Phase cycling EXORCYCLE

Pure 2D absorption line shapes are readily obtained in heteronuclear 2D /-resolved spectra. The incorrect setting of 90° and 180° pulses can, however, cause ghost peaks that can be removed by a phase cycling procedure, appropriately named Exorcycle (Rutar, 1984b). A... 

A minimum two-step phase cycle was used by inverting the selective 90° pulse and the receiver phases on alternate scans. Exorcycle could also be applied on all 180° pulses. 

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. 

Figure 9.23. Solvent suppression schemes based on presaturation (a) presaturation alone, (b) ID NOESY and (c) FLIPSY, Sequence (b) makes use of the conventional NOESY phase cycle whereas FLIPSY uses EXORCYCLE on one (or both) of the 180° pulses (i.e. pulse = X, y, —X, —y, receiver = x, —x,...

EXORCYCLE Phase cycle to suppress ghost and phantom artefacts in spin-echo sequences 7.2.2... 

A two step cycle, consisting of 0°, 180° for the 180° pulse and 0°, 0° for the receiver, can easily be shown to select all even values of Ap. This reduced form of EXORCYCLE is sometimes used when it is necessary to minimise the number of steps in a phase cycle. An eight step cycle, in which the 180° pulse is advanced in steps of 45°, can be used to select the refocusing of doublequantum coherence in which the transfer is from p = +2 to -2 (i.e. Ap = -4) or vice versa. 

Both of these phase cycles can be extended by EXORCYCLE phase cycling of the 180° pulse, resulting in a total of 16 steps. 

It is also common to add EXORCYCLE phase cycling to the I spin 180° pulse, giving a cycle with eight steps overall. 

Another example of CT pathway selection using phase cycling is the Exorcycle phase cycle which is used to suppress artefacts due to imperfect refocusing in a spin echo. Check it 2.33.3 shows how it is possible to compensate for an incorrect pulse length, 140° instead of a 180° pulse, using a four step phase cycle. 

EXORCYCLE 4-slep phase cycle for spin echoes... 

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