Characterization - Lipid Nanoparticles: Production, Characterization and Stability - page 46

4.1.2 Laser Diffraction
Laser diffraction (LD), also called laser light scattering, can be used in com-
bination with PCS. LD is a powerful tool that has a much wider detection range
(20 nm-2,000 µm) and is a better choice for lipid nanoparticles in the upper
nanometer and micrometer size ranges (Keck and Müller 2008 ). The two com-
bined are often used to give a complete particle size distribution from ultra-small
to large particles.
The operational principle of LD is based on the complex patterns derived due to
Fraunhofer, Mie and Rayleigh scattering from an illuminated particle. These pat-
terns are dependent on the particle size of the sample to be analysed. The particle
radius is based on the correlation between the angle of diffraction and the particle
radius. The light scattered from an illuminated particle is detected by an array of
detectors in a laser diffractometer which determines its angular distribution. Large
particles predominantly scatter laser light in the forward direction. Smaller parti-
cles give a more spherical distribution of scattered light. Thus, the particle size is
determined from the geometric distribution of the scattered light. The intensity of
the scattered light is also influenced by the particles sizes, and weakens with the
cross sectional area of the particle. It can thus be concluded that larger particles
scatter light at contracted angles (with higher intensities) as against small particles
that scatter light at broader angles (with lower intensities) (Müller et al. 2000 ).
A laser diffractometer also gives a fair estimation of polydispersity of parti-
cles. This technique is well suited for characterization of large microparticles. The
Fraunhofer approximation is used to ascertain the diameters of particles in the
micrometer and millimetre size ranges. The particle size can be calculated using
Mie theory which is a complex combination of optical parameters and angle of
scatter. The main drawback of this theory is that its application to nanoparticles
requires knowledge of optical parameters (the real and imaginary real refractive
indices at the wavelength of measurements) of samples. Particle size distribution
is highly influenced by these optical parameters. Optical parameters are not neces-
sary for particles that are 5-6 times larger than the incident wavelength (Keck and
Müller 2008 ).
The use of LD also presents limitations. This technique is less useful in sam-
ples containing several populations of variable particle sizes. Uncertainties might
develop in instances where particles are non-spherical. Like PCS, an assump-
tion is made that the particles under investigation are spherical. It should also be
acknowledged that LD and PCS use light scattering effects to estimate the particle
size rather than directly measuring the particle size.
The LD data obtained with an instrument equipped with polarization intensity
differential scattering (PIDS) technology has been used to provide more informa-
tion on particle size distribution (Jores et al. 2004 ). PIDS technology combines
wavelength dependence with polarization effects. This combined approach greatly
enhances the sensitivity of LD to smaller particles. However, simultaneous use
of PCS and LD for size measurement is recommended. LD has been used by a