Changing the magnetic field

The basic instrument for performing routine NMRD experiments must contain (1) a variable field magnet (usually an electromagnet) (2) a frequency synthesizer, 

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Figure 4.1 Schematic diagram showing the thermodynamics of the demagnetization process. By changing the magnetic field from the initial value Hl to the final one Hf, with Hf < Hj a gain in magnetic entropy, ASm, occurs during the isothermal process from...

Heinrich Lenz studied the direction of the current that is induced in a conductor as a result of changing the magnetic field near it. You can think of this as the change in a system in electromagnetic equilibrium. Lenz published his law in 1834. It states that when a conductor interacts with a magnetic field, there must be an induced current that opposes the interaction, because of the law of conservation of energy. Lenz s law is used to explain the direction of the induced current in generators, transformers, inductors, and many other systems. 

Most spectroscopic techniques (e.g. infrared and Raman spectroscopy) provide a snapshot view of the structure of a liquid because the timescale of the techniques is of the order of lattice vibration. However, NMR can probe much lower frequency motions, motions which are important in the glass transition and the viscosity of a silicate liquid. In addition, the timescale of the NMR experiment may be varied (by changing the magnetic field, or the type of experiment, T or T fJ, or observing quadrupolar effects) from a few hertz to several hundred megahertz. 

Why would you need to move the spectral window upfield or downfield The lock system changes the magnetic field strength of the spectrometer (B0) slightly to center the 2H frequency of the solvent at the null point of the lock feedback circuit. Changing the field changes all of the resonant frequencies of the spectrum by the same amount, effectively moving the whole spectrum upfield or downfield by as much as 5 ppm when you... 

Sato and co-workers presented the first example of the ability to optically change the magnetic field in a single crystal. The magnetic field is a macroscopic property, but the principle behind the switching mechanism is photoinduced electron transfer within the crystalline lattice. Sato and co-workers employed a Prussian Blue complex with the stoichiometry Ko.2Coi,4[Fe(CN)6] 6.9 H2O. It formed the rock-salt... 

Here we should also stress another feature of the polymer problem. In standard critical phenomena only the relevant parameters are controlled eas ily, for instance by changing the magnetic field or the temperature. The irrelevant parameters are determined by the material at hand, withdittle chance of systematic variation. If we want to control more parameters, we must turn to more complicated systems like multicomponent solutions. For polymer solutions, however, besides the relevant parameters we can control... 

All of the experiments described above were performed by the field-sweep method, in which the radiofrequencies i and a>2 are passed through the spectrum by changing the magnetic field. This method has serious disadvantages when removal of large couplings is required, and, under these conditions, recourse should be had to frequency-sweep experiments. It is also possible to introduce more than one decoupling field and, hence, to effect triple-resonance experiments. 

Another way to perform field cycling is to actually change the magnetic field without moving the sample. Most magnets are very sluggish because of the large inductance of the coil... 

The NMR spectrum is obtained by slowly changing the magnetic field in the region of resonance, and recording the change in rf power absorbed by the sample. 

Figure 3.2. Separation of two ions of different miq by a magnetic sector. The two ions that emanate from a point source with an angular divergence of 2a are brought to focus on the focal plane at points 1 and 2. Changing the magnetic field allows ions to exit the slit sequentially for their detection.

MSBs should be cleaned from time to time as small particles, for example activated carbon or zeolite powders may be transferred from the sorbent basket to the permanent magnet. Here they can change the magnetic field permanently and by this cause systematic uncertainties or errors of measurement. Once these are detected, they may be corrected by recalibrating the zero load position of the balance, cp. Fig. 4.12. 

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