Abstract
A Dynamic Nuclear Polarisation (DNP) enhanced solid-state Magic Angle Spinning (MAS) NMR spectrometer which uses a 187GHz (corresponding to 1H NMR frequency of 284MHz) Extended Interaction Klystron (EIK) amplifier as the microwave source is briefly described. Its performance is demonstrated for a biomolecule (bacteriorhodopsin), a pharmaceutical, and surface functionalised silica. The EIK is very compact and easily incorporated into an existing spectrometer. The bandwidth of the amplifier is sufficient that it obviates the need for a sweepable magnetic field, once set, for all commonly used radicals.The variable power (CW or pulsed) output from the EIK is transmitted to the DNP-NMR probe using a quasi-optic system with a high power isolator and a corrugated waveguide which feeds the microwaves into the DNP-NMR probe. Curved mirrors inside the probe project the microwaves down the axis of the MAS rotor, giving a very efficient system such that maximum DNP enhancement is achieved with less than 3W output from the microwave source. The DNP-NMR probe operates with a sample temperature down to 90K whilst spinning at 8kHz. Significant enhancements, in excess of 100 for bacteriorhodopsin in purple membrane (bR in PM), are shown along with spectra which are enhanced by ≈25 with respect to room temperature, for both the pharmaceutical furosemide and surface functionalised silica. These enhancements allow hitherto prohibitively time consuming experiments to be undertaken. The power at which the DNP enhancement in bR in PM saturates does not change significantly between 90K and 170K even though the enhancement drops by a factor of ≈11. As the DNP build up time decreases by a factor 3 over this temperature range, the reduction in T1n is presumably a significant contribution to the drop in enhancement.
Highlights
Magic Angle Spinning (MAS) NMR is widely used for obtaining structural and dynamic information about solids, its use is often limited by sensitivity both because of the small nuclear magnetic moment and the fact that many experiments involve isotopes of low natural abundance
The subsequent development of commercial MAS Dynamic Nuclear Polarisation (DNP) NMR spectrometers based on gyrotron sources [3] has led to rapid developments in both applications and theoretical understanding
We describe a MAS DNP spectrometer which uses a 187 GHz Extended Interaction Klystron (EIK) amplifier driven by a solid state multiplier chain as the microwave source
Summary
Magic Angle Spinning (MAS) NMR is widely used for obtaining structural and dynamic information about solids, its use is often limited by sensitivity both because of the small nuclear magnetic moment and the fact that many experiments involve isotopes of low natural abundance. Most MAS DNP NMR experiments use nitroxide based radicals at between 100 and 110 K with a gyrotron microwave source. Whilst these sources perform well they require an additional superconducting magnet which must be located some distance from the NMR magnet so requiring significant extra space. We describe a MAS DNP spectrometer which uses a 187 GHz Extended Interaction Klystron (EIK) amplifier driven by a solid state multiplier chain as the microwave source. This corresponds to a 1H NMR frequency of 283.7 MHz to suit our existing probes. The performance of the spectrometer is demonstrated for a biomolecular system, a pharmaceutical and a surface functionalised material
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