. In the latter case, the beam is direaed through the transparent cell. 

Fraunhofer rules do not include the influence of refraction, reflection, polarization and other optical effects. Early Iziser particle analyzers used Fraunhofer approximations because the computers of that time could not handle the storage cuid memory requirements of the Mie method. For example, it has been found that the Fraunhofer-based instrumentation cannot be used to measure the particle size of a suspension of lactose (R.I. = 1.533) in iso-octane (R.I. = 1.391) because the relative refractive index is 1.10, i.e.- 1.533/1.391. This is due to the fact that diffraction of light passing through the particles is nearly the same as that passing around the particles, creating a combined interference pattern which is not indicative of the true... 

Methods for analysis of the particle size distribution in the aerosol cloud include techniques such as time of flight measurement (TOE), inertial impaction and laser diffraction. Dynamic light scattering (photon correlation spectroscopy) is confined to particles (in suspension) in the submicron range. In addition to the size distribution, the particle velocity distribution can be measured with the Phase Doppler technique. 

Third, in selecting a particle sizing technique, one must consider the size of the material itself. Most techniques have effective upper and lower limits of detection. Laser diffraction, for example, can measure particles from approximately 0.01 pm to several millimeters. With optical microscopy and image analysis, it becomes very difficult to resolve features that are smaller than a 0.3 pm because of the wavelength of light used in conventional optical microscopes.1... 

Actual measurements deviate from theory at the large particle end because the angle of the diffracted flux is not sufficient to be distinguishable from the unscattered beam for very large particles. Deviation at the small particle end occurs as the particle size approaches the wavelength of the light source. These limitations... 

Submicron particle size analysis employs a scatter theory not completely described by Fraunhofer diffraction. The small particle range down to about 0.1 micrometer in diameter utilizes a combination of Fraunhofer diffraction and Mie theory for the forward scattered light and 90-degree Mie scatter at three (3) wavelengths and two (2) polarizations of each wavelength. Because of its need for a technology more involved than diffraction theory, submicron measurements are influenced by the index of refraction of the material making up the particulates to be sized. 

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