Transcript Particle Size 4 Particle sizing by Laser Diffraction

Particle Size
Sizing Technique 3:
Laser Diffraction
Photon Correlation Spectroscopy
Kausar Ahmad
Kulliyyah of Pharmacy, IIUM
http://staff.iiu.edu.my/akausar
Physical Pharmacy 2
1
Light Scattering
Reflected
When light strikes a
particle, the light is then
scattered in all direction.
Diffracted
Refracted
Physical Pharmacy 2
2
Light scattered
Physical Pharmacy 2
3
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.
Physical Pharmacy 2
4
Static Light Scattering
Yield information
about size of
particles
• Rayleigh scattering
• size << wavelength
• Mie scattering
• size ~ wavelength
• Fraunhofer diffraction
• size >> wavelength
Physical Pharmacy 2
5
Particle Sizing by Laser Diffraction
Laser diffraction theories
• Fraunhofer theory
• Mie theory
Size range
• 0.04 to 2000 m
Physical Pharmacy 2
6
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
Physical Pharmacy 2
7
Laser Diffraction Configuration
Physical Pharmacy 2
8
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.
Physical Pharmacy 2
9
Examples
Instrument
Laser Source
Detector
Horiba LA-920
He-Ne laser at 632.8
nm for large particles,
Ring detector
array of 75 highpurity silicon
elements
tungsten lamp at 405
nm for small particles.
Coulter LS100
Malvern
Mastersizer E
Lens
Consists of 126
45 mm: 0.1 – 60 m
2 mW He-Ne laser at
632.8 nm
100 mm: 0.5 – 120
m
300 mm: 1.2 – 300
m
The number of
detectors is critical
for high resolution
Physical Pharmacy 2
10
Dynamic Light Scattering
A.k.a. Photon Correlation Spectroscopy
Yield information
about motions of
particles
• Brownian motion
• Diffusion coefficient
• Hydrodynamic size: 0.003 to 5
m (NICOMP 380/DLS)
• Oriented motions
• Electrophoretic mobility/zeta
potential
Physical Pharmacy 2
11
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.
Physical Pharmacy 2
12
PCS setup
Physical Pharmacy 2
13
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.
Physical Pharmacy 2
14
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.
Physical Pharmacy 2
15
Factors influencing scattering intensity
Concentration
Refractive index
difference
between diluent
and particles
Particle mass
Particle size
Light
wavelength
(light source)
Scattering angle
State of
polarisation
Physical Pharmacy 2
16
Physical Pharmacy 2
17
Physical Pharmacy 2
18
Physical Pharmacy 2
19
Physical Pharmacy 2
20
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.
Physical Pharmacy 2
21