Category Archives: Solid-state Microwave Source

Efficient 263 GHz magic angle spinning DNP at 100 K using solid-state diode sources #DNPNMR

Sergeyev, Ivan V., Fabien Aussenac, Armin Purea, Christian Reiter, Eric Bryerton, Steven Retzloff, Jeffrey Hesler, Leo Tometich, and Melanie Rosay. “Efficient 263 GHz Magic Angle Spinning DNP at 100 K Using Solid-State Diode Sources.” Solid State Nuclear Magnetic Resonance 100 (August 2019): 63–69.

The development of new, high-frequency solid-state diode sources capable of operating at 263 GHz, together with an optimized stator design for improved millimeter-wave coupling to the NMR sample, have enabled low-power DNP experiments at 263 GHz/400 MHz. With 250 mW output power, signal enhancements as high as 120 are achieved on standard samples – approximately 1/3 of the maximal enhancement available with high-power gyrotrons under similar conditions. Diode-based sources have a number of advantages over vacuum tube devices: they emit a pure mode, can be rapidly frequency-swept over a wide range of frequencies, have reproducible output power over this range, and have excellent output stability. By virtue of their small size, low thermal footprint, and lack of facility requirements, solid-state diodes are also considerably cheaper to operate and maintain than highpower vacuum tube devices. In light of these features, and anticipating further improvements in terms of available output power, solid-state diodes are likely to find widespread use in DNP and contribute to further advances in the field.

Static DNP-NMR Spectroscopy to Characterize Active Pharmaceutical Ingredients #DNPNMR

Dynamic Nuclear Polarization in general is no new method, but the focus of modern applications has initially been on bio-macromolecules under magic-angle-spinning (MAS) conditions.

One application that came out-of-the-blue was using DNP-NMR spectroscopy to study surface materials by DNP-NMR spectroscopy (for example Lafon et al., 2011) opening up a complete new research area within material science that traditionally struggled with very low signal-to-noise (S/N) ratios.

Even the application of DNP-NMR spectroscopy to study small molecules was not immediately evident, but as demonstrated in Rossini et al, 2012 DNP offers the possibility to record 13C correlation spectra of unlabeled molecules such as glucose in just 16 hours. Without DNP this experiment would require months of spectrometer time.

The majority of the DNP-NMR experiments that have been reported in recent years use gyrotron-based DNP-NMR systems and MAS-DNP probes operating at about 100 K. Alternatively, there is a small group of researchers that use DNP systems based on a solid-state microwave source. These systems have are typically limited by their output power, which ranges between >80 mW at 263 GHz (400 MHz 1H NMR) to < 200 mW at 197 GHz (300 MHz 1H NMR). At lower frequencies the output power increases and > 500 mW can be reached for systems operating at 95 GHz. A comprehensive overview of low-power DNP-NMR systems can be found in Siaw et al., 2016.

Because of the limited output power, DNP experiments are performed at temperatures < 20 K, which requires cooling with liquid helium (very common for example in EPR experiments) and can be cost-effective when using a cryostat (e.g. at 10 K the consumption is about 0.5 l/hr). Furthermore, with the increasing popularity of cryogen-free systems some cryostats don’t require any liquid cryogens anymore for cooling. The main advantage is the reduced cost since a solid-state source based DNP-NMR system typically comes at a 10th of the cost of a gyrotron-based system.

At first sight it seems as if the applications of static DNP are very limited. However, when I was at ENC this year I listened to a talk by David A. Hirsh entitled “35Cl Dynamic Nuclear Polarization Solid-State NMR of Active Pharmaceutical Ingredients”. David is a graduate student in the group of Rob Schurko, University of Windsor and gave a very nice talk on using DNP-NMR spectroscopy to characterize Active Pharmaceutical Ingredients (API) using 35Cl solid-state NMR spectroscopy. Since 35Cl is a quadrupole nucleus the corresponding NMR spectra are typically very broad. MAS does only have a small effect, mainly on the center transition, and traditionally wide-line spectra of static solids are recorded.

To overcome sensitivity issues, the group has developed pulse sequences such as WURST-CPMG or BRAIN-CP to rapidly record broad 35Cl patterns even at moderate magnetic field strengths (e.g. 9.4 T, 400 MHz 1H NMR). However, recording a single spectrum often requires several hours of signal averaging to achieve a sufficiently high signal-to-noise (S/N) ratio. With the aid of DNP these acquisition times can be dramatically reduced to just minutes. In his talk at ENC David described using a grytron-based DNP-NMR system, equipped with a MAS-DNP probe head in his experiments. Polarizing the sample is done while the rotor is spinning, but the rotor is stopped prior to recording the wide-line NMR spectrum. 

