Quantitative size exclusion chromatography of polypropylene II: Analysis systems

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Abstract
Quantitative size exclusion chromatography (SEC) was considered a system with the following components: sample preparation, fractionation, detection, calibration, and resolution correction. Four systems were evaluated: I was 3 columns with “conventional single detector interpretation”; II was 4 columns with concentration correction and detector assessment; III was concentration correction applied to the data of I; IV was two development “mixed bed” columns. Analysis of polystyrene standards included calculation of their molecular weight averages and use of the Glöckner “T” measure of resolution as well as “specific resolution index.” Systems I and III provided the best high molecular weight results. System IV allowed 12–20 min analysis times and provided a highly linear calibration curve with very good reproducibility; however, it showed significantly worse resolution at high molecular weights. Plots of molecular weight averages determined from SEC versus their known true values were particularly useful. Concentration correction using the Rudin model moved different concentration data toward a common universal calibration curve and generally lowered molecular weight averages. Narrow polystyrene standards required relatively high concentrations for precise molecular weight averages and therefore, their averages were not good indicators of the need for concentration correction. Analysis of polypropylene samples corroborated the lower high molecular weight resolution of System III. Concentration correction did significantly change the polypropylene molecular weight distribution but did not affect the result of the kinetic model fitting.

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