Microwave-gated dynamic nuclear polarization.

Dissolution dynamic nuclear polarization (D-DNP) has become a method of choice to enhance signals in nuclear magnetic resonance (NMR). Recently, we have proposed to combine cross-polarization (CP) with D-DNP to provide high polarization P(13C) in short build-up times. In this paper, we show that switching microwave irradiation off for a few hundreds of milliseconds prior to CP can significantly boost the efficiency. By implementing microwave gating, 13C polarizations on sodium [1-13C]acetate as high as 64% could be achieved with a polarization build-up time constant as short as 160 s. A polarization of P(13C) = 78% could even be reached for [13C]urea.