CAVEATS REGARDING THE "SOLVENT/NON-SOLVENT'' AND SELS APPROACHES IN SIZE-EXCLUSION CHROMATOGRAPHY AND RELATED METHODS

A rarely documented, yet industrially popular, approach to the size-exclusion chromatography (SEC) and related analysis of polymers that do not dissolve in common solvents (i.e., those solvents that are employed as mobile phases in high-throughput SEC systems) is to dissolve these analytes in a different solvent and to then inject this solution onto an SEC system that employs a non-solvent as mobile phase. A variant of this approach, known as solvent enhanced light scattering (SELS), is employed to compensate for low optical contrast between analyte and mobile phase. Both the "solvent/non-solvent'' and SELS approaches can provide results for a given sample in the form of chromatograms from which, using an in-place calibration curve, molar mass averages and distributions may be calculated. There appear to be no reports, however, on the accuracy of the solvent/non-solvent and SELS approaches. To this effect, the former were evaluated here (with an eye on implications for the latter), using well-characterized narrow dispersity polystyrene (PS) and poly(methyl methacrylate) standards and broad dispersity PS samples. A variety of approaches to the solvent/non-solvent method were employed, most with disastrous results, except for the trivial case when both solvent and mobile phase are good solvents for the polymer, are miscible with each other, and there is little difference in the specific refractive index increment of the analyte in each. This last case notwithstanding, based on the results shown here it is recommended that the solvent/non-solvent and SELS approaches be abandoned immediately, as they are likely to provide a false sense of confidence in inaccurate results.