Bulk susceptibility effects

Fig. 16. Single voxel STEAM spectra of the SOL muscle (top) and the TA muscle (bottom). Different fibre orientation in those muscles results in clearly different patterns of the lines in the spectra In SOL (feathered muscle with oblique fibres), IMCL and EMCL signals show lower frequency separation than in TA (spindle-shaped muscle) due to bulk susceptibility effects. Furthermore, in SOL the Cr2 doublet merges into one resonance, the Cr3 triplet is less resolved, and TAU is shifted towards TMA.

In general, two different solvent effects on NMR spectra can be distinguished (a) shifts due to a difference in the bulk volume magnetic susceptibility x of the solute and the solvent (b) shifts arising from intermolecular interactions between solute and solvent molecules. Since the bulk susceptibility effect depends on the shape of the sample and, therefore, is not of chemical interest, some form of correction for it is applied. For two... 

Only shifts observed in excess of this amount may then be attributed to intermolecular interaction effects. Use of an internal standard provides an automatic compensation for the bulk susceptibility effect, but for comparison of shifts measured in this way in different solvents it must be kept in mind that the standard itself may be subject to solvent effects. These are minimized in and NMR spectroscopy by the use of tetra-methylsilane (TMS) as an internal standard. 

Table 3 presents Mg line width and chemical shift data obtained as a function of the concentration of MgBr2 and MgCl2 [41]. Also included in Table 3 are the viscosities of the solutions investigated. The chemical shifts, when corrected for bulk susceptibility effects, vary over a total range of less than 1 ppm. 

Data from ref. 61 l4N spectra at 4-3345785 MHz ( 0-5 Hz), sample temperature 30 2°C concentric spherical sample containers are used in order to eliminate bulk susceptibility effects on shifts and signal shape external standards are used, CH3NO, (neat liquid) and, for signals within 15 ppm of that of neat CH,N02, tetranitromethane (neat liquid) reported shifts represent values obtained from iterative fitting of theoretical and experimental lineshapes using a differential saturation method, ref. 63, reported errors are standard deviations for the fitting of at least 200 data points, and represent 68% confidence limits for shifts which are recalculated from values referred to C(N02)4, the error of the shift of C(N02)4 relative to 0H,NO2 is included. 

In order to provide a means for the precise recalculation of nitrogen chemical shifts reported since 1972, it is necessary to have accurate values of the differences in the screening constants between neat CH3N02 and the large number of reference compounds which have so far been used. Table VII shows the results of precise, 4N measurements (61) which have been carried out in concentric spherical sample and reference containers in order to eliminate bulk susceptibility effects on the shifts. Since the technique adopted (61, 63) involves the accumulation of a large number of individually calibrated spectra with the subsequent use of a full-lineshape analysis by the differential saturation method, (63) the resulting random errors comprise those from minor temperature variations, phase drifts, frequency instability, sweep nonlinearity, etc. so that systematic errors should be insignificant as compared with random errors. 

Ref. 61, l4N spectra, neat CH3N02 external reference in concentric spherical sample containers in order to eliminate bulk susceptibility effects differential saturation technique and full lineshape analysis as in ref. 63, errors quoted are standard deviations for about 200 data points. 

Ref. 67, l5N natural-abundance spectra, concentric cylindrical sample tubes, no correction for bulk susceptibility effects referred to what is reported as ca. 10 m HisNO, from data reported for this standard sample relative to (CH3)4N Cle in subsequent papers (34, 36) its shift is ca. +6-1 ppm from neat CH3N02 this corresponds to a HN03 concentration of between 1 m and 2 m, as reckoned from data in Table VII errors quoted are 0-2 ppm, but comparison of results with those from ref. 36, suggests 1 ppm as more realistic limits. 

Ref. 36, l5N natural-abundance spectra, coaxial cylindrical sample tubes, no correction for bulk susceptibility effects referred externally to what was reported as ca. 12 m (CH3)4N C10... 

Estimated accuracy 10%. Not corrected for bulk susceptibility effects. ... 

In Hz at 30°C. Not corrected for field inhomogeneity or bulk susceptibility effects. 

In Table VII are the relative H chemical shifts of water in Nafion at several water contents. The experiments were conducted on specially prepared Nafion spheres in order to eliminate bulk susceptibility effects. These spheres behaved the same as the corresponding Nafion films and powders in limited 23Na NMR experiments. The H linewidths are sufficiently narrow to allow accurate measurement of the chemical shift. With decreasing water content, the resonance shifts upfield, suggesting the breakup of water-hydrogen bonding as for NaCl. The relative shift of pure water and water in saturated Nafion is not known at this time. The increased linewidth indicates decreased water mobility, as seen for the sodium ions. Additional experiments using model electrolytes and a chemical shift standard are warranted. 

Key Words Nuclear magnetic resonance spectroscopy, Solvent, Water suppression. Radiation damping. Demagnetization field. Bulk susceptibility effect. Presaturation, Watergate, Purge, Metabonomics. 

Prof. Levitt uses the term demagnetization field effect (as will we) but terms such as bulk nuclear-paramagnetic-susceptibility-induced transients, dipolar field, or bulk susceptibility effects have also been used. 

For convenience, internal standards are most frequently used. Tetramethylsilane (TMS) is almost universally adopted for resonances, and is usually sufficiently soluble in non-aqueous solvents. The standard texts also describe the use of external standards and the application of bulk susceptibility corrections. Some factors influencing the susceptibilities of mixed solvents and the calculation of bulk susceptibility effects are discussed in sect. 4.24.1. 

Value corrected [uncorrected] for bulk susceptibility effects and extrapolated to infinite dilution [59]... 

Medium effects (corrected for the bulk-susceptibility effect) have been reported for four commonly used solvent-internal reference systems in a paper by Stephen and his co-workers, who point out that the difference in chemical shifts between internal and external reference signals may be as great as 0.54 p.p.m. A novel type of noise-modulated, heteronuclear decoupling has been used by Evelyn and Hall to simplify the n.m.r. spectra of sugar derivatives containing the — CHgF group. By suitable adjustment of the modulation bandwidth and the radio-frequency intensity, the resonances of the coupled nuclei are broadened sufficiently... 

These data are quoted from systematic studies on dilute solutions where bulk susceptibility effects have been taken into account and the solvents used were chosen to encompass a broad range of properties. 

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