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Laser diffraction is a widely used particle sizing technique for materials ranging from hundreds of nanometers up to several millimeters in size. The main reasons for its success are:

  • Wide dynamic range - from submicron to the millimeter size range.
  • Rapid measurements - results generated in less than a minute.
  • Repeatability - large numbers of particles are sampled in each measurement.
  • Instant feedback - monitor and control the particle dispersion process.
  • High sample throughput - hundreds of measurements per day.
  • Calibration not necessary - easily verified using standard reference materials.
  • Well established technique - covered by ISO13320 (2009).

Principles

Laser diffraction measures particle size distributions by measuring the angular variation in intensity of light scattered as a laser beam passes through a dispersed particulate sample. Large particles scatter light at small angles relative to the laser beam and small particles scatter light at large angles, as illustrated below. The angular scattering intensity data is then analyzed to calculate the size of the particles responsible for creating the scattering pattern, using the Mie theory of light scattering. The particle size is reported as a volume equivalent sphere diameter.

Optical properties

Laser diffraction uses Mie theory of light scattering to calculate the particle size distribution, assuming a volume equivalent sphere model.

Mie theory requires knowledge of the optical properties (refractive index and imaginary component) of both the sample being measured, along with the refractive index of the dispersant. Usually the optical properties of the dispersant are relatively easy to find from published data, and many modern instruments will have in-built databases that include common dispersants. For samples where the optical properties are not known, the user can either measure them or estimate them using an iterative approach based upon the goodness of fit between the modeled data and the actual data collected for the sample.

A simplified approach is to use the Fraunhofer approximation, which does not require knowledge of the optical properties of the sample. This can provide accurate results for large particles. However it should be used with caution whenever working with samples which might have particles below 50µm or where the particles are relatively transparent.

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Masterclass 4: Configuring optical parameters for laser diffraction particle size analysis Webinar - Recorded (English)

Selecting the appropriate optical properties for particle size distribution measurement by laser diffraction can appear to be a difficult task, especially when characterizing unknown or mixed materials. In this webinar, we describe the av...

Date recorded:
June 19 2014
Language:
English

Keywords: English Particle size Laser diffraction Webinar - Recorded Eastern Time 

Optical Properties of Organic Compounds: obtaining the refractive index and absorption values for use in laser diffraction particle size analysis Webinar - Recorded (English)

Determination of the optical properties of organic compounds is important when carrying out laser diffraction particle size measurements. However, many users struggle to obtain realistic values. In this presentation, we present methods for me...

Product:
Mastersizer 3000
Date recorded:
June 3 2014
Language:
English

Keywords: English Mastersizer 3000 Laser diffraction Webinar - Recorded Eastern Time 

Laser Diffraction Masterclass 3: Developing dry powder dispersion methods Webinar - Recorded (English)

This webinar presented by Dr. Anne Virden, covers method development for samples measured in dry dispersion. Dry dispersion offers the advantage of measuring larger sample masses much more quickly as well as different dis...

Date recorded:
May 20 2014
Language:
English

Keywords: English Particle size Laser diffraction Webinar - Recorded Eastern Time 

Masterclass 2: Developing wet or liquid dispersion methods using laser diffraction Webinar - Recorded (English)

This webinar will cover method development for samples dispersed in liquids. This will include a discussion of factors affecting the measurement, such as sampling and dispersion, and the relative importance of these factors in different particle size...

Date recorded:
April 3 2014
Language:
English

Keywords: English Particle size Laser diffraction Webinar - Recorded Eastern Time 

Setting pharmaceutical product specifications for particle size analysis Webinar - Recorded (English)

The end point of any particle size measurement is to provide improved product or process understanding and/or control. It order to achieve this, it is important that the size specifications selected are relevant to those products or processes. In thi...

Product:
Mastersizer range,Morphologi G3
Date recorded:
March 19 2014
Language:
English

Keywords: English Mastersizer range Morphologi G3 Particle shape Particle size Image analysis Laser diffraction Webinar - Recorded Eastern Time Pharmaceutical development 
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