LASERS - PARTICLE SIZE ANALYSIS Dr/ Wisam Abu -Jadayil

Different methods adapted for particle size analysis:

Methods applied for the size range of particles:

Disadvantages of other methods: Sieves: This is readily usable for large particles such as are found in mining.Not possible to measure sprays or emulsions. Cohesive and agglomerated materials e.g. clays are difficult to measure. Sedimentation: This has been the traditional method of measurement in the paint and ceramics industry and gives relatively low answers! The applicable range is 2-50 microns. Electrozone sensing: This technique was developed in the mid 1950’s for sizing blood cells. The method requires calibration standards which are expensive, difficult to measure emulsions and Porous particles give significant errors. Microscopy: It is not suitable as a quality or production control technique.

LASER DIFFRACTION: Applicable range: 0.1-3000 μm Principle: The method relies on the fact that diffraction angle is inversely proportional to particle size. Instruments consist of: 1)A laser as a source of coherent light of known fixed wavelength. Eg: He-Ne gas lasers (λ=0.63μm). 2) A suitable detector Eg: A slice of photosensitive silicon with a number of discrete detectors.

Laser diffraction (LD) is a method used for particle size measurement that is based on the properties of particles to scatter light. This method has become the preferred standard in many industries for characterization and quality control. There are two principal optical models for analysis of diffraction spectra: Fraunhofer Scattering, Mie Scattering.

Fraunhofer Scattering: This method is based on diffraction scattering only, and it won’t take into the consideration of others like absorption, refraction and reflection, where as Mie scattering will. The scattering pattern produced by a single spherical particle consists of a series of light and dark concentric bands that decrease in intensity with increasing radial position.

Contd… 3). The equation describing this distribution of intensity (I), is called the Airy function and is given by where J1 is the first-order spherical Bessel function,I(0) is the scattered intensity on the optical axis, and X is a dimensionless parameter given by the following expression, where f is the focal length of the receiving lens and R is the radial distance in the focal plane as measured from the optical axis.

2) MIE SCATTERING: The Mie model takes into account both diffraction and diffusion (absorption, refraction and reflection) of the light around the particle in its medium. The dimensionless size parameter (X) (here we need refractive index of the particle) where d is the particle diameter, and m is the refractive index of the particle.

Malvern mastersizer 2000: Mastersizer 2000 technical specifications: Size range Materials in the range 0.02μm to 2000μm Measurement principle Mie scattering Detection systems Red light: forward scattering, side scattering, back scattering Blue light: wide angle forward and back scattering. Light sources : Red light: helium-neon laser Blue light: solid-state light source.

Laser diffraction gives the following advantages: Flexibility. This method is rapid producing an answer in less than one minute, unlike other techniques. A wide dynamic range(0.1- 2000 microns) The method is non-destructive and non-intrusive. Hence samples can be recovered if they are valuable. Dry powders can be measured directly, although this may result in poorer dispersion than using a liquid dispersing medium. This method gives more detailed particle size distributions than the other techniques stated.

Contd… Speed of measurement -- single measurements can be made in 400 [micro]sec, allowing the dynamics of drug delivery from aerosol devices to be followed. Measurement repeatability -- the ability to acquire data rapidly allows many thousands of measurements to be averaged when reporting a single result, delivering excellent repeatability when compared with techniques that deliver results based on one-off measurements. Range of applicability -- sprays, dry powders, suspensions and sprays can all be characterized using the same technique, allowing different formulation types to be compared in a realistic way.

APPLICATIONS Laser diffraction-based particle characterization is widely accepted as a standard technique in the pharma industry. Application of laser Diffraction Particle Size Analyzer in Silicon Carbide. Application of Laser Diffraction Particle Size Analyzer in Food Industry. Cementitous powders. Duodenal digesta.

BIBLIOGRAPHY www.wikipedia.org www.cilasus.com www.nbtccornell.edu www.malvern.co.uk References: An article by Advanced Combustion Engineering Research Center (ACERC) at Brigham Young University.

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