Low field (10 mT) pulsed dynamic nuclear polarization

EPR irradiation by a train of inverting pulses has potential advantages ove r continuous-wave EPR irradiation in DNP applications; however, it has prev iously been used only at high field (5 T), This paper presents the design a nd testing of an apparatus for performing pulsed DNP experiments at 10 mT w ith large samples (17 ml). Experimental results using pulsed DNP with an aq ueous solution of a narrow-linewidth Paramagnetic probe are presented. A ma ximum DNP enhancement of about -36 with a train of inverting pulses (width 500 ns, repetition time 4 mu s) was measured. A preliminary comparison show ed that, when the same enhancement value is considered, the pulsed DNP tech nique requires an average power that is about three times higher than that required with the CW irradiation. However, for in vivo DNP applications it is very important to minimize the average power deposited in the sample. Fr om the experimental results reported in this work, when considering the max imum enhancement, the pulsed technique requires only 2% of the average powe r necessary with the CW DNP technique. We believe that this reduction in th e average power can be important for future DNP studies with large biologic al-samples. (C) 1999 Academic Press.