Category Archives: Field Cycling

A temperature-controlled sample shuttle for field-cycling NMR

Today, something that has nothing to do with DNP-NMR spectroscopy, but is a cool piece of equipment.

Hall, Andrew M.R., Topaz A.A. Cartlidge, and Giuseppe Pileio. “A Temperature-Controlled Sample Shuttle for Field-Cycling NMR.” Journal of Magnetic Resonance 317 (August 2020): 106778.

We present a design for a temperature-controlled sample shuttle for use in NMR measurements at variable magnetic field strength. Accurate temperature control was achieved using a mixture of waterethylene glycol as a heat transfer fluid, reducing temperature gradients across the sample to <0.05 °C and minimising convection. Using the sample shuttle, we show how the longitudinal (T1) and singlet order (TS) relaxation time constants were measured for two molecules capable of supporting long-lived states, with new record lifetimes observed at low field and above ambient temperatures.

X-nuclei hyperpolarization for studying molecular dynamics by DNP-FFC #DNPNMR

Gizatullin, Bulat, Carlos Mattea, and Siegfried Stapf. “X-Nuclei Hyperpolarization for Studying Molecular Dynamics by DNP-FFC.” Journal of Magnetic Resonance 307 (October 2019): 106583.

Dynamic Nuclear Polarization methods are used for improving the quality of the NMR data, opening new possibilities by increasing both the sensitivity and the selectivity in NMR relaxation experiments. Recently, Fast Field Cycling relaxometry combined with DNP was introduced, demonstrating that molecular dynamics studies in the presence of natural or artificial radicals are indeed feasible under conditions where the signal-to-noise ratio is frequently critical. In this work, the extension of NMR relaxation dispersion beyond 1H NMR, by hyperpolarization of X-nuclei, is demonstrated. Overhauser effect via nitroxide radicals in simple (low viscous) liquids and saline solutions was observed for 2H, 7Li and 13C nuclei at ambient temperature. Substantial NMR signal enhancement up to several hundred was achieved for the studied samples. An advanced approach for reconstructing of the original relaxation dispersion of pure substances is used to eliminate the effect of the additional radical relaxivity of the X-nuclei.

Enhanced Dynamic Nuclear Polarization via Swept Microwave Frequency Combs #DNPNMR

Ajoy, A., R. Nazaryan, K. Liu, X. Lv, B. Safvati, G. Wang, E. Druga, et al. “Enhanced Dynamic Nuclear Polarization via Swept Microwave Frequency Combs.” Proceedings of the National Academy of Sciences 115, no. 42 (October 16, 2018): 10576–81.

Dynamic nuclear polarization (DNP) has enabled enormous gains in magnetic resonance signals and led to vastly accelerated NMR/MRI imaging and spectroscopy. Unlike conventional cw-techniques, DNP methods that exploit the full electron spectrum are appealing since they allow direct participation of all electrons in the hyperpolarization process. Such methods typically entail sweeps of microwave radiation over the broad electron linewidth to excite DNP but are often inefficient because the sweeps, constrained by adiabaticity requirements, are slow. In this paper, we develop a technique to overcome the DNP bottlenecks set by the slow sweeps, using a swept microwave frequency comb that increases the effective number of polarization transfer events while respecting adiabaticity constraints. This allows a multiplicative gain in DNP enhancement, scaling with the number of comb frequencies and limited only by the hyperfine-mediated electron linewidth. We demonstrate the technique for the optical hyperpolarization of 13C nuclei in powdered microdiamonds at low fields, increasing the DNP enhancement from 30 to 100 measured with respect to the thermal signal at 7T. For low concentrations of broad linewidth electron radicals [e.g., TEMPO ((2,2,6,6- tetramethylpiperidin-1-yl)oxyl)], these multiplicative gains could exceed an order of magnitude.

A compact X-Band resonator for DNP-enhanced Fast-Field-Cycling NMR #DNPNMR

Neudert, O., C. Mattea, and S. Stapf, A compact X-Band resonator for DNP-enhanced Fast-Field-Cycling NMR. J Magn Reson, 2016. 271: p. 7-14.

A new probehead was developed enabling Dynamic Nuclear Polarization (DNP)-enhanced Fast-Field-Cycling relaxometry at 340mT polarization field strength. It is based on a dielectric cavity resonator operating in the TM110 mode at 9.5GHz, which is suitable for both transverse and axial magnet geometries with a bore access of at least 20mm. The probehead includes a planar radio frequency coil for NMR detection and is compatible with standard 3mm NMR tubes. The resonator was assessed in terms of the microwave conversion factor and microwave-induced sample heating effects. Due to the compact size of the cavity, appreciable microwave magnetic field strengths were observed even with only moderate quality factors. Exemplary DNP experiments at 9.5GHz and 2.0GHz microwave frequency are compared for three different viscous samples, demonstrating the advantage of DNP at 9.5GHz for such systems. This new probehead enables new applications of DNP-enhanced Fast-Field-Cycling relaxometry of viscous and solid systems.

