Water for Particle Analyzers
Application Overview
Particle size analysis is an important area in research and development, as well as in quality control in many industries such as pharmaceuticals, semiconductors, cement, ceramics, paints, emulsions, etc. Particle size and size distribution have a significant effect on the mechanical strength, density, electrical and thermal properties of the finished product. Significant production losses can be incurred due to high rejection rates if size and size distribution of powders being used in a process are not adequately controlled.
Particle size and size distribution analysis can be carried out using numerous commercially available instruments, or using instruments designed for operations in very specific environments. Some instruments can be used in an on-line (integrated into the manufacturing process) or off-line mode of operation. Most instruments operate in a batch mode of analysis, whereas there are only a few instruments that can be used in a continuous mode.
The methods used for particle analysis can be grouped into three:
- Ensemble methods
- Counting methods
- Separation methods
Ensemble Methods
In the ensemble methods, all particles in the sample are measured at the same time. The size distribution data is extracted from a combined signal for all particles. The following techniques fall under the ensemble category:- Low angle laser light scattering (LALLS, also known as laser diffraction). This technique uses a laser beam passing through a sample of particles in suspension. Light intensity data are collected at different angles away from the axis of the laser beam. The basic components of a laser diffraction instrument are a light source, typically a low power (approximately 10 mW Helium-Neon, in the region of 632 nm wavelength) laser source, optical elements to process the incident beam, a sample cell within which the sample is introduced, and a detection system (Figure 1).
Figure 1. Schematic of a laser diffraction instrument for particle size measurements
Light scattered from the sample is focused on to a detection system, that can be a multi-element array or numerous detectors placed at discrete locations. The detectors convert the scattered light intensity incident upon them into electrical signals that are then processed to obtain information about the particle size and size distribution.
- Dynamic light scattering (DLS, also known as quasi-elastic light scattering, QELS). This technique measures Brownian motion of sub-micron particles as a function of time.
- Ultrasonic attenuation spectroscopy (UAS). This is based on the principle that plane sound waves moving through a suspension of particles are attenuated in a predictable manner according to size and concentration of particles in suspension, the spacing of transmitter and receiver, and other physical parameters.
Counting Methods
In the counting methods, individual particles are measured and counts of similar size particles are placed in bins to come up with a distribution. The following are counting methods:- Electrozone counter. In this technique, particles are suspended in an electrically conductive liquid and forced to flow through a small orifice. Conductors are placed in the fluid on either side of the orifice, and the electrical resistivity of the orifice is measured as particles pass.
- Microscopy counting. Microscopy-based techniques are a powerful tool for characterization of particle size, size distribution and morphology. They involve direct observation of particles and the consequent determination of size based on a defined measure of diameter. A significant advantage of this technique compared to others is its ability to determine the particle shape, in addition to making a direct measurement of size.
Separation Methods
In separation methods to analyze particles, an outside force or process is used to separate particles according to size.- Capillary hydrodynamic fractionation (CHDF). This method uses a very fine capillary tube, a few µm internal diameter, to carry a flow of emulsifier in water. At the start of the analysis, a very dilute suspension of particles is added to the flow upstream of the capillary. As the particles move down the capillary, they diffuse across the capillary bore. Large particles reach the end of the capillary first, and the smallest particles last. A detector at the end of the capillary measures the concentration of the particles as they exit the capillary.
- Sedimentation field flow fractionation (SF3). In this technique, the separation force is established by a centrifugal field that is generated by spinning the complete circular SF3 channel at a high rate. The bigger and denser particles are forced more in the direction of the bottom or outer wall than the smaller particles.
- Differential centrifugal sedimentation (DSC). At the start of the analysis, a dilute sample is injected into the center of the disc centrifuge. The time for the particles to reach the detector beam versus beam intensity is converted to a size distribution data using Stoke’s Law and Mie theory light scattering calculations.
More Information
You may find more information related to particle size measurements in the following web sites:- http://www.upstate.edu/pathenvi/introduction.html
- http://www.patentstorm.us/patents/5426501/description.html
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