Echo-antiecho method

Figure 5.69. Gradient-selected TOCSY. Sequence (a) is suitable for absolute-value presentations with a 1 1 gradient combination selecting the N-type spectrum. Sequence (b) provides phase-sensitive data sets via the echo-antiecho method for which separate P- and N-type data are collected through inversion of the first gradient.

The Echo/Antiecho method [2.45] In the introduction to the discussion of phase and amplitude modulated 2D data sets, see Table 2.8, it was emphasized that frequency discrimination in the fl dimension was inherent in phase modulated data sets. However the lineshapes was phase twisted because of the absorptive and dispersive contributions to the real part of s(coi, CO2) which describes the lineshapes in the final spectrum. 

The quadrature detection mode is given as a suffix to the main sequence name me magnitude calculated, TPPI time proportional phase increment, E/A Echo / Antiecho The term "selective" is reserved exclusively for sequences using selective pulses. If selectivity is achieved using other methods this is defined using a different term. 

Finally, we note an alternative approach to quadrature detection known as echo-antiecho selection [11], which is applicable only to pulsed field gradient selected 2D methods and which now finds widespread use. As this involves a quite different procedure, it will not be considered further here but will be introduced in Section 5.5.2 after field gradients have been described. 

A second solution is offered by the whole shifted-echo method, whereby a selective n pulse is added after a time delay <5 at the end of the refocusing block to create an additional spin echo on the CT (Fig. 12b) [139, 164, 167]. In this case, the echo and antiecho signals are delayed by... 

If the phase cycling used selects only one coherence transfer pathway, a two-dimensional spectrum with phase-twist lineshapes is obtained. However, the experiment can be easily modified to ensure that pure absorption (pure phase) lineshapes are obtained. The most commonly used modified acquisition schemes that allow pure-phase MQMAS spectra to be obtained include amplitude-modulated experiments, with hyper-complex (States) acquisition, and phase-modulated experiments, with delayed acquisition such as shifted echo or antiecho and split-methods. 

In this type of experiment, the echo and antiecho are linearly combined with the same amplitude to yield an amplitude-modulated signal in Pure absorption lineshapes may then be obtained in the frequency domain spectrum after a two-dimensional Fourier transform is performed. The disadvantage of this method is that it is not possible to discriminate the sign of the MQ coher-... 

An important shortcoming of the two-pulse methods is the difficulty of balancing the echo and antiecho amplitudes to obtain undistorted two-dimensional lineshapes, especially for nuclei with spin larger than 3/2. The two coherence-transfer pathways being different, it has been shown that their amplitude depends on the quadrupole couphng and the orientation of the crystallites. In a powder, it is nearly impossible to reach a perfect equalisation of these amplitudes. 

To overcome these difficulties, the z-filter experiment was adapted to MQMAS by Amoureux et al. [24]. In this three-pulse scheme the two hard pulses (excitation of the MQ coherences and conversion into OQ coherence) are followed by a short delay during which the magnetisation is stored along the z-axis as zero-quantum coherences and then transferred into observable IQ coherences using a selective n/2 pulse (Fig. 5a). The symmetrisation of the echo and antiecho pathways during the two hard pulses (p=0—> 3—>0) forces an equal intensity of the echo and antiecho signals, leading to amplitude-modulated FIDs and, thus, to pure absorption spectra. This is a robust method, easy to optimise. 

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