Trim pulses

Figure 7.6 (a) A ID TOCSYexjjeriment with Gaussian excitation, (b) A ID TOCSY experiment with half-Gaussian excitation and TR (trim) pulses. (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527, copyright (1991), with permission from John Wiley and Sons Limited, Baffins Lane, Chichester, Sussex P019 lUD, England.)... 

Fig. 1. Pulse sequences of ID selective experiments. The shaped 90° pulses were half-Gaussian. The 180° selective pulses were produced by a DANTE-Z pulse train [44]. T denotes the trim pulse of phase ip. tnoe stands for the NOE-mixing time.

Fig. 9. ID TOCSY-ROESY. (a) H spectrum of oligosaccharide 3 (5 mg/0.5 ml D2O). (b) ID TOCSY spectrum acquired using the pulse sequence of fig. 7(b) and a selective excitation of H-lc by a 49.2 ms 270° Gaussian pulse. Duration of the spin lock was 132.7 ms including two 2.5 ms trim pulses. 32 scans were accumulated, (c) ID TOCSY-ROESY spectrum acquired using the pulse sequence of fig. 7(d) with initial selective TOCSY transfer from H-lc and selective ROESY transfer from H-4c. Parameters for the TOCSY part were the same as in (b). A 49.2 ms 270° Gaussian pulse was used at the beginning of the ROESY transfer. A 500 ms ROESY spin-lock pulse ( yBi/2 K = 2.8 kHz) was applied 1000 Hz downfield from the H-4c resonance. The time used for the frequency change was 3 ms. 128 scans were accumulated. A partial structure of 3 is given in the inset. Solid and dotted lines represent TOCSY and ROESY transfers, respectively.

A = 9.09 ms, 2.5 ms trim pulse and the total mixing time of 87 ms. The relaxation delay was 2 s and the acquisition time was 1.4 s. A = 0 in the first and A = 1 in the second experiment. Number of scans was 128 in both spectra. (Reprinted with adaptation with permission from ref. [38]. Copyright 1993 ESCOM Science Publisher B.V.)... 

Fig. 9.1. (A) Gaussian (a) and sine (b) excitation profiles. (B) Composite (G3) Gaussian pulse. (C) Train of soft pulses modified after the DANTE sequence to achieve selective off-resonance excitation. (D) Redfield 21412 sequence. (E) Binomial 11, 121, 1331, 14641 sequences. (F) JR (a) and compensated JR (or 1111) (b) sequences. (G) Watergate sequence. (H) Weft (Superweft) sequence. (I) Modeft sequence. (J) MLEV16 sequence. (K) NOESY sequence with trim pulse. (L) MLEV17 sequence with trim pulses. (M) Clean-TOCSY sequence.

Many tricks have been applied (trim pulses, z filters, gradients, etc.) to remove these antiphase terms, leaving only the pure phase Ia and Ib terms (Fig. 9.43, right). For example, a z filter is a 90 — A — 90° sequence that puts the desired magnetization on the z axis and then allows a bit of evolution to occur for the undesired terms ... 

In conventional two-dimensional Hartmann-Hahn experiments, only the transfer of a single magnetization component a is used. In order to avoid phase-twisted lineshapes, the orthogonal magnetization components B and y are eliminated with the use of trim pulses or other filters (see Section XII). If two magnetization components can be transferred with identical transfer functions 7) (t) = the sensitivity of multidimen-... 

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... 

In addition to the selected magnetization component (e.g., 7 ), several terms in the density operator survive the application of trim pulses (or z filters). For example, if a trim pulse is applied along the x axis of the rotating frame, all terms of the density operator that commute with remain unaffected, that is, in addition to the in-phase operators and (x magnetization), antiphase combinations like (lyS - I Sy) or (I SyTy + also survive the trim pulses. In the effective field frame, these terms represent operators with coherence order p = 0. Modified z filters and spin-lock pulses that are able to suppress these zero-quantum-type terms will be discussed in Section XII.B. 

TTie TOCSY sequence (ii) [85], supplemented with trim pulses and z-filter [24],... 

Fig. 4. Special pulse schemes for 2D- X, "Y H) correlations. The same notation as before is used A denotes a fixed delay of length ( /( H,Y(X)) l (a) HMQC sequence for indirect detection of spin-1 nuclei. (b) INEPT-HMQC. (c) INEPT-HETCOR. (d) HMQC-TOCSY si denotes an MLEV spinlock sequence of duration t which is framed by trim pulses. ...

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. 

Figure 5.68. Two practical schemes for implementing TOCSY based on (a) the MLEV-17 mixing scheme and (b) the DIPST2 isotropic mixing scheme. The MLEV sequence is bracketed by short, continuous-wave, spin-lock trim pulses (SL) to provide pure-phase data. In scheme (b) this can be achieved by phase-cycling the 90° z-filter pulses that surround the mixing scheme. This demands the independent inversion of each bracketing 90° pulse with coincident receiver inversion, thus (p =x, —X, X, —x (j) = X, X, —X, —X and (j)r = x, —X, —X, X. The S periods allow for the necessary power switching.

In Check it 5.4.2.1 the ID selective COSY, ID selective relayed COSY without and with z-filter and a ID selective TOCSY spectrum are simulated for the same spin system and the results compared. As already mentioned the spinlock for isotropic mixing can be generated in different ways and this has lead to the development of improvements and elements being added to the spinlock sequence. Of these improvements the trim pulse and z-filter, adapted to the spinlock sequence [5.154], are the most popular. 

MLEV-17 spinlock sequence with z-filter and trim pulses. 

Whilst the z-filter helps to eliminate experimental phase errors further spectral distortions can be suppressed by using two trim pulses and an additional pulse in the MLEV-17 pulse sequence. There are two main sources for these distortions ... 

In the MLEV-17 sequence a 180° pulse (or a 60° pulse) is appended to the MLEV-16 sequence. The additional 180° pulse inverts the magnetization that is not perfectly aligned with a particular axis of the rotating frame so that after an even number of MLEV17 cycles the magnetization is perfectly aligned. Any residual magnetization which is not perfectly parallel to the selected axis to which the spins are locked are defocused by the two trim pulses. 

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