Field gradient NMR

Field gradient NMR (often abbreviated as PFG-NMR from English pulsed-field-gradient nuclear magnetic resonance ) is a special technique of nuclear magnetic resonance spectroscopy (NMR spectroscopy).

functionality

While in normal NMR spectroscopy signals are measured as a function of frequency, i.e. energy -resolved, in a homogeneous magnetic field , inhomogeneous magnetic fields are deliberately used in field gradient NMR. ${\ displaystyle B_ {0}}$

This is done by generating linear field gradients of known size using additional magnetic field coils and superimposing them on the main field . The result is a locally varying magnetic field in which the resonance frequency of a core is described by a known function of the position of the core in the magnetic field. ${\ displaystyle B_ {0}}$ ${\ displaystyle B_ {0} (x, y, z)}$ ${\ displaystyle \ omega _ {0}}$ ${\ displaystyle \ omega _ {0} (x, y, z)}$

In this way it is possible to carry out spatially resolved NMR experiments, whereby one can obtain local information such as the position or spatial displacement of "NMR-visible" (mostly hydrogen-containing) particles. This technology is also the basis for spatial resolution in magnetic resonance imaging (MRT) and diffusion tensor imaging .

Maxwell coils are used to generate the gradient in the direction of the main field, so-called Golay coils perpendicular to this .

Depending on whether the magnetic field gradient is applied all the time or is only switched in the form of short pulses, a distinction is made between the method of static field gradients (SFG) and pulsed field gradients (PFG). The PFG method is more complex in terms of equipment, but offers the advantage of significantly higher signal intensities and signal curves that are easier to interpret.

Applications

The PFG-NMR is used, in addition to the imaging method, above all to measure changes in the position of the observed particles during an adjustable and thus defined time, such as B. in flow movements and self-diffusion processes . In a spin-echo experiment, the movement of molecules relative to the locally varying magnetic field is measured via a phase shift (in the case of directed flow movements) or an attenuation (in the case of diffusion processes) of the NMR signal. In contrast to other measurement methods for self-diffusion coefficients , field gradient NMR manages without special tracer substances and can therefore also be used for non-destructive measurements in porous materials. In addition, the observation time over which the diffusion shift is measured can be varied (usually from a few milliseconds to a few seconds). Conclusions about the structure of the system can be drawn from the dependence of the measured diffusion coefficient on the observation time (e.g. by evaluating the anomalous diffusion in a pore system). Thus, the PFG-NMR plays an outstanding role in geological applications and especially in the petroleum industry in the non-invasive determination of pore sizes and shapes in rocks and sediments of different porosity .

While diffusion is incoherent, translational motion of particles, flow motions are coherent. Coherent motions can be studied using the same PFG-NMR methods. So z. B. blood flow measurements, or on-line flow measurements in process engineering systems with the PFG spin echo method can be carried out. The measurement of the coherent movement of electrically charged particles (ions, charged macromolecules) is carried out in the so-called "electrophoretic NMR" (ENMR) , where the migration speeds or mobility of these particles in the electric field can be determined. Here the PFG-NMR experiment is carried out in the presence of an electric current in the sample being examined. It is very interesting that in complex mixtures of different charged particles, due to the selectivity of NMR, the particles of a certain type can be specifically tracked. The electrophoretic NMR can u. a. can be used to study the charge transport through membranes in fuel cells and to measure the electrical charge of certain macromolecules ( polyelectrolytes ).

The technique of field gradient NMR can also be used as a contrast mechanism in magnetic resonance tomography: Diffusion contrasts are of interest , for example, for diagnosing brain damage caused by a stroke .

literature

Malcolm H. Levitt: Spin Dynamics: Basics of Nuclear Magnetic Resonance . 2nd Edition. John Wiley & Sons, New York 2008, ISBN 0-470-51117-6 , pp. 77-78 .
Paul T. Callaghan : Principles of Nuclear Magnetic Resonance Microscopy . Clarendon Press, Oxford 1991, ISBN 0-19-853997-5 , pp. 162–165 ( limited preview in Google Book search).
Siegmar Braun, Hans-Otto Kalinowski, Stefan Berger: 150 and More Basic NMR Experiments. A Practical Course . Wiley-VCH, Weinheim 1998, ISBN 3-527-29512-7 ( limited preview in the Google book search).
Manfred Holz: Contact-free measurement of mass transport by NMR . In: Nachr. Chem. Tech. Lab . tape 34 . VCH, Weinheim 1986, p. 858-864 .

Individual evidence

^ Garrett, Milan Wayne: Thick Cylindrical Coil Systems for Strong Magnetic Fields with Field or Gradient Homogeneities of the 6th to 20th Order . In: Journal of Applied Physics . tape 38 , no. 6 , 1967, p. 2563-2586 , doi : 10.1063 / 1.1709950 .
^ ^A ^b Manfred Holz: Field-Assisted Diffusion Studied by Electrophoretic NMR In: Paul Heitjans , Jörg Kärger (Eds.): Diffusion in Condensed Matter. Methods, Materials, Models. Greatly enlarged and completely revised edition. Springer, Berlin a. a. 2005, ISBN 3-540-20043-6 , pp. 717-742.

[1] Garrett, Milan Wayne: Thick Cylindrical Coil Systems for Strong Magnetic Fields with Field or Gradient Homogeneities of the 6th to 20th Order . In: Journal of Applied Physics . tape 38 , no. 6 , 1967, p. 2563-2586 , doi : 10.1063 / 1.1709950 .

[Holz-2] A ^b Manfred Holz: Field-Assisted Diffusion Studied by Electrophoretic NMR In: Paul Heitjans , Jörg Kärger (Eds.): Diffusion in Condensed Matter. Methods, Materials, Models. Greatly enlarged and completely revised edition. Springer, Berlin a. a. 2005, ISBN 3-540-20043-6 , pp. 717-742.