Detection at ultralow field field applied along the y-axis. Following this, a zero-field mixing step can be introduced to transfer polarization from Ŝy to Ky . After the ZF mixing stage, a DC magnetic pulse is applied along the z-axis, with a pulse duration t p . By adjusting t p to satisfy t p B p γ 15N = π/2 or t p B p γ 1H = π/2, the sequence can be used to generate NMR signal for either N or 1 H spins.