Size-exclusion chromatographic determination of polymer molar mass averages using a fractal calibration.

The characterization of polymers by size-exclusion chromatography basically consists of the determination of the weight-average molar mass (Mw), number-average molar mass (Mn), and polydispersity index (I). An accurate estimation of these magnitudes requires the use of a reliable and trusted calibration curve. Three procedures for building up a calibration curve are analyzed in this work. The first is the classical universal calibration (UC), based on the elution of tetrahydrofuran-polystyrene in a system as reference. The second is based on the proper calibration curve made with standards of the sample under study. However, two main drawbacks arise when using these methodologies: the nonfulfilment of the UC when secondary mechanisms, other than pure size-exclusion, are present in the separation process; and the lack of a broad set of narrow standards of the sample under analysis in the second procedure. In order to circumvent these difficulties, a third, recently-proposed approach based on fractal considerations is applied. The accuracy and reliability of this method is proven through the calculation of the deviations observed in the estimation of the Mw values for polymer samples in different solvent-gel chromatographic systems. Whereas the classical UC shows a mean deviation of approximately 80% relative to the values given by the manufacturer, the fractal calibration yields a mean deviation of approximately 16%, similar to that obtained from the proper calibration. Moreover, the fractal procedure only needs one polymeric sample to generate the calibration curve.