Absolute molar mass determination in mixed solvents. 1. Solving for the SEC/MALS/DRI trivial case
Andre M. Striegel, Pritish Sinha
Size-exclusion chromatography (SEC) with on-line static light scattering, specifically multi- angle static light scattering (MALS), and differential refractometry (DRI) detection remains the premier method by which to determine absolute, calibrant-independent molar masses of polymers. The method is restricted to the use of either neat solvents or solvents with a small amount of additive, however. This is because, in mixed solvents, preferential solvation (i.e., the enrichment, within the hydrodynamic volume of the polymer in solution, of one solvent over the other as compared to the solvent ratio outside said volume) leads to errors in the areas of the MALS and DRI chromatograms, as the solvent baseline does not accurately represent the solvent contribution to these detectors peaks. A seemingly trivial way by which to overcome this problem is through the use of an isorefractive solvent pair. This trivial solution is complicated, however, by the fact that the solvents in the pair must be miscible with each other in all proportions; the individual solvents as well as the mix must be able to fully dissolve the analyte; the solvents must possess sufficient optical contrast with the solution so as to generate an adequate detector signal; the solvent mix must be compatible with the chromatographic stationary phase, such that enthalpic contributions to the separation are minimal and analyte recovery from the columns is quantitative; and the difference in the Rayleigh ratios of the solvents can be ignored. Herein, we present the analysis of narrow dispersity polystyrene and poly(methyl methacrylate) samples, across a four-fold range in molar mass, using SEC/MALS/DRI in a mix of tetrahydrofuran (THF) and methyl isoamyl ketone, solvents which are shown to be isorefractive with each other at the temperature and wavelength of the experiments. Molar mass averages and dispersities are demonstrated to be statistically independent of solvent composition and to correspond well to
and Sinha, P.
Absolute molar mass determination in mixed solvents. 1. Solving for the SEC/MALS/DRI “trivial” case, Analytica Chimica ACTA, [online], https://doi.org/10.1016/j.aca.2018.11.051
(Accessed November 29, 2023)