BURP pulse

Figure 3 Selective HMQC pulse sequence employing a time reversed q-SNEEZE pulse for de-excitation of X nuclei qs is a q-SNEEZE pulse and re is a RE-BURP pulse...

Figure 4.5 Time (right) and frequency O ft) profiles of some soft tt pulses used for narrow band excitation in NMR spectroscopy, (a) Gaussian pulse Hermite pulse and RE-BURP pulse. It is supposed that before the pulse the magnetization was in the z direction, (b) Typical magnetization trajectories for a inversion RE-BURP pulse with distinct frequency offsets. Adapted with permission from References [12,15] (Copyright 2007 American Physical Society and Elsevier).

The last three pulses listed in table 1, namely the two E-BURPs and the are non-universal pulses designed to transfer Zeeman polarization Iz) into detectable transverse magnetization I or ly), but not to achieve the reverse operation. The conversion of or ly into Iz can be achieved by using the time-reversal of the considered pulse shape. An alternative... 

Fig. 1. Computer simulations of four selective excitation pulses. (Top) Pulse shapes. From left to right 90° rectangular pulse, 270° Gaussian truncated at 2.5%, Quaternion cascade Q, and E-BURP-1. The vertical axis shows the relative rf amplitudes, whereas the horizontal axis shows the time. (Middle) Trajectories of Cartesian operators in the rotating frame...

Better profiles may sometimes be needed, in which case an E-BURP-1 is recommended, particularly for band-selective experiments. The calibration is relatively easy and very narrow transition regions are very useful (fig. 1). If a universal pulse is needed, the E-BURP-1 may either be replaced by a pulse, or if only a conversion of or ly into Iz is required, by a time-reversed E-BURP-1. 

The quality of the profile pertaining to the selected region can be improved by a modulation of the pulse lengths within the pulse train, which mimics the amplitude modulation of a simple soft pulse [8]. Simple modulation schemes can be devised for attenuating or even suppressing the side-lobes in the vicinity of the selected region (as shown in fig. 3). Alternatively, more elaborated modulation schemes as those of the BURP family [9] can be run in the DANTE-Z mode [10]. This mode actually offers a... 

The double-selective TOCSY-ROESY and TOCSY-NOESY techniques are particularly useful. They allow one to measure NOE and ROE correlations in spectra with high degree of overlap as often found in carbohydrates. In addition to the DANTE, DANTE-Z [66], and Gaussian pulses as described earlier for selective excitation, self-refocusing shaped pulses such as BURP (EBURP and UBURP) [67] have also been used for this purpose [64]. 

FIGURE 9.6 Illustration of the E-BURP selective excitation pulse, (a) Depiction of the time excitation function. (b) Frequency domain excitation profiles for absorption (solid line) and dispersion... 

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... 

From equation (8) three modes of adiabatic spin inversion using RF pulses may be defined as (a) amplitude modulated pulses, e.g. I-BURP [13], G3[16], I-SNOB [17], (b) frequency modulated pulses, e.g. chirp [18,20], tangential sweep [20,21] and (c) both amplitude and frequency modulated pulses, e.g. the hyperbolic secant [22] or WURST (Wide band Uniform Rate Smooth Truncation) [23] pulse. 

Such excitations have been called shaped pulses, and a considerable effort has been expended in an attempt to optimize their shapes. A simple Gaussian shape is a considerable improvement over a rectangular pulse, but is not entirely effective in achieving an optimal peak shape. The use of more elaborate mathematical functions improves the shape of the signal, although with increasing loss of intensity. The BURP Band selective. Uniform Response, Pure phase) family utilizes an exponentially dependent sinusoidal series of Gaussians with considerable success in a variety of situations (EBURP for REBURP for 180 ). 

BURP Band-selective, uniform-response, pure-phase (selective) pulse 9.3.1... 

More elaborate pulse shapes that aim at producing a near top-hat profile yet retain uniform phase for all excited resonances within a predefined frequency window have been developed over the years. These operate without the need for purging pulses or further modifications, allowing them to be used directly in place of hard pulses. They are typically generated by computerised procedures, which result in more exotic pulse envelopes (see Fig. 10.18, and acronyms ) that drive magnetisation vectors along rather more tortuous trajectories than the simpler Gaussian-shaped cousins. Trajectories are shown in Fig. 10.22 for two members of the BURP family of pulses (Sand-selective, Uniform Response,... 

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