NMR Field Cycling Relaxometry
NMR field cycling relaxometry is a versatile tool to monitor slow dynamics in many different materials. The main difference compared to standard NMR equipment is a fast switchable magnetic field. This page gives a short introduction of the field cycling relaxometry method. For information on exemplary applications, please refer to the applications section.
In NMR, nuclear magnetic moments are used to measure specific physical and chemical properties of materials. For instance, nuclear resonance frequencies depend on the molecular environment. Furthermore, nuclear spin-lattice relaxation rates depend on local molecular mobility.
Well known applications are high resolution NMR in liquids and NMR imaging. The former method is capable to resolve the structure of molecules with a high accuracy by detecting tiny frequency shifts caused by chemical bonds. The latter delivers spatially resolved information like maps of the human body or of material samples.
Relaxation of the nuclear spin system is crucial for all NMR applications. The relaxation rate depends strongly on the mobility (fluctuations, diffusion) of the microscopic environment and the strength of the applied magnetic field. As a role of thumb, strong magnetic fields lead to increased sensitivity on fast dynamics while low fields lead to increased sensitivity on slow dynamics. Thus, the relaxation rate as a function of the magnetic field strength is a fingerprint of the microscopic dynamics.
Figure 1: A typical field cycle as function of time. The blue curve indicates the magnetic field,
which is controlled by the relaxometer, the green curve the longitudinal magnetization parallel
to the magnetic field, and the red curve the free induction decay, which is measured by the relaxometer.
Standard NMR equipment is designed to operate at moderate and high static magnetic fields, ranging from 0.5 T up to more than 20 T. However, such equipment is not able to detect relaxation rates at fields in the µT and mT range due to insufficient sensitivity. Hence the strong effect of slow molecular dynamics on the relaxation process at these low magnetic fields cannot be detected by these fixed frequency spectrometers.
A solution of this problem is provided by NMR field cycling relaxometry. This method utilizes a field cycle, that consist of three periods with different magnetic fields as shown in the figure 1:
The variable magnetic field is provided by a resistive electromagnet. Fast current switching, stable and homogeneous magnetic fields, high maximum and low minimum field strengths are key factors for the performance of NMR field cycling relaxometers.
Figure 2 shows a typical result of an NMR field cycling relaxation measurement.
Figure 2: Comparision of the T1 dispersion (rate) of two samples taken from o-ring seals of different age.
Note the difference at low frequencies due to deterioration of the aged material.
The discrepancy at low frequencies shows rubber degradation. The aged rubber sample has hardened and become more brittle. The microscopic dynamics (relaxation rate) is a clear fingerprint of the macroscopic properties.