Pushing NMR sensitivity limits using dynamic nuclear polarization with closed-loop cryogenic helium sample spinning

Abstract

The cooler the better. We report a strategy to push the limits of solid-state NMR sensitivity far beyond its current state-of-the-art.

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

“…We currently do not have the necessary infrastructure for helium recycling. Other laboratories are developing low-temperature MAS NMR systems that incorporate helium recycling [41–44]. …”

Ultra-Low Temperature Nuclear Magnetic Resonance

“…The work from MIT [12] and Osaka [16] showed that reducing the sample temperature tends to increase the DNP-enhancement, as expected. We investigate further this effect at high field (9.4 T) and high-power μw irradiation by combining NUMOC with a Bruker Biospin ULT-MAS-DNP prototype probe and the commercial DNP-NMR spectrometer in Grenoble [13,31]. Using the nitroxide biradical AMUPol [19], one of the current best-performing polarizing agents, DNPenhancements 1Hεon/off ~300 are commonly achieved for model systems under standard MAS-DNP conditions, i.e.…”

“…The lowest sample temperature for "safe" spinning conditions is currently ~30 K. This regime is illustrated by the DNP enhanced NMR spectrum recorded at MAS frequencies of 13, 17.5 and 25 kHz for sample temperatures of 36, 50 and 90 K (see spinning rate curve in Fig. 3), that we recently reported [13,31]. It has to be reminded here that the maximum possible MAS frequency with N2 gas at the minimum possible sample temperature of 100 K is only 15 kHz using the same probe.…”

“…Very special thanks to my colleague Daniel Lee that I was inspired by his writing through papers [13] & [31]. …”

“…Extending this technique to the larger magnetic fields required for high-resolution solid state ssNMR studies occurred with the introduction of higher frequency and high-power continuous-wave microwave (μw) sources [8] and their combination with hardware providing low temperature samples under MAS [12], both facilitating saturation of the electron spin resonance and accordingly improving MAS-DNP efficiencies. Presently, commercial equipment supplying a full experimental setup of MAS-DNP is available at magnetic field strengths of 9, 14, and 19 T, with sample temperatures down to ~100 K. We [13,31], as well as others [12,[14][15][16], are working to demonstrate the further improvement in sensitivity and resolution by performing MAS-DNP at even lower temperatures. It implies the use of helium gas to cool down the spinning sample and technical solutions are far from trivial.…”