# Adiabatic Heteronuclear Isotropic Mixing in Low-Field Nuclear Magnetic Resonance

## TL;DR

This paper shows how adiabatic pulses improve polarization transfer in low-field NMR, making it more reliable for real-world applications.

## Contribution

The study introduces adiabatic pulses for efficient and robust isotropic mixing in low-field NMR.

## Key findings

- Adiabatic WURST pulses achieved 50% polarization transfer with minimal degradation under B1 miscalibration.
- DIPSI-2 pulses had higher efficiency but lower signal-to-noise and were more sensitive to miscalibration.
- Simulations predicted experimental results within 18%, validating their use for pulse design in low-field NMR.

## Abstract

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% B
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.