We continually develop home-built NMR equipment to open up new areas for applications. Central goals in this endeavor are the application of strong field gradients to achieve high spatial resolution or to measure slow molecular diffusion and the use of fast field cycling for broadband dynamical studies.

Picture: AG Vogel

Strong field gradients for high spatial resolution

We employ magnetic field gradients to perform spatially resolved studies of structurally and dynamically inhomogeneous samples and develop versatile sample positioning systems. In this way, we achieve a very high 1D resolution of ca. 2 micrometer.

Demonstration of high spatial resolution in NMR experiments with strong static field gradients: The 1H NMR signal amplitude increases rapidly when the excited slice in the sample is moved from the inorganic substrate to a cavity filled with an organic liquid.
Demonstration of high spatial resolution in NMR experiments with strong static field gradients: The 1H NMR signal amplitude increases rapidly when the excited slice in the sample is moved from the inorganic substrate to a cavity filled with an organic liquid.

In regular NMR experiments, the signal and, thus, the information is obtained from the entire volume of the sample. A spatial resolution can be achieved by applying magnetic field gradients. Under such circumstances, the strength of the applied magnetic field and, hence, the Larmor frequency of the nuclear magnetic moments vary across the sample. Therefore, the resonance condition for the transitions between the Zeeman levels, which is a prerequiste for the generation of a signal, is only met in a specific spatial region. This means that various regions can be addressed selectively. As a main application, this effect is used for MRI scans in medical diagnostics. In 2003, the Nobel Prize in Medicine was awarded to P. Lauterbur and P. Mansfield for the development of the MRI method. We employ magnetic field gradients to perform spatially resolved studies of structurally and dynamically inhomogeneous samples.

In the clinical application, suitable pulsed field gradients are applied to record 3D images. Thereby, the spatial resolution of commercial MRI machines is typically in the millimeter range. We apply much higher static field gradients and develop versatile sample positioning systems. In this way, we achieve a very high 1D resolution of ca. 2 micrometer. This allows us to selectively address a particular region of the sample, e.g., to ascertain properties at interfaces. In the framework of a collaborative research center (SFB 1194), we exploit this possibility to study concentration gradients in evaporating droplets, which are of high relevance for many technological processes, including printing.

Selected publications:

  • B. Kresse et al., “One Dimensional Magnetic Resonance Microscopy with Micrometer Resolution in Static Field Gradients,” Journal of Magnetic Resonance 307 (October 2019): 106566, https://doi.org/10.1016/j.jmr.2019.106566.
Picture: AG Vogel

Strong field gradients for diffusion measurements

Compared to their pulsed counterparts, static field gradients can be much stronger so that smaller diffusion coefficients are accessible. We exploit this capability of static field gradient (SFG) experiments to observe slow diffusion in viscous, soft, and solid materials.

The application of a magnetic field gradient results in a spatial variation of the NMR resonance frequency, which, in turn, enables the measurement of molecular diffusion coefficients. Specifically, under such circumstances, the translation motion of a molecule with respect to the field gradient leads to a detectable frequency change. For the frequency encoding, pulsed or static field gradients can be applied. To detect the frequency changes, the results of frequency measurements at two times are correlated.

The magnitude of the used magnetic field gradient determines the dynamic range of NMR diffusometry. Explicitly, the strength of the field gradient determines the position dependence of the frequency and, thus, the minimum translational displacement required for a detectable frequency change. Compared to their pulsed counterparts, static field gradients can be much stronger so that smaller diffusion coefficients are accessible. We exploit this capability of static field gradient (SFG) experiments to observe slow diffusion in viscous, soft, and solid materials.

Selected publications:

  • Max Weigler et al., “Static Field Gradient NMR Studies of Water Diffusion in Mesoporous Silica,” Physical Chemistry Chemical Physics 22, no. 25 (2020): 13989–98, https://doi.org/10.1039/D0CP01290D.
Self-diffusion coefficients of anions and cations in viscous ionic liquids
Self-diffusion coefficients of anions and cations in viscous ionic liquids
Picture: AG Vogel

Fast field cycling for broadband studies of dynamics

In our lab, a home-built field-cycling setup is continually improved to enhance the accessible frequency range. With an in-house active shielding system, it is possible to perform NMR experiments at the currently worldwide lowest Larmor frequency of 3 Hz.

The spin-lattice relaxation time T1 characterizes the time scale of the buildup of nuclear magnetization in an applied magnetic field B0. Here, the term spin-lattice relaxation refers to the fact that energy is transferred from the nuclear spins to the embedding lattice during the relaxation process. The efficiency of this transfer is determined by the molecular dynamics in the sample. Explicitly, T1 depends on the spectral density of the molecular motion at the Larmor frequency. Thus, T1 measurements for continuously varied Larmor frequency allow one to map out the shape of the spectral density.

A variation of the Larmor frequency requires a change of the B0 field strength. This approach fails when employing regular NMR magnets, which have fixed magnetic fields produced by the superconducting coils. However, the applied magnet field can be switched rapidly when using suitable electromagnets. In particular, fast field-cycling relaxometers enable T1 measurements in wide frequency ranges. In our lab, a home-built field-cycling setup is continually improved to enhance the accessible frequency range. To reach very low B0 fields and, thus, very low Larmor frequencies, it is not sufficient to compensate the Earth's magnetic field. Rather, it is also necessary to shield the fluctuating magnetic fields produced by the alternating electric currents in the lab. With an in-house active shielding system, it is possible to perform NMR experiments at the currently worldwide lowest Larmor frequency of 3 Hz.

Some publications

  • B. Kresse et al., “1 H NMR at Larmor Frequencies down to 3 Hz by Means of Field-Cycling Techniques,” Journal of Magnetic Resonance 277 (April 2017): 79–85, https://doi.org/10.1016/j.jmr.2017.02.002.