How can the polydispersity index be determined?

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The polydispersity index of a chitosan sample can be determined using size exclusion (= gel permeation) chromatography (SEC = GPC) which separates molecules according to their sizes, in combination with a refractive index detector (RID) and a Multiple Angle Laser Light Scattering (MALLS) detector. This method gives two measures for the average molecular weight of the sample (namely the weight-average molecular weight Mw and the number-average molecular weight Mn) which can be used to calculate the polydispersity index as IP = Mw/Mn.