Fast-field-cycling ultralow-field nuclear magnetic relaxation dispersion

Abstract

Optically pumped magnetometers (OPMs) based on alkali-atom vapors are ultra-sensitive devices for dc and low-frequency ac magnetic measurements. Here, in combination with fast-field-cycling hardware and high-resolution spectroscopic detection, we demonstrate applicability of OPMs in quantifying nuclear magnetic relaxation phenomena. Relaxation rate dispersion across the nT to mT field range enables quantitative investigation of extremely slow molecular motion correlations in the liquid state, with time constants > 1 ms, and insight into the corresponding relaxation mechanisms. The 10-20 fT/$$\sqrt{{\rm{H}}}{\rm{z}}$$ H z sensitivity of an OPM between 10 Hz and 5.5 kHz 1H Larmor frequency suffices to detect magnetic resonance signals from ~ 0.1 mL liquid volumes imbibed in simple mesoporous materials, or inside metal tubing, following nuclear spin prepolarization adjacent to the OPM. High-resolution spectroscopic detection can resolve inter-nucleus spin-spin couplings, further widening the scope of application to chemical systems. Expected limits of the technique regarding measurement of relaxation rates above 100 s−1 are discussed.

Meridional composite pulses for low-field magnetic resonance

“…The performance of the spin-selective inversion pulses is measured by applying a composite pulse, then immediately applying a dc pulse of flip angle 90°along + x. The peak field of the composite pulse is controlled such that I-BURP-1 pulse lengths are of comparable length for different N : (τ p /κ) ∼ 8 ms for 1 H. An alkali-metalvapor magnetometer 27 adjacent to the flow cell detects the resulting 1 H free precession signal (FID).…”

“…In this paper we illustrate the above re-utilization concept to derive error-tolerant pulses for magnetic resonance experiments where orientation of total magnetic field is unconstrained in the laboratory frame of reference. The case includes Earth's field nuclear magnetic resonance (NMR) [20][21][22] as well as the emerging area of zero and ultralow-field (ZULF) NMR 23,24 , which presents attractive regimes for nuclear spin hyperpolarization 25,26 , relaxometry 27,28 and precision spectroscopy [29][30][31] in fields ranging from nT to µT.…”

Decoupling of Spin Decoherence Paths near Zero Magnetic Field

“…Between prepolarization and the π/2 pulse, fast field cycling was used to expose the spins to ultralow fields | B z | < 500 nT. A detailed description of the procedure is given in ref . Additionally, to produce spatially homogeneous magnetic fields along x and y axes for the XY4 sequences, two saddle coils of 13 cm diameter, 13 cm length, and relative orientation 90° were added inside the innermost layer of the magnetic shield (6 turns/loop, 29 American wire gauge, 80 μT A –1 ).…”

“…27 Because 1 H spins couple to one another strongly, the relaxation of their total magnetization ∝T 10 (I) is quantified by a single exponential time constant, T 1 ( 1 H). The dependence of T 1 ( 1 H) on B z is measured using a fast-field cycling protocol 32 and presented in Figure 4b. The method involves first premagnetizing the spins in a B z = 20 mT magnetic field, allowing them to relax for a time τ relax near B z = 0, and finally returning to B z = 4.46 μT where a free-precession signal is detected, of amplitude proportional to exp(− τ relax /T 1 ( 1 H)).…”

Zero- to Ultralow-Field Nuclear Magnetic Resonance Enhanced with Dissolution Dynamic Nuclear Polarization

“…To estimate the effect of the unpaired electrons in TEMPOL on nuclear spin relaxation [39,40], we performed relaxometry experiments on a sample of 5.2 M [ 13 C]sodium formate diluted in D 2 O using the zero-field spectrometer, for which we add TEMPOL in increasing concentration.…”