Despite the many advantages of ZULF NMR, its main limitation is low sensitivity: at zero field there is negligible spin magnetization, and therefore an external source of spin polarization is required. In early ZULF-NMR experiments, the sample was prepolarized in a magnetic field where spins reach thermal-equilibrium polarization, obeying the Boltzmann distribution of state populations, after which the sample was brought to zero field for detection . The disadvantage of this method is the low polarization levels that can be obtained, on the order of 10 -6 at tesla-level fields.