# Kratky plot

A Kratky plot is obtained by plotting scattering intensity as $\scriptstyle I(q) \times q^2$ vs. $\scriptstyle q$ (instead of simply $\scriptstyle I(q)$ vs. $\scriptstyle q$). This representation divides-out the decay of the scattering, making certain other features more evident.

In particular, a Kratky analysis is often performed on polymer solutions. Here, the shape of the curve in the Kratky plot helps identify the conformation of the polymer chain:

• A rise to a plateau indicates an unfolded chain (random coil)
• A distinct peak is indicative of a compact or folded conformation for the chain (in biological studies, this may be a protein in a well-defined/folded state).
• Other conformations can also potentially be distinguished:
• Pseudo-linear rise for rod-like conformation
• High-q upturn from the plateau indicates worm-like chain
• In polymer gels, a peak may indicate the presence of inhomogeneities, such as clustering.
• etc.

## Practical

The validity of a Kratky analysis is strongly dependent on data quality. Only with a very careful and representative background subtraction can the analysis be considered reliable.