On-line SEC-MR-NMR hyphenation: optimization of sensitivity and selectivity on a 62 MHz benchtop NMR spectrometer
The development of sophisticated synthetic routes for polymeric materials and more complex formulations used in current products require more advanced analytical techniques. As simple 1D experiments do not suffice to provide the necessary information. The development of coupled techniques, especially liquid chromatography (LC) in conjunction with molecular spectroscopy, is one promising approach to fit these requirements. The focus of this article is the optimization of a medium resolution (MR), benchtop 1H-NMR spectrometer coupled to a SEC instrument, where the NMR acts as an on-line chemical selective detector. The approach is to retain typical SEC selectivity while obtaining NMR on-line data with the highest possible sensitivity through full optimization of the entire set-up, pulse sequence and numerical data evaluation. A detailed description will be provided on how each part has been improved, including instrumental demands e.g. custom designed flow cells. Application examples of a PS/PMMA blend and PS-b-PMMA block copolymer are given to illustrate the potential of the hyphenated technique and the necessity of optimization. It is shown that the MR-NMR can be successfully hyphenated to SEC as an information rich chemical detector, providing the average chemical composition (CC) as a function of molar mass distribution (MMD), for polymers at isocratic mobile phase conditions.