Towards the universal use of DOSY as a molar mass characterization tool: temperature dependence investigations and a software tool to process diffusion coefficients

Abstract

Two aspects of molar mass determination via diffusion ordered spectroscopy (DOSY) are described in this work. Firstly, we investigated how far the temperature of measurement affects the outcome of the DOSY experiment. For this, we performed molar mass calibrations of diffusion coefficients obtained for a series of narrowly distributed polystyrene samples in the temperature range of 0 to 40 °C. While a linear calibration is obtained at each temperature, a profound dependence of the obtained diffusion coefficient on temperature is first identified. We then demonstrated that this effect is an artifact created from convection in an NMR tube during the experiment and is dependent on the pulse sequence program. Using the dstebpgp3s pulse sequence, the available molar mass range and the temperature window for calibration are extended, and a reasonable agreement of all data with the Stokes–Einstein equation is found. To verify the validity of the chosen pulse sequence, we further determined the radii of gyration for different polymers via small-angle neutron scattering (SANS) experiments. SANS confirms the expected change in the radius with the molar mass, and no significant temperature dependence of the coil size is seen, in agreement with the results obtained using the dstebpgp3s pulse sequence. Secondly, we discuss different modes of calibration that scientists can use to determine molar masses from their individually measured diffusion coefficients. In addition, we provide a freely available software tool that allows one to directly transform diffusion coefficients into molar masses by applying a variety of calibrations and by guiding researchers as to which calibration is most suitable for their specific case.