Interleaved acquisition

Artifacts may be roughly categorized into those due to inherent limitations (e.g. pulses cannot excite unlimited bandwidths even if all hardware components work perfectly) and those that result from improper set-up of the experiment or nonideal functioning of the NMR spectrometer system. In this chapter we will mainly focus on the latter two. These artifacts are more likely to appear in multiple-pulse experiments. Quite often, they are avoided by clever programming of the experiments (e.g. interleaved acquisition of data for NOE spectra, use of pulsed-field gradients instead of phase-cycling). 

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). 

It proved helpful for the purpose of noise reduction to perform relaxation experiments in an interleaved fashion, as one pseudo-3D experiment, where the 2D planes in the F2 dimension correspond to various relaxation delays. The acquisition order (3-2-1) is selected so that cycling through various relaxation delays (in R1 or R2 experiments) or through NOE/NONOE 2D planes is performed prior to incrementing the evolution period in the indirect dimension (FI) (see e.g. Ref. [16]). The resulting pseudo-3D spectrum can be processed as a set of 2D spectra in tl and t3 dimensions, and then analyzed in the usual way. This procedure reduces the noise arising from switching from one 2D experiment to the other and helps minimize temperature variations between the spectra acquired... 

Figure 11. The interleaved sampling scheme used to increase the efficiency of data acquisition in time-resolved CS spectrometer experiments (see text for details). Reproduced with permission from Ref. 86.

With a suitable instrument, the above three acquisition experiments (precursor ion scan, neutral loss scan, and MRM) can be cycled as having during all the experiment time (usually between 2 and 3 min) the interleave of each of them. 

Figure 4 (A) Left Different elementary slices subjected to the decreasing RF field produced by the gradient coil. Right corresponding nutations assuming that nuclear magnetization was initially at thermal equilibrium. (B) Left The RF gradient pulse train with interleaved detection windows leading to the acquisition of a pseudo-FID. Right the Fourier transform of this pseudo-FID yields a profile representative of the object shape (see A).

The effects t < tocm and t < team were unexpected to us because t should only be occurring while being in Degraded Mode (due to transition te). Somehow, there was an interleaving in between that caused a transition to the Orbit Control Mode or to the Earth Acquisition Mode. The effect matrices furthermore showed that other transitions in the TT C were enabled by other unexpected transitions. This indicated that our model did not handle the mutual exclusion of the TT C with other components correctly. This was corrected in our final model. 

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