Quadrature detection phase cycling

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. 

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. 

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. 

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. 

Fig. 3. Basic pulse sequences for 2D- X,"T H correlations. Tbe same notation as in Hg. 1 is used. Minimum phase cycles for selection of correlation signals are given, more elaborate schemes for quadrature detection in FI and phase-sensitive spectra may be applied following standard rules. (a) HETCOR (without 180° pulses)/INEPT (with 180° pulses), the refocusing delays A are optional in both experiments setting the mixing pulses 8 to 45°/135° instead of 90° allows to determine coupling signs in ABX-type spectra. (b), HSQC. (c), HMQC the refocusing delay A2 is optional.

The solid-state H CRAMPS NMR measurements were performed on a Chemagnetics CMX 300 spectrometer equipped with a CRAMPS probe with 5 mm rotor. Quadrature-phase detection was carried out according to the phase-cycling technique proposed by Burum et al. Here, we used the MREV-8 pulse sequence for homonuclear decoupling. The experimental conditions are the same as those described in the previous section. 

Figure 5.40. The DQF-COSY experiment and coherence transfer pathway. The pulses are phase-cycled as described in the text to select the pathway shown with quadrature detection observing the p = — 1 magnetisation. The period 8 allows for rf phase changes and is typically of only a few microseconds.

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 H-NMR experiments were performed using the quadrupolar echo pulse sequence ti/2x-T,-7i/2y-T2-acquisition with phase-cycling and quadrature detection. A Bruker MSL 400... 

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