Mixing period

After another short mixing period ( 15 min) the expl compn is poured into the desired molds. Vacuum casting has not been found necessary to obtain good density, provided that the mold design is not too intricate or the height-to- diam ratio not too great (Ref 38)... 

All these experiments involve at least three distinct time periods preparation (tp), evolution (tl), and detection (t2) these periods are usually separate by rf pulses. Some experiments (e.g., NOESY, RELAY) further contain an additional "Mixing period, tm, between the evolution and detection periods. 

Figure 3.1 The various time periods in a two-dimensional NMR experiment. Nuclei are allowed to approach a state of thermal equilibrium during the preparation period before the first pulse is applied. This pulse disturbs the equilibrium ptolariza-tion state established during the preparation period, and during the subsequent evolution period the nuclei may be subjected to the influence of other, neighboring spins. If the amplitudes of the nuclei are modulated by the chemical shifts of the nuclei to which they are coupled, 2D-shift-correlated spectra are obtained. On the other hand, if their amplitudes are modulated by the coupling frequencies, then 2D /-resolved spectra result. The evolution period may be followed by a mixing period A, as in Nuclear Overhauser Enhancement Spectroscopy (NOESY) or 2D exchange spectra. The mixing period is followed by the second evolution (detection) period) ij.

A number of 2D NMR experiments, such as NOESY, have a mixing period incorporated in their pulse sequence. In principle, precession of j magnetization also occurs during the mixing period. Why do we not need to have a third Fourier transformation to monitor the precession frequencies that occur during the mixing period ... 

D, = relaxation delay (preparation period) t ,ta,t etc = fixed mixing periods NS == number of transients per FID DS = number of preparatory dummy transients per FID ... 

Precession of > 5i-magnetization may also occur during the mixing period. This is normally a constant effect that does not vary from one FID to the next, since the mixing period is a constant time period and changes taking place during it contribute equally in every FID. 

In three-dimensional experiments, two different 2D experiments are combined, so three frequency coordinates are involved. In general, the 3D experiment may be made up of the preparation, evolution (evolution times, t and <2, and the detection time %. This is illustrated in Fig. 6.1. 

Mixing period The third time period in 2D experiments, such as NOESY, during which mixing of coherences can occur between correlated nuclei. 

Fig. 3 Experimental results measured for [U-13C, 15N]-alanine based on the MCSP approach. The polarization of C after the mixing period increases as the number of R-TOBSY mixing periods increases to four. Accordingly, the C) 1 is nullified eventually. (Figure and caption adapted from [81]. Copyright (2006), with permission from Elsevier)...

As mentioned above, 2D-NMR (or more generally multidimensional NMR) is based on the transfer of magnetization during the evolution/mixing period. 

In nutrient removal SBR systems the process phase (tP) consists of aerated period (tA) and mixed period (tM) which can be anoxic (tDN) and anaerobic (tA ) depending on presence of nitrate. Hence ... 

Fig. 10.18. IDR (Inverted Direct Response)—HSQC-TOCSY pulse sequence. The experiment first uses an HSQC sequence to label protons with the chemical shift of their directly bound carbons, followed by an isotropic mixing period that propagates magnetization to vicinal neighbor and more distant protons. The extent to which magnetization is propagated in the experiment is a function of both the size of the intervening vicinal coupling constants and the duration of the mixing period. After isotropic mixing, direct responses are inverted by the experiment and proton detection begins.

The pulse sequence for ID TOCSY is a ID modification of the original TOCSY experiment [2] introduced by Braunschweiler and Ernst. The TOCSY experiment was also referred to as HOHAHA (which stands for HOmonuclear HArtman-HAhn) by Bax and Davis [3]. The ID TOCSY experiment was proposed by Bax and co-workers [4, 5], and by Kessler et al. [6]. The essential features of the pulse sequence involve the use of selective excitation of a desired resonance, followed by a homonu-clear Hartman-Hahn (or isotropic) mixing period [2, 7]. That is, the unit -Pnonsei - in the 2D TOCSY pulse sequence is replaced by Fsei -where P stands for a pulse (or pulses), ti is the evolution period in the 2D experiment and r is a fixed delay. 

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