In high-field NMR, nuclear spins are polarized by the large magnetic field; in zero-field NMR of bulk samples, an external source of nuclear spin polarization is required. We used chemical reaction with para-enriched hydrogen gas for this demonstration; a number of alternative approaches have been shown in the context of ZULF NMR . A closely related analogue to hydrogenative PHIP (including its heterogeneous version ) is signal amplification by reversible exchange (SABRE) , in which the hyperpolarized spin order of para-enriched hydrogen is transferred to other molecules via reversible exchange on a suitable metal-containing catalyst. Another option is to pre-polarize the molecules in an external magnetic field prior to detection. The field of hyperpolarized NMR is growing rapidly, and yet more polarization techniques are being explored for application to ZULF NMR.