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Particle Size Sizing Technique 3:

Laser Diffraction Photon Correlation Spectroscopy

Kausar Ahmad

Kulliyyah of Pharmacy, IIUM

http://staff.iiu.edu.my/akausar

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Light Scattering

Reflected

Diffracted

Refracted

When light strikes a particle, the light is then scattered in all direction.

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Light scattered

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Scattering Intensity

illuminated by a monochromatic light source.

Results from scattered electromagnetic radiation from different portions of the particle

scattering intensity is detected by a detector

scattering intensity pattern is specific to a material with certain physical properties and particle size distribution.

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Static Light Scattering

• Rayleigh scattering • size << wavelength

• Mie scattering • size ~ wavelength

• Fraunhofer diffraction • size >> wavelength

Yield information about size of

particles

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Particle Sizing by Laser Diffraction

Laser diffraction theories

• Fraunhofer theory• Mie theory

Size range

• 0.04 to 2000 m

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Mie Theory Application for particle size analysis

Knowledge of complex refractive index, m=n-ik

• n=refractive index, • ik=absorption coefficient

Particle-related problems

• Applicable for spherical particles only

• Unknown n and ik• Change in n during

production process due to concentration and temperature change

• Mixtures of particle with different m and unknown mixing ratio

• Influence of coated particles

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Laser Diffraction Configuration

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Laser Diffraction Sizing Principle

Particles pass through monochromatic laser

source

Light is diffracted, the angles inversely related

to size.

The scattered light is collected by a Fourier

optical system.

The detector is a silicon wafer, with etched concentric rings of increasing radius,

corresponding to the angle of diffracted light.

The signal from each detector is amplified and translated into size using

Fraunhofer and Mie theories.

A volume size distribution is

obtained.

ExamplesInstrument Laser Source Detector Lens

Horiba LA-920 He-Ne laser at 632.8 nm for large particles,

tungsten lamp at 405 nm for small particles.

Ring detector array of 75 high-purity silicon elements

Coulter LS100 Consists of 126

Malvern Mastersizer E

2 mW He-Ne laser at 632.8 nm

45 mm: 0.1 – 60 m

100 mm: 0.5 – 120 m

300 mm: 1.2 – 300 m

The number of detectors is critical for high resolution

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Dynamic Light ScatteringA.k.a. Photon Correlation Spectroscopy

• Brownian motion• Diffusion coefficient• Hydrodynamic size: 0.003 to 5

m (NICOMP 380/DLS)• Oriented motions

• Electrophoretic mobility/zeta potential

Yield information about motions of

particles

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DLS Principle

particles undergo Brownian motion

causes fluctuations of local concentration of particles

results in local inhomogeneities of refractive index.

results in fluctuations of intensity of the scattered light.

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PCS setup

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Characteristics of PCS/DLS

Extremely broad range of size.

The observed intensity is a result of scattering properties of individual particles.

Particles scatter light independently.

No effects of interaction.

A sufficient number of particles sized within a short time.

Measure angular pattern of scattered light.

Analyse angular pattern by matrix conversion to recover size distribution.

Size range can be varied by using different laser source.

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Laser Source for DLS

Instrument model: NICOMP 380/DLS

• 15mW red laser diode (635nm wavelength) and a 20mW green single mode laser diode to extend the lower sizing limit of the 380 to below 10nm

• 50mW and 100mW single mode green laser with a 532nm wavelength to extend the lower size limit of the 380 to below 5nm.

Shorter wavelength for small particles.

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Factors influencing scattering intensity

Concentration

Refractive index difference

between diluent and particles

Particle mass

Particle sizeLight

wavelength (light source)

Scattering angle

State of polarisation

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References

http://www.photocor.com/photon_correlation_spectroscopy.htm

SD Duke, RE Brown, EB Layendecker, “Calibration of spherical particles by light scattering”, Duke Scientific Corporation, Technical Note-002B, May 15, 2000.