Fig. 2.

Concept diagram for dissolution dynamic nuclear polarization (d-DNP) and parahydrogen-induced polarization (PHIP) polarization techniques. (a) In d-DNP, the source of ¹³C nuclear polarization (P) is the approximately unity electron polarization (P) at low temperature and high magnetic field (curves plotted for 3.35T) (i). This is transferred to ¹³C via microwave excitation (ii), predominantly mediated via the thermal mixing effect. (iii) Prototype commercial cryogen-free d-DNP system reported in Ardenkjaer-Larsen et al.⁴⁰ (original photo courtesy of Jan Henrik Ardenkjaer-Larsen, Technical University of Denmark and GE Healthcare). (b) In PHIP, the source of ¹³C polarization is the inherent spin order of the parahydrogen spin isomer of hydrogen, which can be generated to very high purity by cooling normal hydrogen in the presence of a paramagnetic catalyst (i). Parahydrogen is reacted with an unsaturated substrate, generating ¹H hyperpolarization, which is subsequently transferred to ¹³C or other target heteronucleus (ii). Several dedicated low-field (mT) polarization systems have been designed for automating the hydrogenation and polarization transfer processes; the example shown is reprinted with permission from Springer Nature (Hövener et al).⁵⁸