Selective pulse experiments, systematic

Two additional systematic errors may arise in selective pulse experiments associated with pulse selectivity and relaxation during excitation. Pulse... 

Syvitski et al43 reported a 3D sequence which uses TPPI to separate the spectra of different coherence orders. The pulse sequence employs a non-selective 2D excitation sequence (as in Fig. 2) but also systematically increments the phase of the first two pulses in the excitation sequence by A

) in the experiment is arbitrary but needs to be at least 2TV+ 1, where TV is the number of spins in the spin system. The 3D interferogram is then a function of t, Fourier transformation over these dimensions6,30,44 affords MQ spectra of various orders cleanly separated in the pseudo-frequency ... 

Selective experiments enabling one to focus on a region of interest within a molecule are extremely useful becau.se of the potential for the exploitation of information present in their fine structures. Many could also be applied in a systematic manner to different spectral regions of given molecules, provided they can be acquired quickly and are suitable for automatic computer processing. Many different experimental schemes have been proposed. The DAISY experiment proposed by Friedrich etal. enables one to determine whether there are two remote spins in the same coupling network. The same authors proposed the use of. soft pulses in order to simplify three-dimensional experiments. 

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. 

The preparation period involves preparing the reservoir of nuclei in a known state by using T processes or saturation pulses. The evolution period, te, (including the mixing period for some 2D experiments) is the time in which the spins are subjected to a preselected interaction as a result of designed pulse sequences. The evolution time is systematically incremented throughout the course of the experiment to accommodate the desired spin interactions. During the detection period, t, a second selected interaction or combination of interactions occurs. Double Fourier transformation yields a 2D NMR spectrum with two frequency axes... 

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