Adiabatic Heteronuclear Isotropic Mixing in Low-Field Nuclear Magnetic Resonance

The heteronuclear isotropic mixing between ¹H and ¹⁹F spins is demonstrated in low-field NMR. The efficient polarization transfer between nuclei expands the application range of low-field NMR in the chemical space, which is being made possible by new techniques of nuclear spin hyperpolarization. The isotropic mixing is demonstrated using a heteronuclear two-dimensional correlation spectrum of 3-fluoropyridine. An adiabatic WURST pulse achieved 50% transfer over the frequency difference of 2149 Hz in a magnetic field of 0.86 mT. While the efficiency of the DIPSI-2 was higher at 63%, it yielded a 26% less signal-to-noise ratio, compared to the WURST pulse experiment. In the presence of a 20% <i>B</i>₁ miscalibration, the DIPSI-2 mixing efficiency degraded to 26%, whereas the adiabatic pulse performance was reduced by only 1% at an amplitude reduced by 62.5%. The smooth amplitude profile at low WURST order increases adiabaticity and mixing performance, when the pulse is sufficiently short to alleviate spin relaxation. The improved performance of the adiabatic pulse under these conditions is important for low-cost and ex-situ applications of low-field NMR spectroscopy. The observed polarization transfer efficiency in density matrix simulations of both sequences predicted the experimental values in both the optimized and miscalibrated conditions within 18%. The agreement indicates that the simulations can be used to design optimal mixing pulses for varied conditions in the low-field NMR experiments. Isotropic mixing sequences in this context may in the future be used for the characterization or identification of chemical compounds on the benchtop, in the field or in environmental applications.