domingo, 25 de julio de 2010

NMR experiments


In ordinary NMR experiments, the material to be investigated, the sample, is placed in a static magnetic field (produced by the main coil, Fig. 1) that polarizes the nuclei (aligns them parallel to the field axis). The larger the field, the bigger the nuclear polarization. Another coil (r.f. coil, Fig. 1), whose axis is perpendicular to that of the main magnet coil, is wound around the sample. The terminals of this sample coil are connected to an amplifier that delivers the magnetic radio frequency (r.f.) tunes created by ingenious designed frequency variations of a synthesizers r.f. signal and the nuclei start their intricate quantum dance.

Fig 1

The NMR experiment:

A current through the main coil (green) generates a strong magnetic field that polarizes the nuclei in the sample material (red). It is surrounded by the r.f. coil (black) that delivers the computer generated r.f. tunes that initiate the nuclear quantum dance. At some point in time, the switch is turned and now the dance is recorded through the voltage it induces, the NMR signal, in the r.f. coil. The signals Fourier transform (FT) shows "lines" for different nuclei in different electronic environments. In our new experiments, the field produced by the main coil is powered by a sudden discharge of a capacitor bank, our power supply.
When the music suddenly stops, the nuclei keep on dancing for a while (~100 μs). From watching their dance through the voltage it induces, the NMR signal, at the same terminals of the sample coil (turn the switch in Fig. 1), we gather important information. Usually from the Fourier transform (FT) of the NMR signal we can identify the nuclei, count them, detect their nuclear neighbors; we can measure the fine details of the electronic structure of the material through the interactions the nuclei have with the surrounding electrons.

As one might expect, the NMR signal from the nuclei is weak. Fortunately, it grows with the strength of the initial nuclear polarization, i.e., the strength of the static field. In addition, our ability to differentiate between the fine details of the nuclear signals, what we call resolution, also grows as the magnetic field is raised. These are the chief reasons why NMR sensitivity and resolution are typically boosted by high field magnets. Ofcourse, the resolution also demands a precise knowledge of the magnets homogeneity over the size of the sample, and stability during the time of the quantum dance. In physics applications, the magnetic field is also used as a tuning parameter (like temperature and pressure). It is particularly interesting when the interaction of the material with the magnetic field leads to dramatic effects like structural or other phase transitions, changes that can easily be detected with NMR.

Up until now, NMR methods have been developed in static magnets, for they provide the easiest environment for basic experiments. In the early days of iron yoke magnets, great effort was put into homogenizing and stabilizing these energy and water (cooling) consuming heavyweights. With the introduction of superconducting magnets, stability and homogeneity were less problematic. Over the last 40 years the maximum fields were increased steadily. However, this steady increase has come to a halt. Currently, there are no superconductors in sight that could be used to raise the field for NMR magnets much above 20 T. This fact has prompted NMR researchers to reconsider the use of resistive magnets. Over the past 10 years, various laboratories have been built worldwide that use Bitter-type magnets (< 33 T), or even hybrids of superconducting (non-persistent) and resistive magnets for NMR up to 45 T. These magnets consume tremendous amounts of energy and cooling water. Any increase in field strength seems unlikely. Can NMR experiments go beyond 45 T? As Pulsed High Magnetic Field Laboratories show, fields in excess of 45 T can be created temporarily with small coils. Is it possible to use such pulsed magnets for NMR? Given the great potential of new NMR methods, why have such experiments not been reported?

As every NMR practitioner knows, finding the inherently weak NMR signal can be challenging at times even in static magnets, since the NMR spectrometer and the probe that holds the sample have to be calibrated. Here, the repetition rate for search and tune-up procedures can be very fast, contrary to pulsed magnets where one has to wait some 40 min. In addition, the magnetic field of pulsed magnets is not precisely known at any instant of time, as required for excitation and observation of NMR The field varies from shot to shot and also during the dance of the nuclei. A very small field noise suffices in quenching the NMR signal. Also, the nuclear polarization build-up is not complete if the nuclear relaxation is not much faster than the rise time of the field pulse. The small size of the magnets coil causes great spatial inhomogeneity that spoils the resolution. Lastly, the beneficial signal averaging which is usually performed in NMR experiments is greatly limited. These and other facts have kept researchers around the world from pursuing NMR in pulsed magnets, and prevented some from reporting positive experiments.

We decided it would be challenging to investigate all these issues more thoroughly by giving the experiment priority. First, we developed a strategy and the necessary hardware to search for NMR in the pulsed field. About one year ago, we were able to record the first Cu NMR spectrum at a moderate 12 T peak field. Despite the low field, this was a hallmark experiment, since it showed that NMR in pulsed magnets is possible, i.e., that one can find the signal in the pulsed field and its properties are such that NMR can be done (e.g., the field is sufficiently smooth in time).
Fig 2

The worlds first 58 T 2H FT NMR signal (this signal was not corrected for the time dependence of the field - work in progress). Although the line is rather broad, we believe that the resolution can be increased substantially by resorting to microcoils.

Our next step aimed at raising the field and increasing the resolution. This brought up new obstacles. For instance, calibrating pulsed fields that are considerably larger than fields available to us with static magnets (< 16 T) required a different strategy. We could no longer test all the equipment in the static field. Also, it became clear that the fields time dependence ultimately hampered resolution. Without going into detail, we will mention two major results: First, we were able to devise experiments that enabled us to calibrate the pulsed magnets at the IFW up to 60 T, and we show the worlds first 58 T NMR spectrum in Fig. 2. Second, in another set of experiments, we could show that the fields time dependence, which alters the dance of the nuclei, could actually be removed from the analysis. This is a very important result as it shows that the inherent time dependence of the pulsed field is not a major obstacle for doing NMR in pulsed magnets. What have we achieved and what do we conclude?

For the first time, NMR was observed in pulsed high field magnets. We set the world record for high-field NMR at 58 T. More specifically, we designed methods and experiments that facilitate NMR in pulsed magnets, e.g., we proved that the time-dependence of the field, a major concern for NMR resolution, can basically be eliminated. This clearly establishes NMR in pulsed high field magnets as a new tool for the structural investigation of materials, especially pertaining to experiments where the extra field is mandatory. Of course, by calibrating the magnets, we also performed the first high-precision diagnostics of pulsed magnets up to 60 T.

NMR in pulsed magnets has many disadvantages over NMR in static fields. Therefore, one might think that NMR in pulsed magnets will be limited to high field applications only. However, we do not think this is true. The simplicity and the small size of pulsed magnets up to about 30 T (no refrigeration at very low temperatures, low initial costs and maintenance costs, sturdiness, transportability, use on demand) along with now available technologies (sub-micrometer manufacturing, micro electronics, highpower electronics) will open up new NMR applications. One day perhaps, we will be able to watch the quantum dance of the nuclei even at home with our very own tabletop device to check the quality of the evening wine.
Nombre: Franklin J. Quintero C.
Asignatura: CRF
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