On-resonance spin lock

Even more efficient single-scan zero-quantum dephasing is possible if adiabatically switched gradients are used in combination with on-resonance spin-locking. Experimental and theoretical details of this technique can be found in the paper by Davis et al. (1993). 

What does this mean for the effect of the spin lock on sample magnetization If the sample magnetization starts on the / axis, for example, the tilted spin-lock axis will destroy the component that is perpendicular to the spin-lock axis and retain the component that is on the spin-lock axis. This preserved component is locked because it is on the axis of the effective field and has no reason to precess around the z axis. So even if the spin is off-resonance, its magnetization does not precess around the z axis during the spin-lock period. Instead, the component that is not on the tilted spin-lock axis precesses around the spin-lock axis until it is destroyed by Bi inhomogeneity, and the component that is on the spin-lock axis is retained. 

Recently a new type of proton assisted recoupling experiments has been developed for coherence transfer where rf irradiation is taking place on all involved rf channels. This embraces the homonuclear proton assisted recoupling (PAR) [45, 140, 141] and the later resonant second-order transfer (RESORT) [142] experiments, as well as the heteronuclear proton assisted insensitive nuclei (PAIN) cross polarization [44] experiments. In PAR and PAIN, spin-lock CW irradiation is applied on both passive ( H) and active spins (13C, 15N) without matching rotary resonance conditions. In RESORT a phase alternation irradiation scheme for the passive spins is used. 

The spin-locking and CP behavior of the most commonly used SQ coherence (CT) in quadrupolar nuclei under static and MAS conditions has been described in detail by Vega using the fictitious spin-1/2 approximation [223]. In a static sample, the Hartmann-Hahn matching condition requires that co = nut where co ut is one of the nutation frequencies associated with the SQ coherence of the quadrupolar S spin (see Sect. 2.3.4). In the simple case of on-resonance SQ-CP this translates to [224]... 

Since water protons are not bound to or nuclei, the water signal is also suppressed by the spin-lock purge pulse. In practice, the suppression of the water signal is sufficient to record HSQC spectra of protein samples dissolved in mixtures of 95% H20/5% D2O without any further water suppression scheme [12]. For optimum water suppression the carrier frequency must be at the frequency of the water resonance. On resonance, the phase of the water magnetization is not affected by imperfections of the first 180°(ff) pulse, so that no solvent magnetization ends up along the axis of the spin-lock purge pulse. 

The pulse sequence for ICP experiments appears simple a 90° proton pulse is followed immediately by a spin lock radio-frequency (rf) field of strength B that is phase shifted by 90° relative to the first pulse. By a spin-lock field is meant a strong rf field B that is on resonance with the given nucleus it keeps magnetization in a spin-locked orientation parallel to the B direction where the decay of magnetization is governed by T p. At present the strong continuous B field is replaced by multipulse sequences that are well known from other spin-lock experiments such as TOCSY, ROESY etc. Simultaneously,... 

In addition to the difficulty of finding the optimal r, the JCP experiment also suffers from extreme sensitivity to the HaHa match. Moreover, the original experiment required both rf fields (Si and H) to be highly homogenous, preferably created by the same transmitter coil, to keep the same ratio of the two fields within the whole active volume of the sample147. To reduce the sensitivity of the enhancement to the HaHa condition and cross-relaxation time, a modified, refocused JCP experiment (RJCP) was suggested151. In this experiment the spin-lock field on the silicon resonance is interrupted at r = j for a duration corresponding to a conventional 90° pulse, and later (r = /) the spin-lock field on protons is similarly interrupted both fields then remain on until r = 2//. 

To understand selective (shaped) pulses and the spin lock, we need to look in detail at the effect of pulses on spins as a function of their resonant frequency, v0, that is to say the position of a resonance within the spectral window. 

In the rotating frame of reference for an on-resonance peak, the B0 field is exactly canceled by a fictitious field created by the rotation of the axes, so that for nuclei that are on-resonance the only field present is the B field during the spin lock (Z eff =B i). If we place the sample magnetization on the y axis of the rotating frame with a 90° hard pulse (phase —jc), the spin lock can be placed on the y axis (phase y). While the spin lock is on, the sample magnetization is locked on the y axis and will not undergo precession, as the only field present is the B field and the sample magnetization is on the same axis as the B field (Fig. 8.37). 

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