Conventional size exclusion chromatography (SEC) yields molar masses relative to a calibration curve, Hyphenation to a light scattering detector (SEC-LS) provides absolute molar masses at each elution volume, yielding true molar mass distributions and averages. SEC-LS requires an LS instrument in conjunction with a concentration detector, typically a RI detector. The information obtained by these detectors are voltages, which have to be converted to the measured physical property. Currently, two approchaes for calibration of SEC-LS exist. Most manufacturers of LS instruments use well-characterized polymer standards for calibration, one manufacturer performs calibration using a reference liquid of known Rayleigh ratio. Therefore, true molar masses are obtained by SEC-LS, even if the standards used to calibrate the SEC-LS setup is not of identical chemical structure as the analyte to be analyzed later on. This contrasts conventional SEC. If calibration is performed using polymer standards, the molar mass and the specific refractive index increment, dn/dc, of the standard need to be known. For molar mass deteremination of unknown analytes, knowledge of their dn/dc is also required Unfortunately the correct dn/dc is often unknown in the solvent applied, or its determination is difficult, e.g. in mixed solvents or in solvents containing salts or additives. This contribution will describe an alternative approach for calibration of an SEC-LS setup and determination of the molar mass of unknown analytes. Neither the dn/dc of the sample, nor of the polymer used for calibration is required. Only the molar mass of the calibrant and the concentrations of the calibrant and the unknown samples are required. The new calibration approach does not require any dn/dc. Consequently, experimental difficulties arising from preferential solvation as they are present in ternary systems (mixed solvent, salt containg eluents) are eliminated. Besides the theoretical approach, the contribution will provide experimental results, demonstrating the suitability of this new approach.

Light scattering without refractive index increment: A new approach to calibrate SEC-light scattering setups

254th National Meeting (2017)

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