Spin-locking elimination

The pulse sequence for the ID ROESY experiment using purged half-Gaussian pulses is shown in Fig. 7.7. The purging is required to remove the dispersive components, since these are not completely eliminated by the weak spin-lock field employed in the ID ROESY experiment. 

Trim pulses are short spin-locking periods without compensation for rf inhomogeneity. Magnetization components orthogonal to the spin-lock axis are dephased. The minimum duration of a trim pulse for elimination of the unwanted magnetization components depends on the rf amplitude and can be estimated by... 

An alternative approach for the elimination of zero-quantum coherence is based on its evolution during periods of spin-locking in inhomogeneous Bq or Bj fields (Titman et al., 1990 Davis et al., 1993). This approach is particularly attractive, because it does not rely on time-consuming phasecycling schemes and variation of z-filter delays. Consider an inhomogeneous rf field with amplitude I f (r) that depends on the position r in the... 

Figure 5.62. A schematic illustration of events during spin-lock mixing. All chemical shift differences between spins are eliminated yet all spin-spin couplings between them remain. This forces the strong-coupling condition on all spins (see text).

There are essentially two approaches based on composite-pulse methods in widespread use for the practical implementation of the TOCSY experiment (Fig. 5.68). The first of these [51] (Fig. 5.68a) is based on the so-called MLEV-17 spin-lock, in which an even number of cycles through the MLEV-17 sequence are used to produce the desired total mixing period. To ensure the collection of absorption-mode data, only magnetisation along a single axis should be retained, so it is necessary to eliminate magnetisation not parallel to this before or after the transfer sequence. In this implementation, this is achieved by the use of trim-pulses applied for 2-3 ms along the chosen axis. 

The third complicating factor specific to ROESY is the attenuation of cross-peak intensities as a function of resonance offset from the transmitter frequency [69]. Off-resonance spins experience a spin-lock axis that is tipped out of the x-y plane (Section 3.2.1) resulting in a reduction in observable transverse signal in addition to a reduction in cross-relaxation rates. This is more of a problem for quantitative measurements, although fortunately mid-sized molecules show the weakest dependence of ROE cross-relaxation rates on offset. The so-called compensated ROESY sequence [69] eliminates these frequency-dependent losses should quantitative data be required. 

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