The variance of the instrumental spreading function

i.e. the spreading factor of mono-dispersed polymer in a SEC column was determined experimentally with narrow MWD polystyrene standard samples by the method of simultaneous calibration. The dependence of the spreading factor on the retention volume deduced from a simple theoretical approach may be expressed by a formula with four physically meaningful and experimentally determinable parameters. The formula fits the experimental data quite well and the conditions for the appearance of maximum spreading factor are explicable.