Phase cycles

Bain A D 1984 Coherence levels and coherence pathways in NMR. A simple way to design phase cycling procedures J. Magn. Reson. 56 418-27... 

TOCSY data are acquired in tbe pbase-sensitive mode using quadrature detection, and aU. tbe data phases are positive. Tbis increases the SNR for the matrix, and the time required for the experiment is short because very Htde, if any, phase cycling is necessary. In some cases a single scan per FID suffices, and the data can be acquired in approximately 10 min,... 

Blade Material Volt Phase Cycle Ratio ... 

Driver Number Kind Type Manufacturer Full Load Speed, RPM Elec. Char.—phase/cycles/volts Rated H.P. ... 

Type and specifications on fan driver, gear types, pow er voltage, phase, cycles. Motors should at least meet specifications equivalent to totally enclosed, fan cooled, or if in explosive hazardous area, TEFC Class I, Group D (except this not acceptable in hydrogen or acetylene atmosphere). Due to moisture conditions around this equipment, it should be protected against moisture penetration and corrosion. 

A simple, two-step phase cycling scheme may therefore be employed The signals of 0° phase and 90° phase pass through signal channels (1) and (2) to sections A and B, respectively, of the computer memory during... 

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.

What is the effect of phase cycling on the appearance of NMR signals ... 

Phase cycling is widely employed in multipulse NMR experiments. It is also required in quadrature detection. Phase cycling is used to prevent the introduction of constant voltage generated by the electronics into the signal of the sample, to suppress artifact peaks, to correct pulse imperfections, and to select particular responses in 2D or multiple-quantum spectra. 

In 2D NMR experiments, the FIDs are relatively short and with fewer data points, so dc correction is more difficult to carry out accurately. Phase cycling procedures are recommended whenever required to remove dc offsets before dc correction, which is carried out before the first (F2) Fourier transform. Since the data points in the transform arise from frequency-domain spectra, no dc correction is normally required (we expect to see unchanging dc components at Fi = 0). 

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... 

Gradient-Accelerated Spectroscopy (GAS)—An Alternative to Phase Cycling... 

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

Gradient-enhanced 2D TOCSY spectrum of 10 mMof sucrose in D..,0 is shown in figure 7.26. The clean spectrum obtainable without any noise and without the nece.ssity of any phase cycling illustrates the power of this new technique in modern NMR spectroscopy. 

Coherence pathway selection Choosing a corresponding phase cycle so that undesired resonances are suppressed and only the desired magnetization is observed. 

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

Phase cycling As employed in modern NMR experiments, repeating the pulse sequence with all the other parameters being kept constant and only the phases of the pulse (s) and the phase-sensitive detector reference being changed. The FIDs are acquired and coadded. The procedure is used to eliminate undesired coherences or artifact signals, or to produce certain desired effects (e.g., multiple-quantum filtration). 

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. 

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