Evolution detection

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.

ACCORDION-evolution, detection period with/without decoupling... 

In the usual preparation-evolution-detection paradigm, neither the preparation nor the detection depend on the details of the Hamiltonian, except in special cases. Starting from equilibrium, a hard pulse gives a density matrix that is just proportional to F. The detector picks up only the unweighted sum of the spin operators, /. It is only during an evolution (perhaps between sampling points in an FID) that these totals need be divided amongst the various lines in the spectrum. Therefore, one of the factors in the transition probability represents the conversion from preparation to evolution the other factor represents the conversion back from evolution to detection. 

The common feature of the 1 D multipulse experiments described above was the time sequence preparation-evolution-detection, whereby the detected signal is only a function of the detection time t2- The important difference in 2D NMR is that the evolution time t is now a variable. In a 2D experiment n separate experiments are performed with incremented values of /]. For each expieriment a free induction decay S(tz) is measured. In this way a matrix S t. t2) S built up. 

Figure 17. Tau-suppression effects in ESEEM spectra and their potential use in identifying and correlating peaks. In a semi-classical model, the procedure entails the locking of the temporal aspects of the pulse sequence, that is, the preparation-evolution-detection re-gionsfsee Ponti Schweiger, 1994) to the precession frequency of a specific ENDOR resonance. One thereby renders the ENDOR transition transparent to the echo modulation inter-ferogram.

The monitoring of carbon-14 labeled carbon dioxide evolution detected by scintillation counting is a very sensitive method but requires the expensive synthesis of radiolabeled test substrates. It is closely related to the ultimate degradation of the test material. H-PE is also used in biodegradation testing [121]. Rate of biodegradation is measured by CO2 production relative to that of compounds of similar chemical structure produced in nature which are known not to accumulate in the environment [117]. 

The experiment was performed inside a UHV chamber with a dynamic atmosphere of 8 X 10" mbar hydrogen. The result is shown in Fig. 2.17, in which the water evolution detected by a QMS is compared to the evolution of the metallic character of the surface, as expressed by the intensity at the Fermi edge monitored by He I UPS (for the shape of the whole spectra, see Section 2.7). The water evolution curve indicates two steps in the reduction process, but only the second step leads to the formation of metallic iron in the region near the surface. This evidence provides further strong support for the suggested two-step nucleation mechanism found for the reduction of magnetite in its modified form, which is now suggested as a model for the activation of the ammonia synthesis catalyst. 

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