Category Archives: NMR Probes

Broadband and multi-resonant sensors for NMR

This article is not directly related to DNP, however, for everyone interested in building NMR probes, this paper gives a nice overview of the different concepts of circuit elements in an NMR probe. While this article focuses on micro-coils, most of the concepts can be applied to RF circuit design for NMR probes in general.

Davoodi, Hossein, Mazin Jouda, Jan G. Korvink, Neil MacKinnon, and Vlad Badilita. “Broadband and Multi-Resonant Sensors for NMR.” Progress in Nuclear Magnetic Resonance Spectroscopy 112–113 (June 2019): 34–54.

It has always been of considerable interest to study the nuclear magnetic resonance response of multiple nuclei simultaneously, whether these signals arise from internuclear couplings within the same molecule, or from uncoupled nuclei within sample mixtures. The literature contains numerous uncorrelated reports on techniques employed to achieve multi-nuclear NMR detection. This paper consolidates the subset of techniques in which single coil detectors are utilized, and highlights the strengths and weaknesses of each approach, at the same time pointing the way towards future developments in the field of multi-nuclear NMR. We compare the different multi-nuclear NMR techniques in terms of performance, and present a guide to NMR probe designers towards application-based optimum design. We also review the applicability of micro-coils in the context of multi-nuclear methods. Micro-coils benefit from compact geometries and exhibit lower impedance, which provide new opportunities and challenges for the NMR probe designer.

Modular, triple-resonance, transmission line DNP MAS probe for 500 MHz/330 GHz #DNPNMR

Reese, Marcel, Christy George, Chen Yang, Sudheer Jawla, J. Tassilo Grün, Harald Schwalbe, Christina Redfield, Richard J. Temkin, and Robert G. Griffin. “Modular, Triple-Resonance, Transmission Line DNP MAS Probe for 500 MHz/330 GHz.” Journal of Magnetic Resonance 307 (October 2019): 106573.

We describe the design and construction of a modular, triple-resonance, fully balanced, DNP-MAS probe based on transmission line technology and its integration into a 500MHz/330GHz DNP-NMR spectrometer. A novel quantitative probe design and characterization strategy is developed and employed to achieve optimal sensitivity, RF homogeneity and excellent isolation between channels. The resulting three channel HCN probe has a modular design with each individual, swappable module being equipped with connectorized, transmission line ports. This strategy permits attachment of a mating connector that facilitates accurate impedance measurements at these ports and allows characterization and adjustment (e.g. for balancing or tuning/matching) of each component individually. The RF performance of the probe is excellent; for example, the 13C channel attains a Rabi frequency of 280 kHz for a 3.2 mm rotor. In addition, a frequency tunable 330 GHz gyrotron operating at the second harmonic of the electron cyclotron frequency was developed for DNP applications. Careful alignment of the corrugated waveguide led to minimal loss of the microwave power, and an enhancement factor =180 was achieved for U-13C urea in the glassy matrix at 80 K. We demonstrated the operation of the system with acquisition of multidimensional spectra of cross-linked lysozyme crystals which are insoluble in glycerol-water mixtures used for DNP and samples of RNA.

Design of a local quasi-distributed tuning and matching circuit for dissolution DNP cross polarization #DNPNMR

Vinther, Joachim M.O., Vitaliy Zhurbenko, Mohammed M. Albannay, and Jan Henrik Ardenkjær-Larsen. “Design of a Local Quasi-Distributed Tuning and Matching Circuit for Dissolution DNP Cross Polarization.” Solid State Nuclear Magnetic Resonance 102 (October 2019): 12–20.

Dynamic nuclear polarization (DNP) build-up times at low temperature for low-gamma nuclei can be unfavorably long and can be accelerated by transfer of polarization from protons. The efficiency of the cross polarization (CP) depends on the B1-field strengths, the pulse sequence chosen for cross polarization and the sample composition. CP experiments rely on high B1-fields, which typically lead to electrical discharge and breakdown in the circuit. This problem is particularly severe in the low pressure helium atmosphere due to easily ionized helium atoms. The purpose of this study is to identify strategies to minimize voltages across components in a tuning and matching circuit of the coil to avoid electrical discharge during CP experiments. Design equations for three tuning and matching network configurations are derived. The results of the study are then used in the design of a single coil double resonance DNP probe operating at 71.8 MHz (13C frequency) and 285.5 MHz (1H frequency). In the current setup we achieve 28% polarization on 13C in urea with a build-up time of 11.6 min with CP compared to 14% and 53 min by direct polarization using TEMPOL as the radical. Different cross polarization sequences are compared.

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