Characterization - Lipid Nanoparticles: Production, Characterization and Stability - page 47

number of researchers with who have reported sizes between 100 and 500 nm
(Das et al. 2011 ; Doktorovova et al. 2011 ; Kovacevic et al. 2011 ; Mitri et al. 2011 ;
Noack et al. 2012 ). These researchers have also reported the presence of micropar-
ticles in the formulation.
4.1.3 Field-Flow Fractionation
Field-flow fractionation (FFF) is a relatively new technique with the advantage of
colloid separation into particle sizes prior to measurement. Particles are separated
based on the difference in their Stokes' radii. FFF involves application of electric
fields to fluid suspensions or solutions. The fields are applied perpendicular to the
direction of flow of the impelling solutions, resulting in separation of particles pre-
sent in the fluid (Jores et al. 2004 ). FFF is often more suitable for particle sizing
than PCS because of better resolution of small particle size differences. The sepa-
ration of particles based on their sizes helps in further characterization of the sepa-
rated particles. Samples characterized by FFF often need to be diluted, so again
there are issues with possible changes in particle size on dilution. Dilution of the
sample, a major limitation of these method, may lead to changes in concentration
of surfactants, salt and other stabilisers thus changing the particle size and/or sta-
bility of the particles (Müller et al. 2000 ).
4.1.4 Other Techniques
Other than light scattering and field-flow fractionation techniques, the Coulter
Counter method can also be used for particle size determination. This method
uses an electrical zone sensing mechanism to determine the absolute number of
microparticles (Freitas and Müller 1998 ; Schwarz and Mehnert 1997 ) passing
through an aperture of a size which only allows particles of smaller size to pass
through. The particle number is determined by measuring electrical resistance
which changes when particles pass through the sensing device. The samples have
to be diluted in a comparatively concentrated salt solution such as normal saline
(0.9 % sodium chloride solutions) prior to measurements. This may interfere with
the electrical double layer and subsequently destabilize electrostatically stabilized
colloidal particles making the technique unsuitable for a large number of different
systems.
Information on particle size may also be obtained from microscopic techniques
(Sect. 4.2 ) such as electron microscopy and atomic force microscopy that are
sensitive in the colloidal size range. Unlike light scattering based techniques, the
Coulter Counter and Microscope techniques measure individual particles so they
have the advantage of a more direct measurement; however they are also quite
subjective in real experimentation due to the nature of particle selection. Modern