Phase cycling CYCLOPS

To suppress other interference effects, the phase of the transmitter pulse is also shifted by 180° and the signals subtracted from sections A and B, leading to the CYCLOPS phase cycling scheme shown in Table 1.4, in which the two different receiver channels differing in phase by 90° are designated as 1 and 2 and the four different receiver pulses (90°, 90°, 90° and 90f,) are called x, y, — x, and —y, respectively. 

Figure 1.43 The first two steps of the CYCLOPS phase cycling scheme. Any imbalance in receiver channels is removed by switching them so they contribute equally to the regions A and B of the computer memory.

CYCLOPS A four-step phase cycle that corrects dc imbalance in the two channels of a quadrature detector system. 

Quadrature images Any imbalances between the two channels of a quadrature detection system cause ghost peaks, which appear as symmetrically located artifact peaks on opposite sides of the spectrometer frequency. They can be eliminated by an appropriate phase-cycling procedure, e.g., CYCLOPS. 

Quadrature images in the Fi dimension can be suppressed by expanding the 8-step phase cycle to 32 steps or 16 steps, respectively, using CYCLOPS [20] or 2-step CYCLOPS [21]. In the CYCLOPS scheme, the phases of all pulses are simultaneously incremented by 90°, 180° and 270°. In the 2-step CYCLOPS scheme, the incrementation of the pulse phases is limited to the 90° step. 

Fig. 16. Pulse sequence used in slow-spinning version of DECODER experiment. Each of the solid rectangles represents a 90° pulse. Standard CYCLOPS and spin-temperature alternation were used for phase cycling, (b) Pulse sequence used in the 3D experiment the phase cycling for the t part was similar to Grans170. (Adapted from Lewis et al.260 with permission.)...

As we see in later chapters, a number of types of phase cycling are critical to the execution of many 2D experiments. The procedures are similar to that used in CYCLOPS, but the details vary depending on the particular type of signal that must be suppressed. Meanwhile, in addition to any phase cycling unique to the 2D experiment, the complete four-step CYCLOPS cycle is often needed to suppress the quadrature detection artifacts, with the result that long cycles (16 to 64 steps) may be needed, with consequent lengthening of experimental time. 

If CYCLOPS is used to eliminate artifacts in quadrature detection, this eight-step cycle must then be nested within CYCLOPS to give a 32-step cycle overall. In this simple treatment we have not taken into account the effect of pulse imperfections, which generate additional coherence pathways from coherences that were found to vanish in the preceding analyses, so that further phase cycling is often necessary. 

Figure 3.8. Quadrature phase sensitive detection along with the phase cycling and signal routing used in CYCLOPS for eliminating quadrature artefacts.

For H NMR spectra, the number of scans should be a multiple of 4, since this is the length of the CYCLOPS phase cycle used to minimize imperfections associated with quadrature signal detection (Section 5-8). Anywhere from 4 to 128 scans are usually sufficient to obtain a good spectrum with a relatively flat baseline. This number, however, depends heavily on the concentration of the sample. Nevertheless, accumulation times of over 1 hour are relatively rare. 

Table 2.2.2 The CYCLOPS sequence for transmitter and receiver phase cycling...

Table 3.1. The four-step CYCLOPS phase cycle illustrated in Fig. 3.26...

Figure 3.26. Phase cycling. The CYCLOPS scheme cancels unwanted artefacts whilst retaining the desired NMR signals. This four-step phase cycle is explained in the text.

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 CYCLOPS phase cycling scheme is commonly used in even the simplest pulse-acquire experiments. The sequence is designed to cancel some imperfections associated with errors in the two phase detectors mentioned above a description of how this is achieved is beyond the scope of this discussion. However, the cycle itself illustrates very well the points made in the previous section. 

If time permits we sometimes add CYCLOPS-style cycling to all of the pulses in the sequence so as to suppress some artefacts associated with imperfections in the receiver. Adding such cycling does, of course, extend the phase cycle by a factor of four. 

There have been no reported applications of the J, J-HMBC experiment published to date. There has, however, been one report of a COSY-type artefact observed in -J. J-HMBC spectra of a cyclopentafurnanone. The COSY-type responses observed are displaced in Fj as a function of the choice of Jscaie-Removing the bipolar gradients flanking the BIRD pulse in the A2 interval of the STAR operator and superimposing a CYCLOPS phase cycle on the BIRD pulse completely suppresses the COSY-type response artefacts associated with the -J, J-HMBC experiment. ... 

By use of the two-step phase cycle, it is therefore possible to compensate for the effects of imbalance in the two receiver channels. It is also possible to remove extraneous signals that may occur, such as from DC offsets in the receiver, by simultaneously inverting the phase of the rf pulse and the receiver thus, as shown in Fig. 3.27, a steps to c and likewise b becomes d. The NMR signals will foUow the phase of the pulse and so will add in the memory whereas offsets or spurious signals will be independent of this and so will cancel. This gives us a second possible two-step phase cycle which, when combined with the first, produces an overall four-step cycle known as CYCLOPS [8] (CYCLically Ordered PhaSe cycle. Table 3.1). This is the standard phase cycle used for one-pulse acquisitions on all spectrometers and is often nested within the phase cycles of 2D experiments again with the aim of removing receiver artefacts. 

These artifacts can be eliminated by a phase cycling scheme called CYCLOPS [46], To make the discussion of this phase cycling easier, let s consider that S is usually sufficiently... 

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