How to Upgrade Your NMR Spectrometers for DNP
You will need four components to upgrade your NMR spectrometer to conduct dynamic nuclear polarization (DNP-NMR):
- High-power, high-frequency terahertz source
- Low-loss transmission line
- NMR probe that allows simultaneous terahertz and radio-frequency irradiation
- Polarizing agent as the source of the high thermal electron polarization
The efficiency of a DNP experiment depends strongly on the magnetic field (B1) at the sample, which is induced by the terahertz irradiation. The strength of this field is proportional to the square root of the product of the applied power and the quality factor Q (B1~sqrt(P*Q)). Since in many cases DNP experiments are performed without a resonant structure (cavity) the quality factor Q is small (<5) and to create sufficiently strong B1 fields high-power terahertz sources such as gyrotrons are required.
Gyrotron oscillators and amplifiers are a vacuum electronic devices (VED) that operate in a static magnetic field. They are fastwave devices that rely on a resonance phenomenon between the modes of an interaction structure and the electron beam in the magnetic field. The interaction structure (resonator) can be overmoded and has therefore physical dimension much larger than the operating wavelength. This permits high power operation even at high frequencies without the risk of damage to the interaction structure, guaranteeing a long lifetime of the device.
Low-loss microwave transmission lines are required to efficiently deliver the terahertz radiation from the source t(e.g. gyrotron) o the sample. Unfortunately, fundamental waveguides are very inefficient at high frequencies. For example at 140 GHz a fundamental rectangular waveguide (WR-08) has 8 dB loss per meter. To minimize losses, overmoded waveguides such as a WR-42 (K-band, 20-40 GHz) can be used. Near the source and the NMR coil the overmoded waveguide is then tapered down to the fundamental mode. Using this approach losses can be minimized to a few decibels.
To ensure minimal transmission losses a corrugated waveguide should be used. Operating in the circular HE11 mode, a corrugated waveguide has almost negligible Ohmic losses, which dramatically increases its efficiency. No additional mode converters are needed, since the gyrotron can be designed to deliver this mode. Since the HE11 mode couples very efficiently to a free-space gaussian beam mode no additional mode converters are necessary and the sample can be directly illuminated.
For DNP-enhanced solid-state NMR experiments a low-temperature, magic-angle-spinning (MAS) NMR probe is required. Cryogenic temperatures are achieved by either using cold nitrogen as the bearing and turbine gas or by using a separated variable-temperature line for sample cooling. Temperatures down to 85 K can be reached using this approach. If lower temperatures are necessary cold He gas, as blow-off gas directly from a liquid He dewar can be used for cooling. To maintain the possibility to tune the rf circuit even at low temperatures a transmission line circuit can be used. Here all variable tuning elements are located outside the probe at room temperature. The terahertz radiation can be introduce either along the rotor axis or perpendicular. In the later case, typically more of the sample is exposed to the terahertz radiation due to sample rotation, increasing the efficiency of the DNP process. Here the radiation is launched through the turns of the coil. A sample eject system can be added to the probe to conveniently change the samples.
Dynamic nuclear polarization (DNP) requires a paramagnetic polarizing agent that present in the sample as the source of the high polarization. This paramagnetic molecule can be either an endogenous radical or an exogenous radical that is added to the sample. For high-field DNP experiments in which 1H nuclei are polarized typically biradicals show a better performance than their monomeric derivatives. For biological applications of DNP, the polarizing agent should be soluble in aqueous media (e.g. glycerol/water). A glycerol/water mixture has the additional advantage that it acts as a cryoprotectant of the sample.