Quadrature phase cycling

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.)...

In practice, hard rf-pulses are used for uniform excitation of broad lines. Our own work has tended to use an echo sequence with the phase cycling first proposed by Kun-war. Turner and Oldfield (1986) which combines quadrature phase cycling with further cycling designed to cancel direct magnetisation (the remaining FID) and ringing effects ... 

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,... 

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. 

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. 

The NMR experiments were performed using the quadrupolar echo pulse sequence 7i/2x—Ti—7i/2y—T2—acquisition with phase-cycling and quadrature detection. A Bruker MSL 400 spectrometer was used for the high pressure studies operating at a resonance frequency of 61.4 MHz. In the liquid-crystalline phase, perdeuterated lipids display NMR spectra, which are superpositions of axially symmetric quadrupolar powder patterns of all C-D bonds.From the sharp edges, the quadrupolar splittings... 

Accurate measurements of the frequency-resolved transverse spin relaxation T2) of Rb NMR on single crystals of D-RADP-x (x = 0.20, 0.25, 0.30, 0.35) have been performed in a Bq field of 7 Tesla as a function of temperature. The probe head was placed in a He gas-flow cryostat with a temperature stability of 0.1 K. To obtain the spin echo of the Rb - 1/2 -o-+ 1/2 central transition we have used the standard (90 - fi - 180y -ti echo - (2) pulse sequence with an appropriate phase-cycling scheme to ehminate quadrature detection errors and unwanted coherences due to pulse imperfections. To avoid sparking in the He gas, the RF-field Bi had to be reduced to a level where the 7T/2-pulse length T90 equalled 3.5 ps at room temperature. 

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. 

The actual pulse sequences used by all modem spectrometers are more complicated than the idealized ones given in this text. Many spectrometers employ a technique known as phase cycling in which the phase of the rf pulse is changed in a regular manner (through a cycle ) for each q increment. These phase cycles are extremely important experimental factors that help remove artifacts and other peculiarities of quadrature detection. We will ignore phase cycling in our pulse sequences and discussions because they do not affect... 

QUADRATURE DETECTION, PHASE CYCLING, AND THE RECEIVER PHASE 209... 

The two copies of the COSY spectrum and the fi = 0 responses can all be separated without phase cycling if one is prepared to sacrifice digital resolution by increasing the fi-spectral width. The following spectmm was collected without phase cycling, with quadrature detection OFF,... 

We consider now an important example of phase cycling that is used in both ID and 2D NMR, namely the suppression of artifacts resulting from imperfections in the hardware used for quadrature phase detection. We detail the principles and procedures involved in this example as a prototype for many more complex phase cycling procedures that we mention more briefly in later chapters. 

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. 

In practice, additional phase cycling is needed in order to generate quadrature signals that can be processed as discussed in Section 10.3 to produce properly phased line shapes. Because of the DQC precession frequency, phase increments of tt/4 are needed to provide the same sort of data usually obtained with tt/2 phase shifts. Hence, a second set of phase-cycled repetitions is needed, with data stored in a separate location. Overall, the following phase cycling can be... 

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.

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