Chemically Resolved Nuclear Magnetic Resonance Spectroscopy by Longitudinal Magnetization Detection with a Diamond Magnetometer

Noninductive magnetometers based on nitrogen-vacancy centers in diamond offer a promising solution for small-volume nuclear magnetic resonance (NMR) detection. A remaining challenge is to operate at a sufficiently high magnetic field to resolve chemical shifts at the part-per-billion level. Here, we demonstrate a Ramsey-<i>M</i> _<i>z</i> protocol that uses Ramsey interferometry to convert an analyte's transverse spin precession into a longitudinal magnetization (<i>M</i> _<i>z</i> ), which is subsequently modulated and detected with a diamond magnetometer. We recorded NMR spectra at <i>B</i> ₀ = 0.32 T with a fractional spectral resolution of ∼350 ppb, limited by the stability of the electromagnet bias field. We resolve the chemical shift structure of ethanol with negligible distortion. Based on the laser illumination volume within the diamond (∼0.9 nL), we calculate an effective analyte detection volume of ∼1 nL. Through simulation, we show that the protocol can be extended to fields up to <i>B</i> ₀ = 3 T, with minimal spectral distortion, by using composite nuclear-spin inversion pulses. For subnanoliter analyte volumes, we estimate a resolution of ∼1 ppb and a concentration sensitivity of ∼40 mM s^1/2 are feasible with improvements to the sensor design. Our results establish diamond magnetometers as high-resolution NMR detectors in the moderate magnetic field regime, with potential applications in metabolomics and pharmaceutical research.