Wednesday, October 3, 2012

Techniques in DNP-NMR Spectroscopy

Check out my second blog post that I wrote for the SpinSights blog. It gives a short overview about the different techniques of DNP-NMR spectroscopy.

Here is a quick summary:

  • DNP can be separated into four different areas based on whether DNP is performed at high or low magnetic field strengths (1,2 vs. 3,4) or in the solid or solution state (1,3 vs. 2,4). In principle it is always possible to directly polarize a sample, but some (direct) polarizing methods are more challenging than others. E.g. at low magnetic fields (cases 3 and 4) a solid-state source can be used, but the output power of such solid-sate microwave/THz sources drops off drastically at higher frequencies. In these cases a gyrotron can be used (cases 1 and 2).
  • For solution-state NMR the situation is more challenging due to the high ohmic losses of the liquid NMR sample and severe heating that occurs (think microwave oven). To prevent this, a resonance structure (resonator) is required to separate the electric field components of the THz radiation from the magnetic field components. This becomes very challenging since at high frequencies (e.g. 400 MHz 1H, 268 GHz e-) the wavelength of the THz radiation is 1.12 mm and therefore fundamental mode devices are very small and can accommodate only very small amounts of liquid sample. This has led to the development of several other techniques:
    • Temperature-Jump DNP (A): In a TJ-DNP experiment the sample (10 – 20 microliters) is polarized in-situ at 90 K using a gyrotron as the THz source. Once the sample is polarized, the sample is rapidly melted using a laser flash, and the NMR experiment can be performed. After the laser is turned off, the sample freezes and can be polarized again.
    • Dissolution-DNP (B): In a dissolution experiment the sample is polarized ex-situ in the solid-state at liquid Helium temperatures at 3.3T using a low-power solid-state 95 GHz microwave source. The sample is then melted using a hot solvent and rapidly transferred into the high-field NMR spectrometer for detection.
    • Shuttle DNP9-10 (arrow C): In a shuttle-DNP experiment, the sample is polarized at a low magnetic field and then physically shuttled into the high-field NMR magnet to acquire the NMR spectrum.