# Guinier Plot

The Guinier Plot allows the determination of the Radius of Gyration $$R_g$$ from the measured scattered intensity $$I_s(q)$$ as a function of the scattering angle $$\theta$$. It is used in X-Ray, neutron and static light scattering (SLS). Using a logarithmic plot of the Guinier equation

\begin{gather}\ln{I_s(q)} = \ln{I_i} - \frac{q^2 R^2_g}{3}\end{gather}

one obtains $$R_g$$  from a simple linear fit.

This is however only true for small scattering angles and within RGD limits:

Related products:

LS Spectrometer

3D LS Spectrometer

• Either scattering angle should be sufficiently small or the wavelength of the scattered wave ($$q R_g \ll 1$$).
• The difference in refractive index n of solvent and scattering unit should not be very large ($$m -1 < 0.3$$).
• As is the case for all SLS measurements multiple scattering must be suppressed or the sample must be sufficiently dilute.

So for colloidal science the RDG limits should be considered more carefully than for polymer science, because the difference in relative refractive index is typically small between a polymer chain and the surrounding solvent.