A spectrometer designed for 6.7 and 14.1 T DNP-enhanced solid-state MAS NMR using quasi-optical microwave transmission

Pike, Kevin J. and Kemp, Thomas F. and Takahashi, Hiroki and Day, Robert and Howes, Andrew P. and Kryukov, Eugeny V. and MacDonald, James F. and Collis, Alana E. C. and Bolton, David R. and Wylde, Richard J. and Orwick, Marcella and Kosuga, Kosuke and Clark, Andrew J. and Idehara, Toshitaka and Watts, Anthony and Smith, Graham M. and Newton, Mark E. and Dupree, Ray and Smith, Mark E. (2012) A spectrometer designed for 6.7 and 14.1 T DNP-enhanced solid-state MAS NMR using quasi-optical microwave transmission. Journal of Magnetic Resonance, 215. pp. 1-9. ISSN 1096-0856

Abstract

A Dynamic Nuclear Polarisation (DNP) enhanced solid-state Magic Angle Spinning (MAS) NMR spectrometer operating at 6.7 T is described and demonstrated. The 187 GHz TE13 fundamental mode of the FU CW VII gyrotron is used as the microwave source for this magnetic field strength and 284 MHz 1H DNP-NMR. The spectrometer is designed for use with microwave frequencies up to 395 GHz (the TE16 second-harmonic mode of the gyrotron) for DNP at 14.1 T (600 MHz 1H NMR). The pulsed microwave output from the gyrotron is converted to a quasi-optical Gaussian beam using a Vlasov antenna and transmitted to the NMR probe via an optical bench, with beam splitters for monitoring and adjusting the microwave power, a ferrite rotator to isolate the gyrotron from the reflected power and a Martin–Puplett interferometer for adjusting the polarisation. The Gaussian beam is reflected by curved mirrors inside the DNP-MAS-NMR probe to be incident at the sample along the MAS rotation axis. The beam is focussed to a ∼1 mm waist at the top of the rotor and then gradually diverges to give much more efficient coupling throughout the sample than designs using direct waveguide irradiation. The probe can be used in triple channel HXY mode for 600 MHz 1H and double channel HX mode for 284 MHz 1H, with MAS sample temperatures ⩾85 K. Initial data at 6.7 T and ∼1 W pulsed microwave power are presented with 13C enhancements of 60 for a frozen urea solution (1H–13C CP), 16 for bacteriorhodopsin in purple membrane (1H–13C CP) and 22 for 15N in a frozen glycine solution (1H–15N CP) being obtained. In comparison with designs which irradiate perpendicular to the rotation axis the approach used here provides a highly efficient use of the incident microwave beam and an NMR-optimised coil design.