NMR depth-profiling/relaxometry

Nuclear Magnetic Resonance (NMR) is used to determine molecular structures from chemical shift spectra in chemical analysis. The same technology is also applied to produce slice-selective images by diagnostic Magnetic Resonance Imaging (MRI) in hospitals and for civil engineering. At ITMC all NMR applications are accommodated in the Magnetic Resonance Center (MARC). MARC is one of a few laboratories worldwide well equipped for investigating a broad range of topics by NMR and to exploit the full potential of NMR technology. The NMR machines serve high-resolution NMR spectroscopy to analyse the structure and dynamics of molecules in solution and in the solid state. Also, MRI is used to study the structure of objects from soft matter and to monitor the progress, localization and effects of chemical processes in chemical, biological and medical engineering. One integral part of MARC is the division of mobile low-field NMR for non-destructive testing and chemical analyses at the site of interest. One set of devices are mobile low-field NMR sensors of the type NMR-MOUSE^®. This NMR sensor has been developed in the chair of Prof. Blümich and is available in different designs allowing a contact free and non-destructive scanning with a penetration depth of up to 25 mm into the objects depending on the exact kind of the sensor. A magnetic field, generated from a set of four permanent magnets separated with a well-chosen gap between all of them, generates a magnetic stray field parallel to the surface of the sensor. This field forces the atomic spins of all ¹H-atoms in a thin slice (typically 10 to 100 µm thick) the within the sample to orientate in this field. By applying a radio frequency (RF) pulse from a RF coil build in the sensor with a corresponding resonance frequency it is possible to excite those spins with an CPMG echo train and read out the decay envelope of the echo train. By moving the sensor on an automated lift away from the sample object discrete depth profiles with the spatial resolution of the slice thickness can be recorded. This read out contains information on the stratigraphy of the layered object and the material properties of the individual layers in terms of ¹H spin density, ¹H NMR relaxation times T₁ and T₂, and the self-diffusion coefficient in the material. NMR-MOUSE has been applied to a large number of cultural heritage objects like the bones of Charlemagne, Ötzi (the ice man), violins, monuments and buildings in Hercolaneo and Ostia antica, the Mackintosh building in Glasgow, the Aachen Cathedral and many more.