Magnetic field strength, resonance

Figure 43. Microwave transmission in a two-mode resonator as a function of the magnetic field strength for measurement of the microwave Hall effect in FeS2 (two measurements with an offset difference).16...

Fig. 1.24 Two examples of frequency-depen-dent relaxation times - 7"i is plotted as a function of the proton resonance frequency V = ou/2 JI, which was obtained from measurements at different magnetic fields strengths. Left polyisoprene (PI) melts and solutions of the same samples at 19wt-% concentration in cyclohexane. Numbers indicate the average molecular weight. The difference between the melt and solution increases towards lower magnetic fields strengths, the frequency dependence is more pronounced for melts.

Depending on how the secondary magnetic field is applied, there are two fundamentally different types of spectrometers, namely, continuous wave (CW) and pulse Fourier transform (PFT) spectrometers. The older continuous wave NMR spectrometers (the equivalent of dispersive spectrometry) were operated in one of two modes (i) fixed magnetic field strength and frequency (vi) sweeping of Bi irradiation or (ii) fixed irradiation frequency and variable field strength. In this way, when the resonance condition is reached for a particular type of nuclei (vi = vo), the energy is absorbed and... 

The proton experiment is a so-called single channel experiment the same channel is used for sample irradiation and observation of the signal, and the irradiation frequency is set (automatically) to the resonance frequency of the protons at the magnetic field strength used by the spectrometer. 

Good X-band resonators mounted into a spectrometer and with a sample inside have approximate quality factors of 103 or more, which means that they afford an EPR signal-to-noise ratio that is over circa three orders of magnitude better than that of a measurement on the same sample without a resonator, in free space. This is, of course, a tremendous improvement in sensitivity, and it allows us to do EPR on biomolecules in the sub-pM to mM range, but the flip side of the coin is that we are stuck with the specific resonance frequency of the resonator, and so we cannot vary the microwave frequency, and therefore we have to vary the external magnetic field strength. 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

Figure 34 shows spectra of TA and SOL from a 24-year-old male volunteer recorded at 1.5 T (a) and 3.0 T (b). Besides the clearly improved SNR, which is elevated by a factor of 1.7 to 1.8 at 3.0 T, distinct differences can be observed for the two magnetic field strengths In both muscles, IMCL and EMCL are clearly better separated at 3.0 T as the methylene resonance of IMCL shows smaller natural linewidths (in ppm). However, EMCL signals remain with a broad lineshape in TA as well as in SOL, since the lineshape is dominated by susceptibility induced static field inhomogeneities. Crs and TMA signals are... 

J. L. Evelhoch, C. S. Ewy, B. A. Siegfried, J. J. H. Ackerman, D. W. Rice and R. W. Briggs, P spin-lattice relaxation times and resonance linewidths of rat tissue in vivo dependence upon the static magnetic field strength. Magn. Reson. Med., 1985, 2,410-417. 

---