DANTE pulse

The principle of multiple selective excitation has been incorporated into a few ID and 2D experiments, the schemes of which are shown below (fig. 1). Depending on the experiment, either a DANTE pulse train (ID TOCSY [2]), frequency selective 180° pulses (ID NOE [3], ID INADEQUATE [4], ID C/H COSY [5] and 2D TOCSY-COSY [6]) or frequency selective 90° pulses (2D HMBC [11]) are applied to selectively perturb and uniquely label selected spins. Besides the DANTE pulse , composed itself of a series of non-selective rectangular pulses, Gaussian-shaped 180° and... 

Fig. 1. A comparison of the profiles relative to the three components of magnetization after application of an inverting DANTE pulse train.

Fig. 2. (a) 400 MHz partial H spectrum of a trisaccharide in CDClj. (b)-(e) ID TOCSY difference spectra with selective inversion of Hj// by a DANTE pulse train, and mixing times of 30, 60, 120 and 180 ms, respectively. IOtz values (of 16 scans each) were used for the 2-filter. (Reproduced with permission from Wessel et al. [26a]. Copyright 1991 Verlag... 

An analytical solution for the DANTE pulse sequence was found by Canet et a/.50 that might be useful for designing new selective pulse sequences. Goldman et a/.51 analysed the NMR frequency shift induced by sample spinning. For spin-i systems in solids, it was shown that the intensities of the two MAS-induced RF fields are only about one-thousandth of the real RF field, hence causing no problems in most experiments. However, it becomes important in NQR experiments, for example in the appearance of geometric phase.52... 

In some spectrometers it is not feasible to generate BURP or related tailored soft pulses. A useful alternative is the DANTE pulse sequence (delays alternating with nutation for tailored excitation). Normal high excitation power is used, but rather than apply a single 90° pulse, the DANTE method uses a sequence of small angle pulses that sum to 90° but with a short time between pulses during which nuclei... 

FIGURE 9.7 Trajectories for the on-resonance magnetization and several off-resonance magnetizations from a DANTE pulse sequence consisting of 18 pulses along the x axis, each of 5°. From Free-... 

DANTE pulse sequences do not require spectrometers equipped with shaped pulses. 

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

Figure 20 Saturation transfer experiment using (a) Selective inversion by a shaped pulse or (b) a DANTE pulse sequence. Do not use the DANTE-Z sequence, Section 3.3, in saturation transfer experiments...

The constant amplitude pulses are usually easier to implement and they do not require special hardware. A typical example and also the most frequently used kind of constant amplitude, frequency selective pulse is the DANTE pulse train [4]. In the simplest case it consists of a sequence of square pulses interleaved with periods of free precession. Unfortimately, the excitation profile of the DANTE sequence has extensive sidelobes. It also produces sidebands at frequencies ... 

The excitation band can be shifted off-resonance by applying a constant phase increment to the DANTE pulses [5,6]. The frequency shift Av is given by ... 

A somewhat better excitation profile can be obtained using so-called binomial pulses [8-10]. These pulses are similar to the DANTE pulses, except their number and length are given by binomial coefficients ... 

An early alternative to soft pulses was the DANTE Delays Alternating with Nutation for Tailored Excitation) experiment, which used a sequence of short, hard pulses of angle a <3C 90°, followed by a fixed delay t to achieve selective excitation. Thus, the pulse sequence is (a-T), ]. Nuclei that are on resonance are eventually driven to the y axis and hence are selected, whereas those more removed from the frequency range are not affected. The sequence of hard pulses can achieve a result similar to that of soft pulses and even can be shaped by modulating the duration of the pulse lengths, but DANTE pulses lead to spectral artifacts not created by soft pulses, such as unwanted sidebands. 

Fig. 6.2.16 [Cho2] 2D DANTE-imaging. (a) Timing of signals. The rf excitation is a DANTE pulse train applied in a read gradient Gx- The Hahn echo of the response is detected while both the phase-encoding gradient Gy and the read gradient Gx are turned on. The slice-selection gradient is Gj. (b) Traces in k space.

Fig. 7.2.16 [Kos3] 2D tagging f liingitudinal magnetization, (a) Sequence of two DANTE pulses applied in orthogonal field gradients, (b) Spin-echo image of water at rest in a tube measured with a delay time of 64 ms between the tagging pul.ses and the start of data acquisition.

Hard/shaped pulse, soft pulse, DANTE pulse train, composite pulse, cw-irradiation, cp decoupling sequence, spinlock, constant or incremented delay... 

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