PHIP (Para-Hydrogen Induced Polarization) is one of the most useful and cost-efficient methods of creating nuclear spin hyperpolarization. Signal enhancement in the PHIP method comes from the non-equilibrium nuclear spin order of the parahydrogen molecule. PHIP does not require expensive equipment and allows amplification of NMR signals by approximately a factor of 104, making it ideal for studying catalytic hydrogenation reactions, observing intermediate states and creating hyperpolarized contrast agents for MRI. An important application of this method is the transfer of PHIP from protons to “insensitive” magnetic heteronuclei, like 13C, 15N, etc.
In this work we study the transfer of PHIP to 13C nuclei at high magnetic fields. To convert PHIP into enhanced magnetization of a heteronucleus, we implement a scheme with resonant RF-excitation at the NMR frequencies of protons and carbon. The RF-field at the proton frequency stays constant, while the field at the carbon frequency is adiabatically decreased from the maximum value to zero in order to pass through anti-crossing of spin levels in the doubly rotating reference frame.
To maximize the efficiency of transfer we study the dependence of the polarization transfer efficiency on the frequency of the 13C RF-field and on the amplitude of proton RF-field. A method is proposed for effective suppression of signals coming from thermally polarized nuclei. The transfer scheme is optimized using “constant adiabacity” passage upon variation of the 13C RF-field. A comprehensive optimization of the scheme allowed us to achieve 41,000-fold signal enhancement, which corresponds to 33.4% of 13C polarization.
Implementation of the scheme with double-frequency RF-excitation makes it possible to efficiently transfer PHIP to the heteronuclei, suppressing unwanted background signals. The conversion of PHIP into the magnetization of heteronuclei will possibly contribute to chemical and biological NMR studies exploiting spin hyperpolarization.