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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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Characterizing Nasal Spray Suspensions for Regulatory and Scientific Purposes Webinar - Recorded (English)

Demonstrating bioequivalence (BE) for complex generics such as nasal spray suspension drug products is a challenging task. Sponsors not only have to consider device requirements but also understand the properties of the API in the presence of functio...

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May 17 2016
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Manufacturing and characterizing spray dried products: Part 1 – An introduction to spray drying Webinar - Recorded (English)

Spray drying, which produces a dry powder from a liquid or slurry, is one of the most popularly-used continuous drying processes in which a liquid in the form of an emulsion, wet slurry or suspension is sprayed via a spray nozzle or a rotary atomizer...

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May 12 2016
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Using particle size analysis and rheology to better understand the mouthfeel of chocolate Webinar - Recorded (English)

Chocolate is an affordable yet luxurious treat with mature markets in Europe and North America and developing markets around the world. The secret of chocolate’s almost universal appeal is in the mouthfeel – the “snap” of the first bite, how the choc...

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Demo at your desk: Mastersizer 3000 Webinar - Recorded (English)

This online demo is designed to get you close up and personal with the Mastersizer 3000. You will be given an insider's look at the instrument and its accessories via our lab webcam, and follow the measurement of typical pharmaceutical samples on the...

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Interpretación de resultados de tamaño de partícula por la técnica de difracción laser Webinar - Recorded (Spanish)

Difracción láser es una técnica ampliamente utilizada para la medición de tamaño de partícula en materiales que varían entre cientos de nanómetros hasta varios milímetros en tamaño. Es una de las tecnologías más fáciles de entender y obtener resulta...

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Glossary