Malvern Zetasizer Nano Series, Руководство пользователя

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N o t e
. The attenuator index and measurement position are automatically
adjusted by the software, though for more experienced users there is an
advanced button that allows both to be set manually.
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Results
This section gives the results of the measurement. The values given here will be
based either on intensity, volume or number, depending on which view tab is
selected.
The results section gives three pieces of information:
Z-average size (also known as the “cumulants mean”)
In dynamic light scattering this is the most important and stable number
produced by the technique. This is the size to use if a number is required
for quality control purposes.
It will only be comparable with other techniques if the sample is
monomodal (i.e. only one peak), spherical and monodisperse (i.e. no width
to the distribution), and the sample is prepared in the correct dispersant.
In any other case, the Z-average size can only be used to compare results
with samples measured in the same dispersant, by the same technique, i.e.
by Dynamic Light Scattering (DLS).
The cumulants analysis only gives two values, a mean value for the size, and
a width parameter known as the
Polydispersity , or the Polydispersity
Index ( PDI ). It is important to note that this mean size (often given the
symbol Z or z-average) is an intensity mean. It is not a mass or number
mean because it is calculated from the signal intensity.
The cumulants analysis is actually the fit of a polynomial to the log of the
G1 correlation function.
Ln[G1] = a + bt + ct
2
+ dt
3
+ et
4
+ ...........
The value of b is known as the second order cumulant, or the z-average
diffusion coefficient. This is converted to a size using the dispersant
viscosity and some instrumental constants.
Only the first three terms a,b,c are used in the standard analysis to avoid
over-resolving the data; however this does mean that the Z-average size is
likely to be interpreted incorrectly if the distribution is very broad (i.e. has a
high polydispersity).
C H A P T E R 5
Z e t a s i z e r N a n o S e r i e s
P a g e 5 . 5