Four step phase cycle

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

The four-step phase cycle of the DANTE-Z sequence. 

Fig. 6. Top 2D MAT sequence for correlating isotopic chemical shift and CSA with two separate experiments P+ and P . All pulses following CP are 90°. A four-step phase cycling is used with 6 = —y, x, —y, x. and 62 = —y, x, x, -y. The receiver phases are x, -x, — y, -y for the P+ pulse sequence and x, —x,y, y for the P pulse sequence. (The sign of receiver phases with an asterisk depends on the relation between the pulse phase and the receiver phase of the particular spectrometer in use. These receiver phases must be changed in sign when the quadrature phase cycle (x,y, —x, -y) of the excitation pulse and the receiver phase in a single-pulse test experiment result in a null signal.) Phase alternation of the first H 90° pulse and quadrature phase cycling of the last 13C 90° pulse can be added to the above phase cycle. The time period T can be any multiple of a rotor period except for multiples of 3. Bottom 2D isotropic chemical shift versus CSA spectrum of calcium formate powder with a three-fold MAT echo extension. (Taken from Gan and Ernst178 with permission.)...

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.

We now need to see if this four step phase cycle eliminates the signals from other pathways. As an example, let us consider a pathway with Ap = 2, which might arise from the transfer from coherence order -1 to +1. Again we draw up a table to show the phase experienced by a pathway with Ap = 2, that is computed as - (2)0... 

E9-6. Write down four-step phase cycles to select (a) Ap = -1 and (b) Ap = +2. Suppose that the cycles are applied to different pulses. Combine them to give a 16-step cycle as was done in section 9.5.4 give the required receiver phase shifts. 

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. 

The four-step phase cycle gives the required performance of artefact suppression and frequency discrimination in the fl dimension. 

Fig. 8.10 Pulse sequence for homonuclear correlation spectroscopy, COSY/GCCOSY [46-48]. Although the gradient version of the experiment is shown, the pulse sequences are the same except for the two gradients and their associated delays. The non-gradient experiment employs a four-step phase cycle the gradient experiment allows the acquisition of data with a...

The dotted line indicates a third possible coherence pathway. If the initial 90° pulse is imperfect, there will be some residual z magnetization which will be raised to coherence level -l-l by the second 90° pulse. Suppression of this pathway requires two extra steps, yielding a four-step phase cycle. Finally, if one also wishes to incorporate a CYCLOPS cycle for 2 quadrature image suppression, the total phase cycle is 4 x 4 = 16 steps. The number of scans in a 2D experiment should be some whole number multiple of the number of steps in the phase cycle. However, with COSY, the sensitivity is high enough that 16 scans are usually more than necessary to acquire good spectra and thus the phase cycle determines the minimum time for the experiment. Fortunately, pulsed field gradient sequences have overcome this problem. 

Table 3 Alternative two-step phase cycles for N-type pathway selection and P-pathway suppression for a COSY spectrum plus a four-step phase cycle which selects the N-pathway while suppressing both P and Z paths...

In this four-step phase cycle, the P-pathway Is cancelled In steps 1 + 2 and in steps 3 + 4 while the Z pathway is cancelled in steps 1 + 3 and steps 2 + 4. 

Fig. 6. Top 2D MAT sequence for correlating isotopic chemical shift and CSA with two separate experiments P+ and P . All pulses following CP are 90°. A four-step phase cycling is used with 6 = —y, x, —y, x. and O2 = —y, x, x, —y. The receiver phases are X, -X, -y for the P+ pulse sequence and x, -x,y, y for the P pulse sequence.

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