Phase cycling artifact suppression

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. 

We recall here that the phases of the pulses in Fig. 8.2A sequence, as well as of all other pulse sequences shown in Fig. 8.2 and described later in this chapter, must be properly cycled to achieve selection of the desired connectivities and suppression of artifacts and of other connectivities due to different types of interactions. The criteria to choose the appropriate phase cycling do not depend on the presence of a paramagnetic center in the molecule, and the reader should refer to the many publications on multidimensional NMR for details. 

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. 

To suppress both types of artifacts, one phase cycle must be nested within the other to create a four-pulse cycle. In addition, many spectrometers also require... 

Yip GN, Zuiderweg ER (2004) A phase cycle scheme that significantly suppresses offset-dependent artifacts in the R2-CPMG 15 N relaxation experiment. J Magn Reson 171 25-36... 

The usual 2D J spectroscopy pulse sequence of Figure 7 was used to collect the time domain data, with the relative phases of the two pulses and of the receiver reference being cycled on successive transients to suppress artifacts due to pulse imperfect ions.38 The phase cycling sequence used was ... 

Spurious signals due to imperfect carbon-13 180 pulses are suppressed if receiver phase imbalance causes zero frequency artifacts, these may be removed by adding receiver phase cycling, extending the sequence to include 32 different sets of phases. 

Changing the phase of the RF pulses and of the receiver in an NMR experiment in a systematic fashion (phase cycling) has been the method used to select only signals of interest, to eliminate artifacts, and to achieve quadrature detection in the t dimension. However, often 4, 8, or even 16 steps are needed for every t increment in a 2D NMR pulse sequence in order to achieve the correct level of suppression of undesired peaks. If the sensitivity of the experiment is such that one scan for each t increment would have been sufficient, then significant time is wasted in the data acquisition. 

Phase cycling is a fundamental procedure in most NMR experiments and is used not only for removing instrument artifacts, but also for selecting or suppressing signals, specially for achievement of specific coherence transfer pathways [5,13]. In NMR experiments, one must be aware of the importance of phase cycling, which sometimes is more difficult to understand than the basic aspects of the pulse sequences. 

Before the development of pulsed field gradient sequences (see below), most NMR pulse sequences included a phase cycle in which the phases of at least one pulse and the receiver were varied systematically. This was needed for one or more of several reasons, e.g. suppression of unwanted signals, suppression of artifacts due to hardware imperfections and/or incomplete return to equilibrium between scans and coherence pathway selection in multidimensional NMR. One example of each the first two kinds of phase cycle is briefly discussed below. 

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