Immobilized free-radical substrates for magnetization of carbon-13 nuclei in flow NMR measurements

Abstract

Nuclear magnetic resonance detection of slowly relaxing 13C nuclei in flowing streams is facilitated by the placement of an immobilized free-radical (IFR) substrate in the flow path adjacent to the NMR probe to equilibrate sample nuclei prior to detection. The distinct advantage of IFR substrates is a 10- to 100-fold reduction in sample volume compared to conventional large volume premagnetization coils often used in flow NMR. IFR activity is sufficient to reduce the effective spin-lattice relaxation times of spin- 1 2 nucleu in the substrate so that large signal-to-noise increases are achieved when compared to spectra acquired in similar experiment times under nonflow conditions. For example, 13C NMR spectra of molecules that contain nonprotonated carbon nuclei with T 1 values in excess of 40 s exhibit a 4- to 9-fold signal enhancement. This S N increase reflects an increased flow experiment duty cycle because saturation effects are eliminated and nuclei achieve near-complete magnetization in the IFR region prior to detection.