Dipolar cross-relaxation modulates signal amplitudes in the ¹H NMR spectrum of hyperpolarized [¹³C]formate

The asymmetry in the doublet of a spin coupled to hyperpolarized ¹³C has been used previously to measure the initial polarization of ¹³C. We tested the hypothesis that a single observation of the ¹H NMR spectrum of hyperpolarized ¹³C formate monitors ¹³C polarization. Depending on the microwave frequency during the polarization process, in-phase or out-of-phase doublets were observed in the ¹H NMR spectrum. Even in this simple two-spin system, ¹³C polarization was not reflected in the relative area of the J_CH doublet components due to strong heteronuclear cross-relaxation. The Solomon equations were used to model the proton signal as a function of time after polarization and to estimate ¹³C polarization from the ¹H NMR spectra.