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Chemically resolved nuclear magnetic resonance spectroscopy by longitudinal magnetization detection with a diamond magnetometer

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-03-03
JournalarXiv (Cornell University)
AuthorsJānis Ơmits, Yaser Silani, Zaili Peng, Bryan A. Richards, Andrew F. McDowell
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”
  • Core Innovation: Demonstration of high-resolution Nuclear Magnetic Resonance (NMR) spectroscopy using a novel “Ramsey-Mz” protocol with Nitrogen-Vacancy (NV) diamond magnetometers, detecting longitudinal nuclear magnetization (Mz).
  • High-Field Capability: The Mz detection method is largely independent of the NMR frequency, overcoming the severe technical challenges (sub-nanosecond pulse repetition intervals) faced by traditional transverse (Mxy) NV NMR detection at moderate magnetic fields (Bo > 0.2 T).
  • Experimental Achievement: NMR spectra of ethanol were recorded at Bo = 0.32 T in a ~1 nL detection volume, successfully resolving ~2 ppm chemical shift splittings with negligible distortion.
  • Resolution and Stability: Achieved a fractional spectral resolution of ~350 ppb, limited primarily by the stability of the electromagnet bias field. This is ~3 times better than previous high-resolution NV NMR works.
  • Projected Performance: With sensor design improvements (e.g., Ramsey-ENDOR protocol and stable permanent magnets), the technology is projected to achieve a spectral resolution of ~1 ppb and a proton concentration sensitivity of ~40 mMs1/2 for sub-nanoliter analyte volumes.
  • Geometric Advantage: The planar sample-sensor interface inherent to diamond magnetometers minimizes susceptibility-related magnetic gradients, enabling superior spectral resolution compared to conventional microcoil geometries.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Bias Magnetic Field (Bo)0.32TExperimental demonstration field.
Fractional Resolution (Achieved)~350ppbLimited by electromagnet stability.
Fractional Resolution (Projected)~1ppbWith optimized sensor design (Ramsey-ENDOR).
Analyte Detection Volume (Vsens)~1nLEffective volume contributing 50% of signal.
Magnetometer Sensitivity (Achieved)~100pTrms s1/2CW-ODMR, magnetic fields oscillating at 100-1000 Hz.
Magnetometer Sensitivity (Projected)~0.1pTrms s1/2Using Ramsey-ENDOR and 14N nuclear spin memory.
NV Center Concentration~4ppmIn 12C-enriched diamond membrane.
Diamond Membrane Dimensions250 x 250 x 60”m3[110]-cut diamond.
Proton NMR Frequency (fp)~13.8MHzAt Bo = 0.32 T.
Electromagnet Bo Stability< 350ppbAchieved over several hours using feedback loop.
Ethanol T1 (Doped)~0.6sReduced using 2 mM TEMPOL radical doping.
Projected Weighted Signal (Bo=3 T)~7pT/MRamsey-Mz with composite pulses.

Key Methodologies

Section titled “Key Methodologies”

  1. Sensor Fabrication and Geometry:

    • A 60 ”m thick, 12C-enriched, [110]-cut diamond membrane (~4 ppm NV) is adhered to a glass slide containing copper microwave (MW) traces.
    • The Bo field is aligned along one of the in-plane NV axes (~55° angle relative to the diamond edge).
    • A 3D-printed photopolymer container holds the liquid analyte (~1 ”L minimum) in direct contact with the diamond surface.

  2. NV Detection (CW-ODMR):

    • Continuous-Wave Optically-Detected Magnetic Resonance (CW-ODMR) is used for broadband magnetic field detection.
    • A dual-resonance protocol applies MW tones simultaneously at all six NV f±,i resonance frequencies.
    • MW tones are frequency-modulated (10.1 kHz rate, ~500 kHz deviation) with synchronized phase to maximize sensitivity to Bz changes while minimizing temperature dependence.

  3. Magnetic Field Stabilization:

    • The lock-in output from the CW-ODMR signal is fed back to secondary trim coils wrapped around the electromagnet poles.
    • This low-frequency feedback loop compensates for environmental field drift, achieving Bo stability of < 350 ppb over several hours.

  4. Ramsey-Mz Protocol:

    • Encoding: Two phase-coherent Radio-Frequency (RF) pi/2 pulses, separated by a variable time tau (τ), are applied at a frequency detuning Δnuc. This converts the phase accumulated by the transverse nuclear spin magnetization (Mxy) into a longitudinal magnetization amplitude (Mz(τ)).
    • Modulation/Detection: A subsequent train of resonant, phase-cycled RF pi pulses modulates the sign of Mz, converting the slowly-decaying Mz signal into an AC magnetic field signal (~500 Hz) detected by the diamond magnetometer.
    • Relaxation: A dead time (td = 0.6 s) is imposed after acquisition to allow the analyte magnetization to approach equilibrium (T1 relaxation).

  5. High-Field Optimization (Simulation):

    • To maintain spectral fidelity at Bo up to 3 T, simulations show that replacing the simple “hard” RF pi pulses with composite Levitt-Freeman inversion pulses (90-180-90) is necessary to mitigate spectral distortion caused by RF field inhomogeneity and detuning errors.

Commercial Applications

Section titled “Commercial Applications”
  • Metabolomics and Chemical Analysis: Enabling high-resolution NMR analysis of complex liquid mixtures using sub-nanoliter sample volumes, crucial for analyzing scarce biological samples or microfluidic systems.
  • Pharmaceutical Research: High-throughput screening and analysis of small molecules where sample volume is highly restricted, particularly for determining chemical shift structure.
  • Microfluidic Devices: The planar, single-sided sensor geometry is ideal for integration into lab-on-a-chip and microfluidic platforms, minimizing susceptibility gradients that plague traditional microcoil NMR.
  • Quantum Sensing Technology: Advances in high-sensitivity NV magnetometry protocols (Ramsey-ENDOR, 14N memory readout) push the limits of diamond-based sensors for magnetic field detection in the moderate field regime.
  • Compact NMR Systems: The ability to achieve high-resolution NMR at moderate fields (Bo < 3 T) allows for the use of compact, stable permanent magnets instead of large, expensive superconducting magnets.
View Original Abstract

Non-inductive magnetometers based on solid-state spins 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-M_z protocol that uses Ramsey interferometry to convert an analyte’s transverse spin precession into a longitudinal magnetization (M_z), which is subsequently modulated and detected with a diamond magnetometer. We record NMR spectra at B0=0.32 T with a fractional spectral resolution of ~350 ppb, limited by the stability of the electromagnet bias field. We perform NMR spectroscopy on a ~1 nL detection volume of ethanol and resolve the chemical shift structure with negligible distortion. Through simulation, we show that the protocol can be extended to fields up to B0=3 T, with minimal spectral distortion, using composite nuclear-spin inversion pulses. For sub-nanoliter analyte volumes, we estimate a resolution of ~1 ppb and concentration sensitivity of ~40 mM s^{1/2} is 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.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

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