Determining and correcting “moment bias” in gradient polymer elution chromatography

Gradient polymer elution chromatography (GPEC) is rapidly becoming the analytical method of choice for determining the chemical composition distribution (CCD) of synthetic polymers. GPEC can be performed in traditional (strict precipitation–redissolution mechanism) or interactive (normal- and reversed-phase) modes, and results may be qualitative, semi-quantitative, or fully quantitative. Quantitative approaches have thus far relied on colligative or end group techniques for determining the values of standards used in constructing the GPEC calibration curve. While the values obtained from said methods are number-averages, they are assigned to the peak apexes of the standards (i.e. assigned as peak averages). This creates a determinate error in the quantitation, referred to herein as “moment bias”. In this paper we determine moment bias for a series of styrene–acrylonitrile (SAN) copolymers, where the distribution and averages of the AN% have been measured using normal-phase (NP) GPEC. We also correct for the effect via statistical treatment of the chromatographic data.