Chapter 21: Fast Field-cycling NMR Experiments with Hyperpolarized Spins

This chapter gives an overview of field-cycling applications for the control of nuclear spin order in hyperpolarized spin ensembles. Such methods are used for strong signal enhancement in NMR spectroscopy, for the preservation of non-thermal order and for the analysis of the polarization process. We focus on techniques that are operative in the condensed phase and exploit processes based on light irradiation. Such techniques include optical nuclear polarization (ONP), chemically induced dynamic nuclear polarization (CIDNP) and optical pumping (OP). Alternatively, we start from the singlet spin order of para-hydrogen, the singlet spin isomer of the H₂ molecule. This approach is known as para-hydrogen-induced polarization (PHIP) and in a slightly different version as signal amplification by reversible exchange (SABRE). Since in all methods polarization is transferred from primarily polarized spins to target spins, we also review existing methods for polarization transfer. After describing the theoretical framework, we present examples of instrumentation for field cycling in the range between 5 nT and 9.4 T, and subsequently discuss characteristic applications. Advantages and challenges of field cycling in spin hyperpolarization are discussed and perspectives in this field are addressed.