Phase cycling axial peak suppression

Scenario (a) transplants acquisition parameters from a typical ID proton spectrum into the second dimension leading to unacceptable time requirements, whereas (b) and (c) use parameters more appropriate to 2D acquisitions. All calculations use phase cycles for f quad-detection and axial peak suppression only and, for (b) and (c), a recovery delay of Is between scans. A single zero-filling in f] was also employed for (b) and (c). 

A simple way of suppressing axial peaks is to select the pathway Ap = 1 on the first pulse this ensures that all signals arise from the first pulse. A two-step cycle in which the first pulse goes 0°, 180° and the receiver goes 0°, 180° selects Ap = 1. It may be that the other phase cycling used in the sequence will also reject axial peaks so that it is not necessary to add an explicit axial peak suppression steps. Adding a two-step cycle for axial peak suppression... 

All standard pulse sequences for these experiments include some mechanism for selecting xH-X pairs and suppressing the other XH signals (phase cycling and/or pulsed field gradients, see below). As a further advantage, residual signal intensity of protons not bound to X (because of imperfect suppression) will not lead to a cross peak in the xH,X plane after Fourier transformation, but merely contribute to axial peaks at the spec-... 

The basic components of the INADEQUATE phase cycle comprise doublequantum filtration and fi quadrature detection. The filtration may be achieved as for the DQF-COSY experiment described previously, that is, all pulses involved in the DQ excitation (those prior to ti in this case) are stepped x, y, —X, —y with receiver inversion on each step (an equivalent scheme found in spectrometer pulse sequences is to step the ftnal 90° pulse x, y, —x, —y as the receiver steps in the opposite sense x, —y, —x, y, other possibilities also exist). This simple scheme may not be sufficient to fully suppress singlet contributions, which appear along fi = 0 as axial peaks and are distinct from genuine C-C correlations. Extension with the EXORCYCLE sequence (Section 7.2.2) on the 180° pulse together with CYCLOPS (Section 3.2.5) may improve this. Cleaner suppression could also be achieved by the use of pulsed field gradients, which for sensitivity reasons requires a gradient probe optimised for C observation. 

The scheme of Fig. 6.31b has been widely used to produce absolute-value shift correlation spectra, and is often referred to as HETCOR or hetero-COSY. Conversion to the preferred phase-sensitive equivalent (of which various forms have been investigated [55]) requires the reintroduction of the simultaneous 180°( H, C) pulses into the midpoints of both Ai and A2 to remove chemical shift evolution during these periods, exactly as in the full refocused INEPT. In addition, the incorporation of the States or TPPI phase cycling of the 90° proton pulse of the polarisation transfer step is required. Suppression of axial peaks is through the phase alternation of the final proton pulse together with the receiver... 

Table 8J. The basic NOESY phase-cycle for the three pulses (Pn) and the receiver (Pr), including the suppression of axial peaks...

When the SHR method is used, axial peaks (arising from magnetization which has not evolved during q) appear at Fl = 0 such peaks can be a nuisance as they may obscure other wanted peaks. We will see below (section 9.5.6) that axial peaks can be suppressed with the aid of phase cycling, all be it at the cost of doubling the length of the phase cycle. 

Fig. 20. Pulse sequence used to produce chemical-exchange correlated two-dimensional spectra. The phases of Pj, Pj, and the receiver may be cycled to suppress axial peaks, J cross-peaks, and allow quadrature detection in both dimensions (Jeener et al., 1979 Macura et al 1981).

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