Rayleigh Scattering - Malvern Zetasizer Nano series User Manual

Molecular weight theory
The standard approach for molecular weight measurements is to first measure the
scattering intensity of the analyte used relative to that of a well described 'standard'
pure liquid with a known Rayleigh ratio.
A common standard used in Static light scattering is Toluene, for the simple
reason that the Rayleigh ratios of toluene are suitably high for precise
measurements, are known over a range of wavelengths and temperatures and,
maybe more importantly, toluene is relatively easy to obtain in a pure form. The
Rayleigh ratio of toluene can be found in many reference books, but for reference
purposes the expression used to calculate the sample Rayleigh ratio from a toluene
standard is given below.
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Rayleigh scattering

The P
sample scattering intensity. The angular dependence arises from constructive and
destructive interference of light scattered from different positions on the same
particle, as shown below.
Zetasizer Nano
I : Residual scattering intensity of the analyte (i.e. the sample intensity – sol-
A
vent intensity).
I : Toluene scattering intensity.
T
n : Solvent refractive index.
o
n : Toluene refractive index.
T
R : Rayleigh ratio of toluene.
T
term in the Rayleigh equation embodies the angular dependence of the

2
I n
A o
--------- - R
R =

T
2
I n
T T
Constructive
Interference
Chapter 12
Destructive
Interference
ill 6746
Page 12-3