Number of Scans per Time Increment

In order to minimize the overall time of a 2D experiment, one wishes to keep the number of scans per increment (ns//) at a value that is sufficient to observe the spectrum of that particular increment. For heteronucleus detection, this number usually is large, but for protons, adequate detection can often be accomplished in 1 to 4 scans. The minimum ns//, however, is determined by the phase cycle (Section 5-8) of the pulse sequence used and may be anywhere from 4 to 64 scans. As a general rule, 8 scans// is a minimum value for H-detected experiments. Longer experiments that require a large ns//, such as the study of dilute solutions (proton detection) or heteronucleus detection, can make good use of interleaved acquisition with a suitable block size (as described in the discussion of the DEPT experiment in Section 7-2b). 

In recent years, gradient versions of many of the basic 2D NMR experiments have become very popular. One of the main reasons is that the use of gradients eliminates the need for phase cycling in the selection of a coherence pathway. Experiments involving detection can, therefore, often be performed with one to two transients per increment. 

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We have mentioned repeatedly that NMR spectroscopists are engaged in a seemingly never-ending conflict between sensitivity, on the one hand, and both time and resolution, on the other. Nowhere are these problems more acute than in the t dimension of 2D experiments. There must, of course, be a sufficient number of scans per time increment to observe a spectrum, but there must also be enough increments to resolve closely situated signals. Both requirements take precious spectrometer time, and spectroscopists are forced to compromise between sensitivity and resolution in a number of 2D experiments. 

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