Magnetic resonance, absorption curve

As we have emphasized in Ch ters I, III, and IV, computational techniques based on the infonnation derived from spectral moments have quite a long history. In the field of magnetic resonance, these can be traced back to the pioneering work of Van Vleck. In a more general form his basic idea can be expressed as follows. The normalized moment of a magnetic resonance absorption curve may be defined by the relation... 

The spectra of the radicals and magnetic nanoparticles were recorded at room temperature using Bruker EMX 8/2.7 X-band spectrometer at a microwave power of 5 mW, modulation frequency 100 kHz and amplitude 1 G. The first derivative of the resonance absorption curve was detected. The samples were placed into the cavity of the spectrometer in a quartz flat cell. Magnesium oxide powder containing Mn ions was used as an external standard in ESR experiments. Average amount of spin labels on protein macromolecules reached 1 per 4-5 albumin macromolecules and 1 per 2-3 thrombin macromolecules. Rotational correlation times of labels were evaluated as well as a fraction of labels with slow motion (x > 1 ns). 

Figure 14 illustrates a recorder curve of the resonance absorption derivative of F for a sample containing 2.7 wt. % of fluorine. The time of scanning the magnetic field through resonance was 2 hours and the time constant of the spectrometer was 90 seconds in order to increase the available signal-to-noise ratio. The F resonance absorption was examined in the concentration range of 0.3 to 12.5 wt. % fluorine. 

Figure 18. Spin temperature Ts obtained by inversion of (6) where M ) is given by the magnetization curves of Fig. 16. The resonant absorption near 0.491 T is superimposed to a non-resonant background absorption corresponding to a nearly constant spin-temperature.

Figure 20. Examples of the field dependence of Flts. The curves are obtained from the measured magnetization curves and Eq, 9, at 6 and 14 GHz, after subtraction of the nearly field independent non-resonant absorption background. The dots represent the fit of these curves using Eqs. 8 and 10 (Gaussian distributions) yielding

Two different techniques of NMR have been applied to measure wood moisture content based on the presence of the hydrogen nuclei in water. In one of these, designated as a steady-state method, the wood is subjected to an alternating magnetic field of constant frequency, with Ho varied slowly so as to resonate 7 with respect to the applied frequency. At resonance a strong absorption of energy occurs, and the width and intensity of this absorption curve give information on the moisture content of the wood (16). 

One very recently published result must be mentioned. Petukhov et al. have reported the detection of the EPR spectrum of micron-sized crystals of Fe8 via their magnetization response as a function of apphed magnetic field, using a Hall-probe magnetometer under either continuous wave or pulsed microwave irradiation at 118 GHz and between 1.4 and 50 K [53]. Dips are observed in the magnetization vs. field curves corresponding to resonant absorption - that is, EPR transitions. This method offers potentially extraordinary sensitivity and, furthermore, manipulation of the magnetization data in the absence and presence of the microwave radiation allows determination of the spin temperature. 

Figure 19 Integrated absorption curves for Br magnetic resonance in a series of mixed single crystals (l-c)KBr-cNaBr. From Ref. 158. Reproduced by permission of the Institute of Pure and Applied Physics.

Lithium metabolism and transport cannot be studied directly, because the lack of useful radioisotopes has limited the metabolic information available. Lithium has five isotopes, three of which have extremely short half lives (0.8,0.2, 10 s). Lithium occurs naturally as a mixture of the two stable isotopes Li (95.58%) and Li (7.42%), which may be determined using Atomic Absorption Spectroscopy, Nuclear Magnetic Resonance Spectroscopy, or Neutron Activation analysis. Under normal circumstances it is impossible to identify isotopes by using AAS, because the spectral resolution of the spectrometer is inadequate. We have previously reported the use of ISAAS in the determination of lithium pharmacokinetics. Briefly, the shift in the spectrum from Li to Li is 0.015 nm which is identical to the separation of the two lines of the spectrum. Thus, the spectrum of natural lithium is a triplet. By measuring the light absorbed from hollow cathode lamps of each lithium isotope, a series of calibration curves is constructed, and the proportion of each isotope in the sample is determined by solution of the appropriate exponential equation. By using a dual-channel atomic absorption spectrometer, the two isotopes may be determined simultaneously. - ... 

Before and after resonance, there are magnetization components of opposite sign 180° out of phase and in phase (+ u and - u direction) with the rf field B1 i (Fig. 1.8). At resonance, there is no magnetization in the u direction. If the receiver coil obtains the inductance current in phase with Bli (the u direction), a dispersion curve (Fig. 1.9) results, called the u mode. When the absorption or out of phase spectrum ( ) reaches its maximum (/ind (oj) = max.), the dispersion or in phase spectrum ( ) goes through zero and changes its sign, as illustrated in Fig. 1.9. 

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