A compact X-Band ODNP spectrometer towards hyperpolarized 1H spectroscopy

Hyperpolarised benchtop NMR spectroscopy for analytical applications

Benchtop NMR spectrometers, with moderate magnetic field strengths (B0=1-2.4T) and sub-ppm chemical shift resolution, are an affordable and portable alternative to standard laboratory NMR (B0≥7T). However, in moving to lower magnetic field instruments, sensitivity and chemical shift resolution are significantly reduced. The sensitivity limitation can be overcome by using hyperpolarisation to boost benchtop NMR signals by orders of magnitude. Of the wide range of hyperpolarisation methods currently available, dynamic nuclear polarisation (DNP), parahydrogen-induced polarisation (PHIP) and photochemically-induced dynamic nuclear polarisation (photo-CIDNP) have, to date, shown the most promise for integration with benchtop NMR for analytical applications. In this review we provide a summary of the theory of each of these techniques and discuss examples of how they have been integrated with benchtop NMR detection. Progress towards the use of hyperpolarised benchtop NMR for analytical applications, ranging from reaction monitoring to probing biomolecular interactions, is discussed, along with perspectives for the future.

A low-field ceramic magnet design for magnetic resonance

We describe the design of a low-field portable magnet, based on two ceramic magnets, separated by a distance, with their magnetic poles aligned to create a large homogeneous region with a field strength of 425 gauss. Ceramic magnets are an uncommon choice compared to Neodymium Iron Boron magnets for low-field magnetic resonance but are preferable for our purposes to create a homogeneous region at lower field strength. The low cost of large ceramic magnets results in an inexpensive design with a large measurement volume. The magnets rest in a 3D-printed structure, which allows for the magnets to be moved by hand so the experimentalist has control over the field topology. To test the utility of the design, we explored an Overhauser dynamic nuclear polarization experiment with an aqueous solution of 4-Hydroxy-TEMPO. We also explored a simple flow measurement employing the ceramic magnets at a 6-degree pitch, creating a 14.6 gauss/cm constant gradient.

A chip-based C-band ODNP platform

In this paper, we present a chip-based C-band ODNP platform centered around an NMR-on-a-chip transceiver and a printed microwave (MW) Alderman-Grant (AG) coil with a broadband tunable frequency range of 528MHz. The printable ODNP probe is optimized for a high input-power-to-magnetic-field conversion-efficiency, achieving a measured ODNP enhancement factor of -151 at microwave power levels of 33.3dBm corresponding to 2.1W. NMR measurements with and without microwave irradiation verify the functionality and the state-of-the-art performance of the proposed ODNP platform. The wide tuning range of the system allows for indirect measurements of the EPR signal of the DNP agent by sweeping the microwave excitation frequency and recording the resulting NMR signal. This feature can, e.g., be used to detect line broadening of the DNP agent. Moreover, we demonstrate experimentally that the wide tuning range of the new ODNP platform can be used to perform multi-tone microwave excitation for further signal enhancement: Using a 10mM TEMPOL solution, we improved the enhancement by a factor of two.

An easy-built Halbach magnet for LF-NMR with high homogeneity using optimized target-field passive shimming method

The aim of this work is to develop a Halbach magnet that possesses characteristics such as easy-built, low cost and high homogeneity for use in a portable low-field NMR (LF-NMR) system. Considering portability, a 4-ring Halbach magnet was designed through simulation and mechanical modelling, which was successfully constructed in a general laboratory setting. The obtained field strength (B0) was 0.169 T, with an initial homogeneity of 8204 ppm within a sphere with a diameter of 20 mm. To enhance robustness, efficiency and effectiveness of shimming, an optimized target-field passive shimming method was proposed. Subsequently, the homemade spectrometer was used to run NMR experiments on the Halbach magnet. The 1H NMR linewidths of water samples became significantly narrower after passive shimming, e.g., the linewidth of a sample with a diameter of 3 mm and a length of 3 mm reduced from 452.3 Hz (62.5 ppm) to 12.9 Hz (1.8 ppm), which was much less than 102 Hz. The NMR results demonstrate that the proposed passive shimming method can achieve high homogeneity, and the developed Halbach magnet is capable of satisfying numerous LF-NMR applications.

Recent advances in microresonators and supporting instrumentation for electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy characterizes the magnetic properties of paramagnetic materials at the atomic and molecular levels. Resonators are an enabling technology of EPR spectroscopy. Microresonators, which are miniaturized versions of resonators, have advanced inductive-detection EPR spectroscopy of mass-limited samples. Here, we provide our perspective of the benefits and challenges associated with microresonator use for EPR spectroscopy. To begin, we classify the application space for microresonators and present the conceptual foundation for analysis of resonator sensitivity. We summarize previous work and provide insight into the design and fabrication of microresonators as well as detail the requirements and challenges that arise in incorporating microresonators into EPR spectrometer systems. Finally, we provide our perspective on current challenges and prospective fruitful directions.

Microfluidic Overhauser DNP chip for signal-enhanced compact NMR

Nuclear magnetic resonance at low field strength is an insensitive spectroscopic technique, precluding portable applications with small sample volumes, such as needed for biomarker detection in body fluids. Here we report a compact double resonant chip stack system that implements in situ dynamic nuclear polarisation of a 130 nL sample volume, achieving signal enhancements of up to - 60 w.r.t. the thermal equilibrium level at a microwave power level of 0.5 W. This work overcomes instrumental barriers to the use of NMR detection for point-of-care applications.

Field-cycling NMR and DNP - A friendship with benefits

Field-cycling relaxometry, or rather its electronic version with a resistive magnet which requires signal detection at a field strength of 1 Tesla or below, remains an inherently insensitive technique due to the construction compromise that goes along with the need for a fast-switching, low-inductance magnet. For the same reasons, signal lifetime is short and frequency resolution is typically not given, at least for the predominantly used hydrogen nuclei. Dynamic Nuclear Polarization (DNP) bears the potential to circumvent these disadvantages: not only has it been demonstrated to enhance magnetization by up to three orders of magnitude beyond its thermal value, but it also provides the possibility to address particular parts of a molecule, thus generating selectivity even in the absence of spectral resolution. At the same time, DNP requires the introduction of stable radicals giving rise to additional relaxation contributions. This article presents a straightforward way to recover the native relaxation rates of the undisturbed system, and shows examples in different research fields where field-cycling relaxometry is traditionally used for refining models of molecular dynamics and interactions.