Sensitivity analysis of magic angle spinning dynamic nuclear polarization below 6 K #DNPNMR

Judge, Patrick T., Erika L. Sesti, Edward P. Saliba, Nicholas Alaniva, Thomas Halbritter, Snorri Th. Sigurdsson, and Alexander B. Barnes. “Sensitivity Analysis of Magic Angle Spinning Dynamic Nuclear Polarization below 6 K.” Journal of Magnetic Resonance 305 (August 2019): 51–57.

Dynamic nuclear polarization (DNP) improves signal-to-noise in nuclear magnetic resonance (NMR) spectroscopy. Signal-to-noise in NMR can be further improved with cryogenic sample cooling. Whereas MAS DNP is commonly performed between 25 and 110 K, sample temperatures below 6 K lead to further improvements in sensitivity. Here, we demonstrate that solid effect MAS DNP experiments at 6 K, using trityl, yield 3.2Â more sensitivity compared to 90 K. Trityl with solid effect DNP at 6 K yields substantially more signal to noise than biradicals and cross effect DNP. We also characterize cross effect DNP with AMUPol and TEMTriPol-1 biradicals for DNP magic angle spinning at temperatures below 6 K and 7 Tesla. DNP enhancements determined from microwave on/off intensities are 253 from AMUPol and 49 from TEMTriPol-1. The higher thermal Boltzmann polarization at 6 K compared to 298 K, combined with these enhancements, should result in 10,000Â signal gain for AMUPol and 2000Â gain for TEMTriPol-1. However, we show that AMUPol reduces signal in the absence of microwaves by 90% compared to 41% by TEMTriPol-1 at 7 Tesla as the result of depolarization and other detrimental paramagnetic effects. AMUPol still yields the highest signal-to-noise improvement per unit time between the cross effect radicals due to faster polarization buildup (T1DNP = 4.3 s and 36 s for AMUPol and TEMTriPol-1, respectively). Overall, AMUPol results in 2.5Â better sensitivity compared to TEMTriPol-1 in MAS DNP experiments performed below 6 K at 7 T. Trityl provides 6.0Â more sensitivity than TEMTriPol-1 and 1.9Â more than AMUPol at 6 K, thus yielding the greatest signal-to-noise per unit time among all three radicals. A DNP enhancement profile of TEMTriPol-1 recorded with a frequency-tunable custom-built gyrotron oscillator operating at 198 GHz is also included. It is determined that at 7 T below 6 K a microwave power level of 0.6 W incident on the sample is sufficient to saturate the cross effect mechanism using TEMTriPol-1, yet increasing the power level up to 5 W results in higher improvements in DNP sensitivity with AMUPol. These results indicate MAS DNP below 6 K will play a prominent role in ultra-sensitive NMR spectroscopy in the future.

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