As the next step, we investigated the influence of paramagnetic oxygen on heteronuclear long-lived spin singlet states, which are protected from intramolecular dipolar relaxation between the two spins . In high-field NMR, this state has been demonstrated for homonuclear spin-1/2 pairs (e.g., parahydrogen), where two coupled spins are magnetically equivalent, and hence can constitute a spin-0 (singlet) and a spin-1 (triplet) systems. The triplet states are symmetric, and the singlet state is anti-symmetric with respect to particle interchange. Since the main relaxation mechanism, originating from the dipolar coupling, is invariant upon exchange of two spins, it is unlikely to cause a singlet-triplet breaking-symmetry transition; thus, the dipole-dipole coupling cannot contribute to relaxation of the singlet state . At a zero magnetic field, coupled heteronuclear spin-1/2 pairs can form singlet states as well . Specifically, zero-field heteronuclear singlet states have been previously reported in C-formic acid and 13 C 1 -benzene with tens of seconds of lifetimes , which are significantly longer than the longitudinal relaxation times T 1 . C-formic acid is one of the standard samples for ZULF NMR due to its long relaxation time and simple molecular structure. The zero-field spectrum of C-formic acid consists of a single peak at about 222.2 Hz, which is the strength of the one-bond 1 H- C coupling, while the hydroxyl-group proton coupling is negligible due to a fast proton exchange. As discussed in detail in Ref. , after thermal prepolarization with the permanent magnet, the sample is transferred to the ULF region, where it is stored in the guiding (storage) field for a given time. Next, the field is suddenly switched off (≈10 µs) to provide a non-adiabatic transfer to the zero field where singlet-triplet spin system occurs and hence generate coherences between the |T 0 ⟩ (total quantum number F = 1, magnetic quantum number M F = 0) and |S 0 ⟩ (F = 0, M F = 0) states (see Ref for a detailed explanation). As described above, the measurements of the width of the ZULF NMR resonances provide access to the transverse relaxation time T 2 , while the dependence of peak amplitudes on the storage time gives information about the longitudinal relaxation time T 1 . In the considered case, the longitudinal relaxation is determined by the so-called slow and fast relaxation, characterized by the relaxation times T s and T f , respectively . The fast decay is caused by the equalization of the population between three triplet magnetic sublevels (|T -1 ⟩ (F = 1, M F = -1), |T 0 ⟩, and |T +1 ⟩ (F = 1, M F = +1), and the slow relaxation is associated with thermalization among the singlet and triplet states. Thereby, the bi-exponential fit to the amplitude data provides information about both relaxation times. Alternatively, relaxation may be investigated by sudden switching off of the leading field and successive application of the pulse of a magnetic field oriented along the y-axis while the magnetometer is sensitive along the z-axis (see details in the Methods section). In this case, the amplitude of the signal is proportional to the population difference between the |T +1 ⟩ and |T -1 ⟩ states . In turn, signal decay is only determined by the ) s. For the case of N-MP dissolved in H 2 O, the line is a bi-exponential decay with T 1 times of 4.8(5) s and 1.24(3), for N-MP and water protons, respectively. lifetime of the triplet states, and hence the relaxation time T 1 can be determined from a single exponent fitting to the amplitude decay (see Methods).