Kratky plot

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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.

See Also