Electron Nuclear Double Resonance experiments require both strong microwave and radio frequency fields at the sample position. A typical DNP setup is optimized for the NMR channel and microwaves are simply sprayed onto the sample in the NMR coil, thereby wasting a significant amount of the incident microwave power. In a standard high field ENDOR probe, the microwave signal channel is optimized using a high Q microwave resonator with an externally mounted NMR coil, resulting in a substantial loss of radio frequency efficiency due to the shielding of the metal resonator. We have developed a new double resonance resonator allowing high efficiency for both microwave and radio frequency irradiation. The design is based on a cylindrical cavity operating in the TEO,, mode. In contrast to the conventional design, the solid body of the microwave cavity has been replaced by a wound flat wire, which forms the NMR coil while maintaining a high quality factor for microwave irradiation (Q10aded - 1000). In DNP experiments using 4-amino TEMPO as polarizing agent, we have shown that the use of this resonator, combined with a low microwave power setup (17 mW) leads to a 'H-signal enhancement of F-=300-400. Previously, this order of magnitude (F-=185) could be obtained only by using our high power microwave gyrotron (1-5 Watt). For ENDOR experiments the RF pulse lengths are reduced by a factor ~! 4 compared to the standard ENDOR setup, which allows for substantial higher sensitivity on low Y-nuclei ENDOR. The performance was demonstrated with 13 C-ENDOR on an iron-cluster protein, where short irradiation time is required due to very short electronic relaxation times of the iron (S=1/2) cluster, as well as with 2 H-ENDOR on a perdeuterated BDPA stable radical.
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