Broadband inversion pulses

Eq. (10.4) also demonstrates that the frequency window of an adiabatic pulse scales with the square of the applied rf amplitude whereas simple hard pulses exhibit only a linear dependence on this, and it is this feature that explains the efficiency of the swept pulses as broadband inversion pulses. 

In this section, three experiments are going to be discussed. Two of them, a broadband inversion and a Hahn spin echo, are well-known in the rotating frame. They need to meet the requirement of the phase coherence in PIPs in order to work properly in the Eigenframe. The third is a composite pulse with offset modulation. 

As shown in Fig. 21a, the simulated broadband inversion profile by the three PIPs resembles the profile by the composite pulse 90°180°90° except for a different excitation region. The inversion profile is severely distorted (Fig. 21b) if the three initial phases, phase relationship in the rotating frame is the wrong one in the Eigenframe. The phase coherence in PIPs needs to be considered even for PIPs with the same frequency shift, A/ = 50 kHz in this case. 

A. J. Pell, G. Kervem, L. Emsley, M. Deschamps, D. Massiot, P. J. Grandinetti and G. Pintacuda, Broadband inversion for MAS NMR with single-sideband-selective adiabatic pulses. /. Chem. Phys., 2011,134, 024117. 

There are many tricks to get around the problem, such as sandwich 180° pulses (e.g., 90 -180 -90 ) and broadband shaped pulses. Figure 8.4 (top) shows the inversion profile for a simple 180° pulse at the highest available power (fp = 28.4 p,s, yB l2it — 17.6 kHz). The profile is obtained using an inversion-recovery sequence (180°x — r — 90° ) with recovery time r = 0. The final 90° pulse frequency and the 13C peak (13CH3l) are both at the center of the spectral window, but the frequency of the 180° pulse is moved in 10 ppm (1500 Hz)... 

We will see that the major application of shaped pulses is to select a narrow region of the spectrum, thus displaying a narrow bandwidth. But there are also shaped pulses designed to do just the opposite — to give even excitation over a very wide range of frequencies. These broadband shaped pulses are specialized for inversion (Sz — S-) or refocusing... 

For many applications, the basis sequence can be iteratively constructed from simplw tarting sequences (Tyko, 1990). MLEV-4-type super cycles RRRR or RRRR (Levitt et al., 1983) are examples of simple iterative schemes for the construction of basis sequences with vanishing effective fields from a starting sequence R, which is a (approximate) composite inversion pulse R. Here, the composite pulse R is identical to R, except that the phases of all square pulses are shifted by 180°. The MLEV-16 super cycle RRRR RRRR RRRR RRRR (Levitt et al., 1983) suppresses effective fields even better. MLEV-4- and MLEV-16-type supercycles are often used in the construction of broadband Hartmann-Hahn mixing sequences. In these sequences, an effective spin-lock field can be introduced by adding an uncompensated additional pulse after each complete supercycle (see Section X). 

Figure 7.17. The absorption-mode 7-resolved sequences for (a) the 7-spectrum and (b) the anti-J spectrum employing the Zangger-Sterk pulse element. The non-selective 180° pulses are applied as broadband-frequency-swept inversion pulses and the selective 180° pulse is cycled according to the EXORCYCLE scheme (0 =x, y, —x,...

The BIPs therefore provide for broadband pulses of shorter duration and higher power than do the adiabatic pulses and may be better suited when longer pulses can be problematic, as mentioned above. They are suitable as single inversion pulses in... 

Most NMR measurements were made on a Bruker Avance 500MHz instrument with a 5mm pulse field gradient (PFG)-Broadband Inverse(BBI)... 

E. Kupce, R. Freeman, Compensated adiabatic inversion pulses broadband INEPT and HSQC, J. Magn. Reson. 187 (2007) 258-265. 

One possible solution to achieving broadband inversion is to use swept-frequency adiabatic pulses, which give very high bandwidths in relation to the RF power used. These pulses are widely used in solution state experiments, for instance for facilitating heteronuclear decoupling, or in many applications in magnetic resonance imaging (MRI) [38]. 

The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

As is the case in most gradient-enhanced pulse sequences, GROESY spectra should preferably be obtained with non-spinning samples. In our spectrometer, a Bruker ARX-400 equipped with an inverse broadband probehead incorporating a shielded Z-gradient coil, we have used the following experimental parameters ... 

E. Kupce and J. Freeman, Adiabatic pulses for wideband inversion and broadband decoupling. J. Magn. Reson. A, 1995,115, 273-276. 

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