Low-field thermal mixing in [1-(13)C] pyruvic acid for brute-force hyperpolarization

Peat, D.T., et al., Low-field thermal mixing in [1-(13)C] pyruvic acid for brute-force hyperpolarization. Phys Chem Chem Phys, 2016. 18(28): p. 19173-82.

We detail the process of low-field thermal mixing (LFTM) between (1)H and (13)C nuclei in neat [1-(13)C] pyruvic acid at cryogenic temperatures (4-15 K). Using fast-field-cycling NMR, (1)H nuclei in the molecule were polarized at modest high field (2 T) and then equilibrated with (13)C nuclei by fast cycling ( approximately 300-400 ms) to a low field (0-300 G) that activates thermal mixing. The (13)C NMR spectrum was recorded after fast cycling back to 2 T. The (13)C signal derives from (1)H polarization via LFTM, in which the polarized (‘cold’) proton bath contacts the unpolarised (‘hot’) (13)C bath at a field so low that Zeeman and dipolar interactions are similar-sized and fluctuations in the latter drive (1)H-(13)C equilibration. By varying mixing time (tmix) and field (Bmix), we determined field-dependent rates of polarization transfer (1/tau) and decay (1/T1m) during mixing. This defines conditions for effective mixing, as utilized in ‘brute-force’ hyperpolarization of low-gamma nuclei like (13)C using Boltzmann polarization from nearby protons. For neat pyruvic acid, near-optimum mixing occurs for tmix approximately 100-300 ms and Bmix approximately 30-60 G. Three forms of frozen neat pyruvic acid were tested: two glassy samples, (one well-deoxygenated, the other O2-exposed) and one sample pre-treated by annealing (also well-deoxygenated). Both annealing and the presence of O2 are known to dramatically alter high-field longitudinal relaxation (T1) of (1)H and (13)C (up to 10(2)-10(3)-fold effects). Here, we found smaller, but still critical factors of approximately (2-5)x on both tau and T1m. Annealed, well-deoxygenated samples exhibit the longest time constants, e.g., tau approximately 30-70 ms and T1m approximately 1-20 s, each growing vs. Bmix. Mixing ‘turns off’ for Bmix > approximately 100 G. That T1m>>tau is consistent with earlier success with polarization transfer from (1)H to (13)C by LFTM.

Fast-field-cycling relaxometry enhanced by Dynamic Nuclear Polarization

Neudert, O., et al., Fast-field-cycling relaxometry enhanced by Dynamic Nuclear Polarization. Microporous and Mesoporous Materials, 2015. 205(0): p. 70-74.

Fast-field-cycling (FFC) NMR relaxometry experiments enhanced by in-situ Dynamic Nuclear Polarization (DNP) were performed for 1H and 13C nuclear spins with a setup based on a commercial electronically switching FFC relaxometer and a recently-built Alderman–Grant type microwave resonator for 2 GHz. DNP-enhanced 1H relaxation dispersion profiles were compared to reference measurements and literature data in order to prove the reliability of DNP-enhanced relaxometry data. The method was then used to investigate the paramagnetic nuclear spin relaxation of 13C in a benzene-13C6,D6 solution of nitroxide radicals. Dispersion profiles of good quality were obtained within 2 h of measurement time from a comparatively small sample of 60 μl. As a prospect for future applications, DNP experiments with a high-molecular weight Poly(butadiene-1,4) melt and BDPA radical were carried out at 2 GHz and 9.7 GHz microwave frequency, showing solid effect DNP enhancements. In-situ hyperpolarization by DNP may provide extended possibilities for FFC relaxometry, e.g. by allowing enhanced detection of dilute or insensitive nuclear spins, additional selectivity or faster measurements of small samples.

Solid State Field-Cycling NMR Relaxometry: Instrumental Improvements and New Applications

Fujara, F., D. Kruk, and A.F. Privalov, Solid State Field-Cycling NMR Relaxometry: Instrumental Improvements and New Applications. Prog. NMR. Spec., (0).

The paper reviews recent progress in field cycling (FC) NMR instrumentation and its application to solid state physics. Special emphasis is put on our own work during the last 15 years on instrumentation, theory and applications. As far as instrumentation is concerned we report on our development of two types of electronical FC relaxometers, a mechanical FC relaxometer and a combination of FC and one-dimensional microimaging. Progress has been achieved with respect to several parameters such as the accessible field and temperature range as well as the incorporation of sample spinning. Since an appropriate analysis of FC data requires a careful consideration of relaxation theory, we include a theory section discussing the most relevant aspects of relaxation in solids which are related to prevailing residual dipolar and quadrupolar interactions. The most important limitations of relaxation theory are also discussed. With improved instrumentation and with the help of relaxation theory we get access to interesting new applications such as ionic motion in solid electrolytes, structure determination in molecular crystals, ultraslow polymer dynamics and rotational resonance phenomena.

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