SEC is a method which uses the principle that a larger molecular size(three dimensional volume) sample elutes faster. In fact, the actual molecular size of the sample is different according to the solvent in which the sample is dissolved. In the case of polymers which have ionic functional groups, the ionic functional groups repel each other and the apparent molecular size of the polymers becomes larger. To prevent such ionic dissociation, the addition of salts such as NaNO3 or NaCl is known to be effective.
Especially, in the case of hydrophilic polymers, the existence of small amounts of ionic functional groups affect the peak pattern and measurement results. Addition of salts to the eluent is recommended even when it is considered that the sample polymer have no ionic functional groups.
(Left): Principle of SEC
(Right, above): Large apparent size because of repelling
(Right, below): Ordinary size