Kiss, Sebastian, Lorenzo Bordonali, Jan G. Korvink, and Neil MacKinnon. “Microscale Hyperpolarization.” In Micro and Nano Scale NMR, by Jens Anders and Jan G. Korvink, 297–351. Advanced Micro and Nanosystems. Weinheim, Germany: Wiley-VCH Verlag GmbH & Co. KGaA, 2018.
Magnetic resonance (MR) is a tremendously powerful technique for obtaining both structural and dynamical information non-invasively and with atomic resolution. The primary limitation of MR is sensitivity, with the detected resonant exchange of energy dependent on population differences on the order of tens of parts per million as dictated by Boltzmann statistics. The MR community has implemented various strategies to overcome this inherent limitation, including maximizing the static polarizing magnetic field and cooling the probe electronics. As discussed throughout this book, an alternative strategy is to miniaturize the MR detector in order to maximize resonant energy exchange efficiency between the sample and the instrument electronics. In this chapter, we discuss approaches that aim to overcome Boltzmann population statistics. These hyperpolarization techniques rely on the transfer of a large polarization source to the target nuclear spin system, or the preparation of pure spin states that are transferred into the target spin system. The archetypal example of the former case is dynamic nuclear polarization (DNP), whereas in the latter case para-hydrogen and optically pumped 3He or 129Xe are examples.