Field Cycling NMR is a special technique in NMR research. It requires not only solid knowledge but also the supports of instruments. Field-dependent NMR research has been developed just a few years after the first NMR experiments by Prof. E. M. Purcell at Harvard University and Prof. F. Block at Standford University in 1946. The demand for field-dependent NMR research had happened quite early.
To carry out field-dependent NMR measurement, two manners to alter magnetic fields were invented. One is to move the sample between two different strength of magnetic environments; the other is to switch the magnetic field directly by electric power. Prof. E. Hahn was the first to use pneumatic systems, Pound and Purcell were probably the first to use capacitor storage techniques when Hebel and Slichter may be the first to use an air-cored magnet combined with batteries and relays. In Europe, the first electrically field-cycling NMR instrument was developed by Prof. R. Kimmich in Prof. Noack's lab at the University of Ulm. In our issue 3, Prof. Kimmich shared his story in the early days of field-cycling NMR development.
When time went to the end of 1959, relaxation times of the studied sample could be measured at the minus-hour range. Such field-dependent relaxation study officially was named "relaxometry". The importance of field-dependent NMR research started to be recognized. In Europe, Stelar funded a project to develop a commercial Fast Field Cycling (FFC) instrument for NMR relaxometry studies in 1994.
Magnetic-field switching can be done majorly in two ways, one is by fast electronic power switching, and by mechanically moving the sample. The electronic switch is that the sample exposes to a Zeeman field B0 generated by a tunable power supply in a B0 magnet system. In this way, the sample experienced a switchable magnetic environment. As for the mechanical switch, the sample is shuttled between two different magnetic fields. Therefore, the crucial technique for the mechanical manner is the traveling time of the sample between two different magnets. Its driving force could be compressed air or a motor. No matter which manner, they both need to fight against the switching time, Δt in the figure.
One of the main challenges of the electronic switching manner is to generate a homogeneous and stable magnetic field. Since the magnetic field is generated by electric power, the magnetic field could be impacted by the instability of the current and the heat generated by the current. Thus, the cooling system of the electronic magnet was a big challenge during instrument development. Nowadays, the instrument is commercially available and widely applicated in material science. Stelar has developed several commercial FFC relaxometers for different applications. The applicated field range is from µ Tesla to 2~3 Tesla. (the detail of product specifications please go to Stelar).
Meantime, the mechanical switching manner has been widely developed in different labs. The direction of the mechanical manner is to utilize the stray field of the superconducting magnet of a high-field NMR spectrometer. In Berlin, Prof. Vieth had constructed a motor-driving system to move the probe head to the stray field of 300MHz NMR spectrometer for reaching the measurement at different magnetic fields. The stray field was used in the bottom of magnet dewar. Since the probe can be shuttled together with the sample, the applications have stepped into diffusion NMR studies.
The sample shuttling to the upper area of the stray field in the NMR spectrometer had been built by Prof. Redfield at University Brandeis. He has built a pneumatic control system to rapidly shuttle the sample in a 500MHz Varian spectrometer. The studies from quadruple relaxation to dynamics of macromolecules. After constructing the pneumatic control system, he realized the instability of force from air. It brought a serious impact on the commercial probeheads. If one would like to have higher resolution spectra, the shuttling mechanism needs to be more stable. Thus, he further built the mechanical control system by using a stepper motor to move the sample in the stray field of the NMR spectrometer. This instrument has been conducted in the research of membrane dynamics and protein dynamics by collaborating with Prof. Roberts and Prof. Kern. The time of this collaboration has been shared with us in issue 1 by Prof. Roberts.
The crucial feature of mechanical shuttling, compared to the FFC, is the applicable field range. Since it has utilized the stray field of high-field NMR spectrometer, the measurable magnetic field is up to tens Tesla range. Meanwhile, this manner is profited for having high-resolution spectra for the installation of high-field NMR. While the instrument development in the University of Brandeis, in Taiwan Prof. Huang's lab, a shuttling system has been developed, meanwhile, for reaching higher stability.
This shuttling system, named "high-field field-cycler" has been installed on several high-field NMR spectrometers, some are equipped with cryo-probe systems. The driving force of shuttling the sample is from the motor power. Because of its unique mechanical design, it can reach high stability during rapid shuttling. Because of its stability, the sensitivity increased by equipped with cryo-probe has been preserved by the usage of the high-field field-cycler. The related research has been published in J Biomol NMR 66, 187–194 (2016). For the detial of this system, please go to this page, HFFC. Hence, for applications that demand high-resolution and high-sensitivity spectra, this high-field field-cycler is a valuable solution. This system is now not only developed inside in personal labs but also currently available here.