transformed into an easy-to-interpret graph with two peaks at the corresponding particle sizes.
There are several things that can affect diffusion speed besides particle
size that need to be controlled for (or at least considered). The first is the
size of the hydration shell around each particle. Since it is the hydrodynamic radius of each particle that is measured, if two particles have equal
size, but one is more strongly coupled to the surrounding media, that
particle will diffuse more slowly and register as larger. Second, the ionic
strength of a solution affects the coupling of the media to the particles, so
ionic strength must typically be set at a standard level. Third, the surface
morphology of a particle can affect diffusion speed. If a particle has long,
comblike appendages on its surface, they will slow its diffusion and make
it appear larger than a smooth sphere of equal size. Finally, changing
particle geometry can affect diffusion speed. If a particle shifts from a
more compact spherical formation to a more extended conformation, it
will be slowed down and again appear larger despite not changing mass at
all. This last effect is of particular interest in protein analysis because it can
sometimes detect protein morphology shifts between different forms.
One important limitation to DLS is that the signal intensity received from a
given particle is proportional to the sixth power of the particle’s diameter.
Thus, if one has a solution containing a 1:1 mixture by molar concentration
of 10-nm particles and 100-nm particles, the peak area corresponding to
the larger particles will be a million-fold larger than that for the smaller
particles despite the solution containing an equal number of each. This
problem only worsens as the size difference increases and serves to limit
the range of particles that can be accurately analyzed in a single solution.
Dynamic light scattering is an excellent method for determining the sizes
of particles present in a solution. It is able to accurately size particles from
the nanometer scale to above the micron scale so long as any one particular solution does not contain widely varying particle sizes. This
technique has found utility in applications ranging from colloidal science
to proteomics, and has become an integral part of many nanoscience
laboratories. Few other methods of particle sizing present such an
attractive combination of ease of use and wide region of sensitivity.
6.3 X-RAY SPECTROSCOPY
Photons at energies above those that interact with valence electrons or
covalent bonds can also be used to obtain structural and compositional
X-RAY SPECTROSCOPY 211
There are several things that can affect diffusion speed besides particle
size that need to be controlled for (or at least considered). The first is the
size of the hydration shell around each particle. Since it is the hydrodynamic radius of each particle that is measured, if two particles have equal
size, but one is more strongly coupled to the surrounding media, that
particle will diffuse more slowly and register as larger. Second, the ionic
strength of a solution affects the coupling of the media to the particles, so
ionic strength must typically be set at a standard level. Third, the surface
morphology of a particle can affect diffusion speed. If a particle has long,
comblike appendages on its surface, they will slow its diffusion and make
it appear larger than a smooth sphere of equal size. Finally, changing
particle geometry can affect diffusion speed. If a particle shifts from a
more compact spherical formation to a more extended conformation, it
will be slowed down and again appear larger despite not changing mass at
all. This last effect is of particular interest in protein analysis because it can
sometimes detect protein morphology shifts between different forms.
One important limitation to DLS is that the signal intensity received from a
given particle is proportional to the sixth power of the particle’s diameter.
Thus, if one has a solution containing a 1:1 mixture by molar concentration
of 10-nm particles and 100-nm particles, the peak area corresponding to
the larger particles will be a million-fold larger than that for the smaller
particles despite the solution containing an equal number of each. This
problem only worsens as the size difference increases and serves to limit
the range of particles that can be accurately analyzed in a single solution.
Dynamic light scattering is an excellent method for determining the sizes
of particles present in a solution. It is able to accurately size particles from
the nanometer scale to above the micron scale so long as any one particular solution does not contain widely varying particle sizes. This
technique has found utility in applications ranging from colloidal science
to proteomics, and has become an integral part of many nanoscience
laboratories. Few other methods of particle sizing present such an
attractive combination of ease of use and wide region of sensitivity.
6.3 X-RAY SPECTROSCOPY
Photons at energies above those that interact with valence electrons or
covalent bonds can also be used to obtain structural and compositional
X-RAY SPECTROSCOPY 211