Josef Granwehr
Lecturer, Faculty of Science
Past Research
NMR Remote Detection
Remote detection of NMR and MRI is one approach for sensitivity and resolution enhancement, whereby encoding and detection are temporally and spatially separated. This enables the independent optimization of the encoding environment for contrast and the detector for sensitivity. During encoding, information about a porous sample is transferred onto the magnetization of a fluid, which acts as a ``spin sensor''. This magnetization is read out after relocating the fluid to the detector.
As a postdoctoral researcher at UC Berkeley we developed, in collaboration with colleagues from Schlumberger-Doll Research, the method of time-of-flight (TOF) remote detection to profile fluid flow. Correlating the arrival time of the encoded fluid in the detector with the encoding location, we were able to trace the position of a gas inside a porous rock as a function of time. Due to the low spin density, gas flow in optically intransparent material is notoriously difficult to study. By combining remote detection and the use of hyperpolarized Xe-129 enabled us to visualize and analyze gas flow with good sensitivity. Additionally, by using the chemical shift of water and octane in the detection volume, i.e. outside the sample, we could image their flow individually despite the lack of resolution due to the susceptibility gradients inside the rock itself.
By using the chemical shift of Xe-129 in the encoding volume as a contrast property, we were able to distinguish between gas flow through connected pores and gas occluded in closed pores inside a nanoporous silica aerogel sample. We also used remote detection to obtain images of flow in microfluidic devices, where a microcoil was used for detection, while standard microimaging equipment was used for image encoding.
Dissolution Dynamic Nuclear Polarization (DNP)
Another approach for sensitivity enhancement is to transfer spin polarization from electron to nuclear spins by dynamic nuclear polarization (DNP), facilitating a sensitivity gain of several orders of magnitude. The dissolution DNP technique performs the polarization step at a temperature of about 1 K. Then the polarized sample is dissolved with a hot solvent, and NMR experiments in the liquid state at room temperature can be performed. We have set up, in collaboration with Oxford Instruments, a magnet with two isocenters specifically designed for dissolution DNP experiments. This design minimizes the relaxation loss during the sample shuttling, since the sample is shuttled in the cold state in the presence of a relatively high magnetic field and dissolved immediately above the liquid-state probe. This significantly extends the range of longitudinal relaxation times accessible with dissolution DNP to below 1 s.
Longitudinally Detected (LOD) EPR
I have implemented LOD EPR setups working at a large range of detection frequencies. This allowed me, for example, to perform frequency-swept EPR experiments using a superconducting magnet with a constant field, and to measure transient relaxation curves of the longitudinal spin magnetization for samples with very short transverse relaxation times. Both of these experiments are very difficult or impossible to perform with conventional detection.