Magnetic susceptibility effects

In summary, it is non-trivial to implement magnetic resonance pulse sequences which allow us to monitor unambiguously the decrease in absolute concentration of reactant species and associated increase in product species, but measures of relative concentrations from which conversion and selectivity are calculated are much easier to obtain. However, if such measurements are to be deemed quantitative the spectra must be free of (or at least corrected for) relaxation time and magnetic susceptibility effects. 

J. Furrer, K. Elbayed, M. Bourdonneau, J. Raya, D. Linial, A. Bianco and M. Piotto, Dynamic and magnetic susceptibility effects on the MAS NMR linewidth of a tetrapeptide bound to different resins, Magn. Reson. Chem., 2002, 40, 123-132. 

The T2 relaxation times of 50 /rsec and 40 msec given in the preceding discussion correspond to line half-widths of 6.4 kHz and 8 Hz, respectively. Whipple et al. (265) concluded that the line widths of several hundred Hz which are obtained in practice must be due to bulk magnetic susceptibility effects. This type of line broadening is removable by MAS (273) and they were the first to obtain high resolution spectra with linewidths similar to those expected from the T2 values. 

At 25 "C, relative to external 3 M aqueous LiBr, unless otherwise stated data are unconnected for bulk magnetic susceptibility effects. Relative to external O.IM LiBr in THF. Local environment in brackets. Relative to internal LiBr. 

Fyfe et al. [61] studied p-xylene and y-butyrolactone in hectorite in 1981 using H and C NMR with no MAS. The relatively good resolution of these spectra indicates a high degree of mobility of the adsorbates in all cases. The C chemical shifts are nearly identical to those in solution spectra of intercalants, suggesting that magnetic susceptibility effects are not large, at least in this clay. 

Villiinger, A., et al., Dynamic imaging with lanthanide chelates in normal brain Contrast due to magnetic susceptibility effects. Magn Reson Med, 1988. 6 p. 164-174. 

Thus attenuation due to magnetic susceptibility effects has to be minimized if measured Tj relaxation times are to be reliable. For this reason, T2 experiments... 

There came back an enthusiastic report of a substantial chemical shift difference between the adsorbate and the material in the liquid phase, to be followed shortly by another communication which stated that the difference disappeared when a magnetic susceptibility correction was applied to the results. This illustrates a problem which arises whenever one attempts to measure a chemical shift value in a heterogeneous system, the problem of a suitable reference and of eliminating magnetic susceptibility effects which are particularly troublesome in such systems of dubious geometry. Differences between chemical shifts in the same molecule, if they can be resolved, are... 

Figure 2 Magnetic dipole field outside a sphere of a diamagnetic material with dipolar moment m induced by the external field Bq (A) static sample (B) rotating sample. (Reproduced with permission from Wind RA and Hu JZ (2003) Magnetic susceptibility effects in nuclear magnetic resonance spectroscopy of biological objects. In Recent Research Developments in Magnetism and Magnetic Materials, vol. 1, pp. 147-169.)...

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