Absolute Molar Mass Determination in Mixed Solvents. 3. Accuracy of ∂n/∂c Values Obtained by Assuming 100% SEC Mass...

Accurate values of the specific refractive index increment (∂n/∂c) are essential to the accurate determination of molar mass averages, distributions, and related macromolecular parameters by, among others, size-based separations with online static light scattering and differential refractive index (DRI) detection. Examined here is the 100% mass recovery method of calculating the ∂n/∂c of dilute macromolecular solutions, when employing mixed solvents in a size-based separation (e.g., size-exclusion chromatography or SEC) with DRI detection. This method has been used successfully in the past for determining the ∂n/∂c of various polyelectrolytes. It is quicker, generally simpler, and less sample-intensive than its offline, batch-mode DRI counterpart. Whether or not the 100% mass recovery method allows for the necessary solvent equilibration within the immediate vicinity of the polymer chain during a chromatographic run, so as to allow for accurate determination of ∂n/∂c in SEC/DRI experiments, is evaluated here. This is done using a set of three narrow-dispersity linear polystyrene standards covering a 40-fold range in molar mass, dissolved in a 25:75 mix of tetrahydrofuran and N,N-dimethyl formamide. Results are compared to those previously obtained by the batch-mode method and from calculations involving the accurately known molar masses of the polymers and refractive indices of the solvents. The 100% mass recovery method of obtaining ∂n/∂c values, while of great help for determining the ∂n/∂c of, e.g., polyelectrolytes, does not appear able to overcome the obstacle of preferential solvation when analyzing macromolecules in a mix of non-isorefractive solvents with dissimilar second virial coefficients.