Malvern Zetasizer Nano Series, Руководство пользователя

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Although the fundamental size distribution generated by DLS is an intensity
distribution, this can be converted, using Mie theory, to a volume distribution.
This volume distribution can also be further converted to a number distribution.
However, number distributions are of limited use as small errors in gathering data
for the correlation function will lead to huge errors in distribution by number.
Intensity, volume and number distributions
What is the difference between intensity, volume and number distributions?
A very simple way of describing the difference between the intensity, volume and
number distributions is to consider a sample that contains only two sizes of
particles (5nm and 50nm) but with equal numbers of each size particle.
The first graph below shows the result as a number distribution. As expected the
two peaks are of the same size (1:1) as there are equal number of particles.
The second graph shown the result of the volume distribution. The area of the
peak for the 50nm particles is 1000 times larger the peak for the 5nm (1:1000
ratio). This is because the volume of a 50nm particle is 1000 times larger that the
5nm particle (volume of a sphere is equal to 4/3
p
(r)
3
).
The third graph shows the result of an intensity distribution. The area of the peak
for the 50nm particles is now 1,000,000 times larger the peak for the 5nm
(1:1000000 ratio). This is because large particles scatter much more light than
small particles (the intensity of scattering of a particle is proportional to the sixth
power of its diameter - (from Rayleigh’s approximation).
It is worth repeating that the basic distribution obtained from a DLS
measurement is intensity - all other distributions are generated from this.
C H A P T E R 1 3
Z e t a s i z e r N a n o S e r i e s
P a g e 1 3 . 5
Relative
%
in
class
5
1 1
10 50 100
Diameter (nm)
Number
Relative
%
in
class
5
1
1000
10 50 100
Diameter (nm)
Volume
Relative
%
in
class
5
1
1,000,000
10 50 100
Diameter (nm)
Intensity
ILL
4022