To test the detection limit of ULF NMR relaxometry, even higher concentrations of CuSO 4 were studied. In water samples containing 1.5 and 2 mM of CuSO 4 , stored in a field of 10 µT, the longitudinal relaxation times T 1 of protons were 0.38 (5) and 0.363 (7) s, respectively. It should be stressed that, despite the fast relaxation, the water-proton peak was clearly visible in the ULF NMR spectrum, allowing relaxation studies. Since the proton relaxation time of the blood is short as well, i.e., on the order of hundreds of milliseconds (see below), this phantom study has shown the feasibility of ULF NMR relaxometry of blood using an atomic magnetometer 3 .