Live magnetic observation of parahydrogen hyperpolarization dynamics.

Hyperpolarized nuclear spins in molecules exhibit high magnetization that is unachievable by classical polarization techniques, making them widely used as sensors in physics, chemistry, and medicine. The state of a hyperpolarized material, however, is typically only studied indirectly and with partial destruction of magnetization, due to the nature of conventional detection by resonant-pickup NMR spectroscopy or imaging. Here, we establish atomic magnetometers with sub-pT sensitivity as an alternative modality to detect in real time the complex dynamics of hyperpolarized materials without disturbing or interrupting the magnetogenesis process.

NMRduino: A modular, open-source, low-field magnetic resonance platform.

The NMRduino is a compact, cost-effective, sub-MHz NMR spectrometer that utilizes readily available open-source hardware and software components. One of its aims is to simplify the processes of instrument setup and data acquisition control to make experimental NMR spectroscopy accessible to a broader audience. In this introductory paper, the key features and potential applications of NMRduino are described to highlight its versatility both for research and education.

Magnetometer-Detected Nuclear Magnetic Resonance of Photochemically Hyperpolarized Molecules.

Photochemically induced dynamic nuclear polarization (photo-CIDNP) enables nuclear spin ordering by irradiating samples with light. Polarized spins are conventionally detected via high-field chemical-shift-resolved NMR (above 0.1 T).

Real-Time Polarimetry of Hyperpolarized C Nuclear Spins Using an Atomic Magnetometer.

We introduce a method for nondestructive quantification of nuclear spin polarization, of relevance to hyperpolarized spin tracers widely used in magnetic resonance from spectroscopy to in vivo imaging. In a bias field of around 30 nT we use a high-sensitivity miniaturized Rb-vapor magnetometer to measure the field generated by the sample, as it is driven by a windowed dynamical decoupling pulse sequence that both maximizes the nuclear spin lifetime and modulates the polarization for easy detection. We demonstrate the procedure applied to a 0.

Decoupling of Spin Decoherence Paths near Zero Magnetic Field.

We demonstrate a method to quantify and manipulate nuclear spin decoherence mechanisms that are active in zero to ultralow magnetic fields. These include (i) nonadiabatic switching of spin quantization axis due to residual background fields and (ii) scalar pathways due to through-bond couplings between H and heteronuclear spin species, such as H used partially as an isotopic substitute for H. Under conditions of free evolution, scalar relaxation due to H can significantly limit nuclear spin polarization lifetimes and thus the scope of magnetic resonance procedures near zero field.

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

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.

Zero-field nuclear magnetic resonance of chemically exchanging systems.

Zero- to ultralow-field (ZULF) nuclear magnetic resonance (NMR) is an emerging tool for precision chemical analysis. In this work, we study dynamic processes and investigate the influence of chemical exchange on ZULF NMR J-spectra. We develop a computational approach that allows quantitative calculation of J-spectra in the presence of chemical exchange and apply it to study aqueous solutions of [N]ammonium (N[Formula: see text]) as a model system.

Scalar relaxation of NMR transitions at ultralow magnetic field.

Nuclear magnetic resonance signals for H in simple chlorinated, brominated and deuterated liquids were detected at field strengths between 1 nT and a few μT to investigate the influence of scalar relaxation of the second kind (SR2K). SR2K describes the acceleration in magnetization decay rate for a spin-1/2 nucleus that is scalar coupled to a fast-relaxing quadrupolar nucleus. In agreement with simple theoretical models, the experimental data show that couplings to nuclei with small, nonzero quadrupole moments (H) give rise to higher transverse relaxation rates at ultralow field than rapidly relaxing quadrupolar nuclei (Cl and Br).

NMR relaxation in porous materials at zero and ultralow magnetic fields.

NMR detection in the ultralow-field regime (below 10 μT) was used to measure the nuclear spin relaxation rates of liquids imbibed into silica pellets with mean pore diameters in the 10-50 nm range. Heptane, formic acid and acetic acid were studied and relaxation rate data were compared with a conventional field-cycling NMR technique. Detection of H-C spin coupling NMR signals at zero field (∼0.

Invited Review Article: Instrumentation for nuclear magnetic resonance in zero and ultralow magnetic field.

We review experimental techniques in our laboratory for nuclear magnetic resonance (NMR) in zero and ultralow magnetic field (below 0.1 μT) where detection is based on a low-cost, non-cryogenic, spin-exchange relaxation free Rb atomic magnetometer. The typical sensitivity is 20-30 fT/Hz for signal frequencies below 1 kHz and NMR linewidths range from Hz all the way down to tens of mHz.

C-Decoupled J-Coupling Spectroscopy Using Two-Dimensional Nuclear Magnetic Resonance at Zero-Field.

We present a two-dimensional method for obtaining C-decoupled, H-coupled nuclear magnetic resonance (NMR) spectra in zero magnetic field using coherent spin-decoupling. The result is a spectrum determined only by the proton-proton J-coupling network. Detection of NMR signals in zero magnetic field requires at least two different nuclear spin species, but the proton J-spectrum is independent of isotopomer, thus potentially simplifying spectra and thereby improving the analytical capabilities of zero-field NMR.

Low-cost, pseudo-Halbach dipole magnets for NMR.

