FFC NMR relaxometry spectrometer (AMU)
FFC NMR relaxometry is a low-field magnetic resonance technique in the range of a few kHz up to around 20 MHz, which allows to study slow molecular motions and collective dynamics, including, the characterization of rotational dynamics, determination of thermal activation energies, identification and distinction between different molecular dynamics e.g. solid/liquid dynamics, fast/slow dynamics, inter-molecular/intra-molecular dynamics, characterization of surface exchange effects with solvents and other small molecules, characterization of porous systems and local diffusion effects, characterization of translational diffusion in bulk and geometrically restricted environments, separation of amorphous and crystalline contributions in solids, and the evaluation of MRI contrast.
The commercially available Fast Field Cycling (FFC) spectrometer (model Stelar - Spinmaster 2000) is designed for the measurements of 1H-NMR spin-lattice (T1) relaxation time in the Larmor frequency range spanning from 8 kHz up to 20 MHz. It is necessary to use two different measurement techniques to study this broad frequency range. For low frequencies (up to 4 MHz) the cycling technique with pre-polarizing field is used, whereas for frequencies in the range 5 - 20 MHz the non-polarizing cycling sequence is employed. The high pre-polarizing field is used in order to increase signal intensity, and is necessary only for low frequencies.
For example the polarization field can be set to 16 MHz (in Larmor frequency units). This high field is applied for time long enough to allow the magnetization to reach the equilibrium value. Then, within just few millisecond, the magnetic field level is electronically switched to a desired lower value (from the range 8 kHz-4 MHz) and the magnetization is allowed to reach the new equilibrium value in the lower field. This process is known as a relaxation and is described by the time constant T1 the so called spin-lattice relaxation time. The amplitude of magnetization is detected in regular intervals and the time constant T1 is evaluated.
As the result of the whole series of measurements for different magnetic field levels one obtains the T1 dispersion profiles - plots of spin-lattice relaxation times as a function of magnetic field intensity (which for convenience is given in Larmor frequency units). Since the values of T1 are influenced by molecular motions the FFC techniques is a very useful tool for studying molecular dynamics.