Baseline distortions

Wlien is very short, which is almost always true with nuclei having/> 1/2, the dipolar contribution to relaxation will be negligible and, hence, there will be no contributions to the integral from either NOE or saturation. However, resonances more than about 1 kHz wide may lose intensify simply because part of the FID will be lost before it can be digitized, and resonances more than 10 kHz wide may be lost altogether. It is also hard to correct for minor baseline distortions when the peaks themselves are very broad. 

The pulse sequences employed in semisoft experiments are shown in Fig. 7.11. The last two sequences are preferred, since the baseline distortions and distortions due to large couplings in multiplets are suppressed. 

Baseline distortions. The relative motion of source and absorber causes periodic variations of the solid angle d and consequently of the incident count rate as a... 

Fig. 3.10 Variation of the spectrometer aperture as a function of the source motion for Mossbauer spectrometers operated in constant acceleration mode with triangular velocity profile, and the resulting nonlinear baseline distortion of the unfolded raw spectra. For simplicity a point-source is adopted, in contrast to most real cases (Rib mm active spot for Co in Rh)...

The primary objective of preprocessing treatments is to remove the nonchemical biases from the spectral information. Scattering effects induced by particle size or surface roughness may lead to offsets or other more complex baseline distortions. Differences in sample density or the angle of presentation may induce overall intensity variations as well as additional baseline distortions. Most samples are not perfectly uniform on the microscopic scale, and these effects may dominate the initial contrast observed in an un-processed chemical image. In some cases this contrast may provide useful information to the analyst however, it is generally removed in order to focus on the chemical information. 

Davis, D.G. 1989. Elimination of baseline distortions and minimization of artifacts from phased 2D NMR spectra. J. Magnetic Resonance 81 603-607. 

Moreover, the low magnetogyric ratio results in very low resonance frequencies and accentuates the effects of spurious signals in the first part of the FID. These signals cause severe baseline distortions in the FT spectrum, thus preventing a precise measurement of spectral parameters (chemical shift, line width and signal amplitude), especially in the case of broad resonances. 

In acquiring 33S FT NMR spectra, further problems arise from the presence of spurious transitory signals at the beginning of the FID that cause baseline rolling in the FT spectrum. These baseline distortions prevent the sufficiently precise measurement of NMR parameters and sometimes the unambiguous identification of resonances. This problem is common to all fast-relaxing nuclei with low... 

Quantitative analysis is possible with good accuracy if the 33S spectra are characterized by a good S/N and if the signals of the species to be determined are well resolved. It must be considered that in 33S NMR spectra, baseline distortions can seriously endanger the final results. 

Other pulse sequences have been derived to remove the baseline distortions introduced by deadtime and ringing. These are detailed and compared extensively in the article by Geronathassis.17 The principle of most of these approaches is that signal and artefacts show differing coherence with the... 

These examples illustrate the source and size of the experimental errors on a two- and a four-site system. They also highlight the importance of estimating the errors, because mechanistic interpretations often require fairly precise estimates of the rate parameters involved. If care is taken to minimize the noise in the spectrum (through signal averaging) and if the electronics of the system are set up to minimize baseline distortion, relative errors <25% should easily be obtained in even complex multisite systems. 

Applications of NMR spectroscopy to structural, thermodynamic, and dynamic processes have been described. A brief discussion of the types of problems appropriate for study by this technique has been included. H and 13C NMR spectroscopy has been applied to define the ligand coordination in complexes. These experiments, combined with 170-labeling experiments, allowed deduction of the coordination number of the vanadium atom. Integration of NMR spectra allowed measurement of the formation constants and equilibrium constants. 2D 13C and 51V EXSY experiments were used in a qualitative and quantitative manner to examine intra- and intermolecular dynamic processes, of which several examples are discussed. The interpretation of the rate matrix and its relationship to the chemical processes under examination were also described. 2D EXSY spectroscopy has great potential as a tool with which to probe mechanisms in complex reactions however, such uses often requires estimation of errors. The major source of error in 2D 51V EXSY NMR studies on a two- and four-site vanadate system were found to be baseline distortion and the errors were estimated. Our results suggest... 

A very similar sequence is the ll sequence, which actually belongs to a family of binomial pulse sequences. Of these sequences, the 1331 (often referred to as 1—3) is the most popular due to its wider water suppression window (see Figure l).22 Due to the frequency-dependent sine modulation of the resonance amplitudes (the so-called excitation profile ), the resonances on either side of the water signal (carrier frequency) have opposite signs. In addition, the binomial sequences suffer from baseline distortions due to a strong linear phase gradient (see Figure 1). This baseline distortion can be particularly troublesome in multidimensional experiments and therefore the binomial sequences have not proved popular for multidimensional NMR studies. 

It should be noted that when using these sequences there is a dead time between the last pulse and the start of the acquisition (due to instrumental limitations). This can, in some cases, lead to baseline distortions as some of the initial points of the free induction decay (FID) may be lost. Such distortions can effectively be removed by (1) introducing a short delay before the final 180° pulse, (2) incorporating a spin echo at the end of the sequence,25 or (3) spectral processing methods.26... 

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