Gel Permeation Chromatography (GPC) is commonly used to determine the relative molecular weight and molecular weight distribution of polymer samples. GPC really measures the molecule volume and shape function, as dictated by inherent viscosity. If comparable standards are employed, this relative data can be used to determine molecular weights to within 5% accuracy. Polystyrene standards with discrepancies of less than 1.2 are often used to calibrate the GPC. Unfortunately, because polystyrene is a rather linear polymer, it can only be used as a standard if it is compared to other linear and similar polymers.