We present designs for compact, inexpensive and strong dipole permanent magnets aimed primarily at magnetic resonance applications where prepolarization and detection occur at different locations. Low-homogeneity magnets with a 7.5mm bore size and field up to nearly 2T are constructed using low-cost starting materials, standard workshop tools and only few hours of labor - an achievable project for a student or postdoc with spare time.

Nuclear magnetic resonance at millitesla fields using a zero-field spectrometer.

We describe new analytical capabilities for nuclear magnetic resonance (NMR) experiments in which signal detection is performed with chemical resolution (via spin-spin J couplings) in the zero to ultra-low magnetic field region, below 1μT. Using magnetic fields in the 100μT to 1mT range, we demonstrate the implementation of conventional NMR pulse sequences with spin-species selectivity.

Transition-Selective Pulses in Zero-Field Nuclear Magnetic Resonance.

We use low-amplitude, ultralow frequency pulses to drive nuclear spin transitions in zero and ultralow magnetic fields. In analogy to high-field NMR, a range of sophisticated experiments becomes available as these allow narrow-band excitation. As a first demonstration, pulses with excitation bandwidths 0.

Analysis of mass-limited mixtures using supercritical-fluid chromatography and microcoil NMR.

A protocol is presented for offline microfluidic NMR analysis hyphenated with supercritical chromatographic separation. The method demonstrates quantitative detection with good sensitivity. Typical sample amounts of 10 nanomoles can be detected in a fast and cost-effective manner.

Hyperpolarized singlet lifetimes of pyruvate in human blood and in the mouse.

Hyperpolarized NMR is a promising technique for non-invasive imaging of tissue metabolism in vivo. However, the pathways that can be studied are limited by the fast T1 decay of the nuclear spin order. In metabolites containing pairs of coupled nuclear spins-1/2, the spin order may be maintained by exploiting the non-magnetic singlet (spin-0) state of the pair.

Recycling and imaging of nuclear singlet hyperpolarization.

The strong enhancement of NMR signals achieved by hyperpolarization decays, at best, with a time constant of a few minutes. Here, we show that a combination of long-lived singlet states, molecular design, magnetic field cycling, and specific radiofrequency pulse sequences allows repeated observation of the same batch of polarized nuclei over a period of 30 min and more. We report a recycling protocol in which the enhanced nuclear polarization achieved by dissolution-DNP is observed with full intensity and then returned to singlet order.

Accessing long-lived nuclear spin order by isotope-induced symmetry breaking.

Nuclear singlet states are nonmagnetic states of nuclear spin-1/2 pairs that may exhibit lifetimes much slower than the relaxation of the component spins in isolation. This feature makes them attractive vehicles for conveying nuclear hyperpolarization in NMR spectroscopy and magnetic resonance imaging experiments and for reducing signal losses in other NMR experiments caused by undesirably fast nuclear spin relaxation. Here we show access to (13)C(2) singlet states in a symmetrical oxalate molecule by substituting one or more (16)O nuclei by the stable nonmagnetic isotope (18)O.

Hyperpolarized singlet NMR on a small animal imaging system.

Nuclear spin hyperpolarization makes a significant advance toward overcoming the sensitivity limitations of in vivo magnetic resonance imaging, particularly in the case of low-gamma nuclei. The sensitivity may be improved further by storing the hyperpolarization in slowly relaxing singlet populations of spin-1/2 pairs. Here, we report hyperpolarized (13) C spin order transferred into and retrieved from singlet spin order using a small animal magnetic resonance imaging scanner.

Direct enhancement of nuclear singlet order by dynamic nuclear polarization.

Hyperpolarized singlet order is available immediately after dissolution DNP, avoiding need for additional preparation steps. We demonstrate this procedure on a sample of [1,2-(13)C(2)]pyruvic acid.

Paramagnetic relaxation of nuclear singlet states.

Nuclear singlet states often display lifetimes that are much longer than conventional nuclear spin relaxation times. Here we investigate the effect of dissolved paramagnetic species on the singlet relaxation of proton pairs in solution. We find a linear correlation between the singlet relaxation rate constant T and the longitudinal relaxation rate constant T.

Singlet nuclear magnetic resonance of nearly-equivalent spins.

Nuclear singlet states may display lifetimes that are an order of magnitude greater than conventional relaxation times. Existing methods for accessing these long-lived states require a resolved chemical shift difference between the nuclei involved. Here, we demonstrate a new method for accessing singlet states that works even when the nuclei are almost magnetically equivalent, such that the chemical shift difference is unresolved.

Unraveling the spectroscopy of coupled intramolecular tunneling modes: a study of double proton transfer in the formic-acetic acid complex.

The rotational spectrum of the hetero dimer comprising doubly hydrogen-bonded formic acid and acetic acid has been recorded between 4 and 18 GHz using a pulsed-nozzle Fourier transform microwave spectrometer. Each rigid-molecule rotational transition is split into four as a result of two concurrently ongoing tunneling motions, one being proton transfer between the two acid molecules, and the other the torsion/rotation of the methyl group within the acetyl part. We present a full assignment of the spectrum J = 1 to J = 6 for the ground vibronic states.

Determination of molecular torsion angles using nuclear singlet relaxation.

The exponential relaxation time constant, T(S), of a nuclear singlet state is influenced by the proximity of neighboring NMR-active nuclei. For methylene groups in particular this dependence is much stronger than the case for other NMR relaxation constants, including the "conventional" relaxation time constant, T(1), of the longitudinal magnetization. This sensitivity provides a new route for determining torsional angles plus other molecular structural details in the isotropic solution phase.