Method for the preparation of a sample comprising highly polarized nuclear spins, comprising at least the following steps: a) bringing a polarization system, which using an electrochemical reaction can be converted into an EPR-active radical form thereof, in contact or close proximity with said sample; b) in situ electrochemically forming an EPR-active radical form of said polarization system; c) applying dynamic nuclear polarization techniques in the presence of a magnetic field by applying electromagnetic irradiation with a frequency adapted to transfer spin polarization from the electrons of the polarization system to the nuclear spins of the sample leading to a highly polarized state thereof. d) in situ electrochemically converting said polarization system, which is paramagnetic and comprises an EPR-active radical into a diamagnetic form without an EPR-active radical.