Quadrature artifacts

A serious problem associated with quadrature detection is that we rely on the cancellation of unwanted components from two signals that have been detected through different parts of the hardware. This cancellation works properly only if the signals from the two channels are exactly equal and their phases differ from each other by exactly 90°. Since this is practically impossible with absolute efficiency, some so-called image peaks occasionally appear in the center of the spectrum. How can you differentiate between genuine signals and image peaks that arise as artifacts of quadrature detection ... 

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 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... 

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 described in the Section 3.3, two supposedly identical detectors are arranged to sample the signal simultaneously along x and y. However, the two detectors are usually not quite identical, and the reference signals to the detectors may not differ by precisely 90°. Also, the sample-and-hold circuits that follow the detectors may have slightly different characteristics. Three types of artifacts result (1) a DC offset between quadrature channels, (2) a gain difference between channels, and (3) a phase difference between channels. In terms of Eq. 3.5, artifact (1) means that there are constant terms of different magnitude added to the sine and cosine terms, while (2) and (3) imply different values of C and 4 rf respectively, for the sine and cosine terms. 

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. 

All signals are collected simultaneously. The RF pulse delivered is generally on the order of watts while the NMR signal collected is on the order of microwatts. The FID signal in the time domain must be converted to a frequency domain spectrum by application of a Fourier transformation or other mathematical transformation. Commercial instruments generally use quadrature phase-sensitive detection to avoid spectrum artifacts... 

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. 

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