Solvent radiation damping

It might be reasoned from Eq. (4) that even if the solvent resonance was perfectly inverted there would be no radiation damping. However, it has been shown that radiation d imping can be initiated in magnetization stored along the -z-axis by residual RF leakage and even thermal noise from the RF coil." ... 

As is evident from Eq. (1), the radiation damping effect is particularly severe for large solvent peaks and can make the solvent return to equilibrium many times faster than the intrinsic longitudinal relaxation time (see Fig. 2). Thus, the effects of radiation damping are a major source of difficulties in manipulating solvent magnetization and need to be considered in the... 

Fig. 7. (A) The WEFT sequence in this sequence the tt pulse is applied to rotate all of the magnetization (i.e. both solute and solvent) to the -z-axis. A delay (I>np) of sufficient length is used to allow the water magnetization to relax to the origin ( >np = InfZ) ) whilst during the same period, by virtue of faster longitudinal relaxation, the solute resonances have reached thermal equilibrium. An excitation pulse (represented here as a tj/2 pulse) is then applied and an almost water-free spectrum is acquired. However, in the presence of radiation damping the water quicldy returns nonexponentially to the equilibrium position at a similar rate to the solute nuclei (see Fig. 2). However, if during D p a series of n very weak and evenly spaced gradient pulses are applied so as to inhibit the effects of radiation damping, the water relaxes according to its natural spin-lattice relaxation rate. This is the basis of the Water-PRESS sequence (B). An example of a spectrum obtained with Water-PRESS is shown in Fig. IB and Fig. 6.

From the practical viewpoint it might be surprising that the WATERGATE sequence is usually applied with the transmitter frequency exactly on-resonance with the solvent signal. The reason for this approach is that it minimizes radiation damping, which decays the transverse magnetization. Because NMR-SIM is based on an ideal spectrometer this radiation damping decay mechanism cannot be simulated. 

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

Unlike the previous section, however, these pulses are used to return solvent (most often water) resonances back to the +Z-axis without the need for selection or coherence destruction, effectively removing much of the echo refocusing, radiation damping, demagnetization, and alternate suppression concerns. 

In 1998, Liu et al. introduced modifications to the 3919 sequence to reduce the width of fhe on-resonance suppression notch while moving the secondary notches further out. The two new pulses were termed W4 and W5 (Figure 9C). It is important to note that there is a typo in the first paragraph incorrectly stating that the 3919 uses 62a = 180°. The value in the abstract and the original 3919 paper is correct (i.e. 26a = 180°). Like the 3919 sequence, W5 involves a series of explicit pulses (i.e. 5) interspersed with delays that control the solvent suppression width and where the secondary suppression notches occur. It is important to remember that these types of suppression schemes have a net zero effect on solvent (solvent resonances are moved off the +Z-axis but return, while solute is inverted) so radiation damping can certainly cause problems. Additional power may be required. 

Fully protonaled solvents may give rise to radiation damping due to the very strong signals and this may also result in a nonlinear decrease of the signal. 

Figure 5 Solvent suppression pulse sequences based on filtering methods. Method (A) uses a 7, filter to discriminate resonances of solvent and solutes. The difference in molecular diffusion coefficients is used in method (B). is the spin-echo time. (C) Double-quantum filtering COSY, which uses the fact that there is no J-coupling between the two equivalent protons in water molecules and thus it cannot be excited to higher quantum coherence. The PFG pulses in (A) and (B) are used to attenuate radiation damping effects and dephase any transverse magnetization. They are used for the desired coherence selection in (C).

When water is used as solvent in H NMR spectroscopy, radiation damping is not avoidable, but its effects can be minimized if a proper solvent... 

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