The principle behind GPC/SEC is that macromolecules are separated by their hydrodynamic radius or volume.

If this is the case, to accurately calibrate a column we need to form a relationship between the hydrodynamic volume and the retention volume (Universal Calibration), as opposed to the molecular weight and retention volume (Conventional Calibration).

A capillary viscometer detector makes this possible because it directly measures intrinsic viscosity (IV), which is inversely proportional to the molecular density of the polymer or protein coil. The first such detector was developed by Viscotek.

The product of molecular mass and intrinsic viscosity provides the hydrodynamic volume:

MW • IV = 5/2 • NA • Vh

The procedure is as follows:

  • Inject a series of narrow standards of known molecular weight.
  • Measure the retention volume of the resulting RI peak apex.
  • Allow the software to calculate the IV of the peak from the viscometer detector signal.
  • Construct a calibration curve of Log(MW • IV) vs. retention volume.

The primary advantage of Universal calibration is that you obtain Molecular information rather than size distributions. And for structural chemists using the Mark Howink plot of IV againt MWt you get information on  changes in the polymer, such as branching and chain rigidity.

The disadvantage is that you need an additional detector to RI or UV in the Viscosity module and extra software/maths to calculate the additional information. Added cost and some extra difficulty.