This experiment seems to be ideally suited for a low-power DNP-NMR system for static solids, using a cryostat for sample cooling. This would greatly simplify the experiment because starting and stopping the rotor is not required anymore. Because the experiment is performed at much lower temperatures, there will be an additional boost in sensitivity and multi-dimensional correlation experiments should be possible, experiments that are close to impossible to perform without the aid of DNP.

In recent years the NMR community has witnessed the transition of DNP-NMR spectroscopy from an exotic method with a limited number of applications to a method with more and more applications. High-field DNP-NMR spectroscopy either based on a gyrotron or using a low-power solid-state source is still a very young method with many possibilities and I’m very excited to see what other applications lie in the future. I am however convinced that DNP-NMR spectroscopy will find their way into many more labs in the future and that the method will become an integral part of the NMR toolbox.

A versatile and modular quasi optics-based 200GHz dual dynamic nuclear polarization and electron paramagnetic resonance instrument

I know it’s still early in the year, but this may be already my most favorite article for 2016.

Siaw, T.A., et al., A versatile and modular quasi optics-based 200GHz dual dynamic nuclear polarization and electron paramagnetic resonance instrument. J Magn Reson, 2016. 264: p. 131-53.

Solid-state dynamic nuclear polarization (DNP) at higher magnetic fields (>3T) and cryogenic temperatures ( approximately 2-90K) has gained enormous interest and seen major technological advances as an NMR signal enhancing technique. Still, the current state of the art DNP operation is not at a state at which sample and freezing conditions can be rationally chosen and the DNP performance predicted a priori, but relies on purely empirical approaches. An important step towards rational optimization of DNP conditions is to have access to DNP instrumental capabilities to diagnose DNP performance and elucidate DNP mechanisms. The desired diagnoses include the measurement of the “DNP power curve”, i.e. the microwave (MW) power dependence of DNP enhancement, the “DNP spectrum”, i.e. the MW frequency dependence of DNP enhancement, the electron paramagnetic resonance (EPR) spectrum, and the saturation and spectral diffusion properties of the EPR spectrum upon prolonged MW irradiation typical of continuous wave (CW) DNP, as well as various electron and nuclear spin relaxation parameters. Even basic measurements of these DNP parameters require versatile instrumentation at high magnetic fields not commercially available to date. In this article, we describe the detailed design of such a DNP instrument, powered by a solid-state MW source that is tunable between 193 and 201GHz and outputs up to 140mW of MW power. The quality and pathway of the transmitted and reflected MWs is controlled by a quasi-optics (QO) bridge and a corrugated waveguide, where the latter couples the MW from an open-space QO bridge to the sample located inside the superconducting magnet and vice versa. Crucially, the versatility of the solid-state MW source enables the automated acquisition of frequency swept DNP spectra, DNP power curves, the diagnosis of MW power and transmission, and frequency swept continuous wave (CW) and pulsed EPR experiments. The flexibility of the DNP instrument centered around the QO MW bridge will provide an efficient means to collect DNP data that is crucial for understanding the relationship between experimental and sample conditions, and the DNP performance. The modularity of this instrumental platform is suitable for future upgrades and extensions to include new experimental capabilities to meet contemporary DNP needs, including the simultaneous operation of two or more MW sources, time domain DNP, electron double resonance measurements, pulsed EPR operation, or simply the implementation of higher power MW amplifiers.

Low-temperature dynamic nuclear polarization with helium-cooled samples and nitrogen-driven magic-angle spinning

Thurber, K. and R. Tycko, Low-temperature dynamic nuclear polarization with helium-cooled samples and nitrogen-driven magic-angle spinning. J Magn Reson, 2016. 264: p. 99-106.

We describe novel instrumentation for low-temperature solid state nuclear magnetic resonance (NMR) with dynamic nuclear polarization (DNP) and magic-angle spinning (MAS), focusing on aspects of this instrumentation that have not been described in detail in previous publications. We characterize the performance of an extended interaction oscillator (EIO) microwave source, operating near 264GHz with 1.5W output power, which we use in conjunction with a quasi-optical microwave polarizing system and a MAS NMR probe that employs liquid helium for sample cooling and nitrogen gas for sample spinning. Enhancement factors for cross-polarized (13)C NMR signals in the 100-200 range are demonstrated with DNP at 25K. The dependences of signal amplitudes on sample temperature, as well as microwave power, polarization, and frequency, are presented. We show that sample temperatures below 30K can be achieved with helium consumption rates below 1.3l/h. To illustrate potential applications of this instrumentation in structural studies of biochemical systems, we compare results from low-temperature DNP experiments on a calmodulin-binding peptide in its free and bound states.

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