The longitudinal relaxation rates 1/T 1 , derived from ULF NMR measurements with non-degassed CuSO 4 water solutions (flamesealed without the freeze-pump-thaw procedure), are presented in Fig. . Overall, no significant dependence of the longitudinal relaxation rate on the magnetic-field strength was observed. The extracted T 1 relaxivity of CuSO 4 was 1.0(1) s -1 /mM. On the other hand, measurements with the non-degassed pure water solution (bottom dashed line in Fig. ) and degassed CuSO 4 water solutions (Fig. ) show that the gradual increase of the storage field from 10 µT to 510 µT resulted in a continuous decrease in the longitudinal relaxation rate 1/T 1 . The possible explanation for this difference can be the simultaneous presence of paramagnetic oxygen and CuSO 4 in solutions. This may accelerate the relaxation process resulting in the disappearance of the weak dependence of water-proton relaxation on the storage field in the ULF regime. T 1 relaxivity of CuSO 4 is calculated as 1.10(5) s -1 /mM using 1/T 1 values averaged over different magnetic fields. In both cases, T 1 relaxivity constant of CuSO 4 in water are in good agreement with literature values obtained in high magnetic field 52 .