light. Similarly, some particles are moving away from the detector and
have their emitted light red-shifted. On average, however, the particles
are not moving relative to the detector and thus the distribution’s center
lies on the wavelength of the incident light.
The observed wavelength of scattered light is not the only thing affected
by Brownian motion. The total observed intensity also fluctuates in time.
This is because Brownian motion causes the distance between any two
adjacent atoms to vary in time. The light emitted from these atoms
experiences interference; the distance between the atoms determines
whether this interference is constructive or destructive. Thus, as this
distance varies for a certain pair of atoms, the net light emitted from them
will fluctuate in intensity. Since there are so many particles in solution,
the net intensity of scattered light approximately evens out and is never
very far from the average intensity. However, it does not perfectly average
out, and the net intensity of signal in solution can be seen to fluctuate.
Critically, the intensity at any given point in time depends on the location
of all of the particles in solution. Thus, if one takes a measurement at time
zero and another measurement very quickly after that, the intensity of the
second measurement will be very similar to that of the first measurement
so long as the particles have not had sufficient time to diffuse far from
their original positions. If, on the other hand, the particles have had
enough time to move significantly, the intensity will be essentially
random. This concept can be expressed as a correlation. That is, if the
second measurement is likely to be more similar than average to the first
measurement, that is a positive correlation. If, however, the second
measurement is likely to be less similar than average to the first measurement, that is called negative correlation. A correlation of zero means
there is no relation between the measurements and the first intensity
gives the experimenter no information about the second intensity. In a
DLS experiment, as the time delay between the two intensity measurements increases from zero to infinity, the correlation decreases from unity
(perfect correlation) to zero (no correlation).
We can take this data and plot correlation versus time delay for solutions
of different particles (Figure 6.15). This graph gives a time scale for how
fast a given particle diffuses. The faster it diffuses, the more quickly it will
reach zero correlation as the particles need less time to move to different
positions from where they started. These correlation graphs can be
used to calculate the diffusion coefficent, D, which we introduced in
Chapter 3.
LIGHT SCATTERING METHODS 209
have their emitted light red-shifted. On average, however, the particles
are not moving relative to the detector and thus the distribution’s center
lies on the wavelength of the incident light.
The observed wavelength of scattered light is not the only thing affected
by Brownian motion. The total observed intensity also fluctuates in time.
This is because Brownian motion causes the distance between any two
adjacent atoms to vary in time. The light emitted from these atoms
experiences interference; the distance between the atoms determines
whether this interference is constructive or destructive. Thus, as this
distance varies for a certain pair of atoms, the net light emitted from them
will fluctuate in intensity. Since there are so many particles in solution,
the net intensity of scattered light approximately evens out and is never
very far from the average intensity. However, it does not perfectly average
out, and the net intensity of signal in solution can be seen to fluctuate.
Critically, the intensity at any given point in time depends on the location
of all of the particles in solution. Thus, if one takes a measurement at time
zero and another measurement very quickly after that, the intensity of the
second measurement will be very similar to that of the first measurement
so long as the particles have not had sufficient time to diffuse far from
their original positions. If, on the other hand, the particles have had
enough time to move significantly, the intensity will be essentially
random. This concept can be expressed as a correlation. That is, if the
second measurement is likely to be more similar than average to the first
measurement, that is a positive correlation. If, however, the second
measurement is likely to be less similar than average to the first measurement, that is called negative correlation. A correlation of zero means
there is no relation between the measurements and the first intensity
gives the experimenter no information about the second intensity. In a
DLS experiment, as the time delay between the two intensity measurements increases from zero to infinity, the correlation decreases from unity
(perfect correlation) to zero (no correlation).
We can take this data and plot correlation versus time delay for solutions
of different particles (Figure 6.15). This graph gives a time scale for how
fast a given particle diffuses. The faster it diffuses, the more quickly it will
reach zero correlation as the particles need less time to move to different
positions from where they started. These correlation graphs can be
used to calculate the diffusion coefficent, D, which we introduced in
Chapter 3.
LIGHT SCATTERING METHODS 209