Boosting NMR sensitivity

Poor sensitivity is one of the drawbacks of nuclear magnetic resonance spectroscopy. A method called dynamic nuclear polarization (DNP) can improve the situation, particularly for solid-state NMR, by transferring spin polarization from unpaired electrons on paramagnetic agents to the nuclei of the molecules being analyzed. This aligns the nuclei and thus boosts the NMR signal. But the electron spins are such strong magnets that they interfere with the magnetization they just induced on the nuclei, causing the boosted signal to decay quickly and broadening the peaks in the resulting spectra.

Alexander B. Barnes and coworkers at Washington University in St. Louis counteract the drawbacks of the electron spins but retain the sensitivity boosts from DNP by decoupling the electron spins from the nuclear spins (J. Am. Chem. Soc. 2017, DOI: 10.1021/jacs.7b02714).

To achieve that decoupling, Barnes and coworkers built a new device called a gyrotron that can rapidly switch between microwave frequencies.

“We first transfer the sensitivity from the electron spins to the NMR spins with DNP,” Barnes says. “Then we change the microwave frequency so we’re continually rotating the electron spins very fast. We’re averaging out the effect of the electron spins.”

The researchers tested their new strategy by analyzing ¹³C-labeled urea immobilized in a glassy matrix. For ¹³C spins, the combination of DNP and electron decoupling increased the signal intensity by 14% and decreased the spectral peak widths by 11% relative to DNP alone.

Barnes predicts the method will be useful for studying drug binding to proteins and for characterizing surfaces in materials science.