Spectral processing

A pre-requisite for the successful extraction of key NMR parameters from an experimental spectrum is the way it is processed after acquisition. The success criteria are low noise levels, good resolution and flat baseline. Clearly, there are also experimental expedients that can further these aims, but these are not the subject of this review per se. In choosing window functions prior to FT, the criteria of low noise levels and good resolution run counter to one another and the optimum is just that. Zero filling the free induction decay (FID) to the sum of the number acquired in both the u and v spectra (in quadrature detection) allow the most information to be extracted. 

Peak-picking appears to be a domain largely occupied by instrument manufacturers. As a consequence, many of the algorithms remain unpublished. 

The partial convolution of two peaks poses special problems and with high signal to noise, it is easy to see how the first derivative would help this process. Differentiation accentuates the noise, so this will not be a universally successful procedure. 

The role of peak-picking is to return only one position per peak, and in the presence of noise, this again poses problems. It is likely that most algorithms will start from a position of intensity maximum, and then search either side for the first point significantly lower, i.e., a multiplier of the noise. This defines a new peak region and the process could then be repeated. Some software additionally offers the facility of a horizontal threshold, above which values are returned and below which they are not. 

Zero filling the FID more than a factor of two does not contribute to information extraction and any features revealed by this are artefacts. In most instances, zero filling by a factor of two amounts to an interpolation procedure benefiting primarily peak-picking. There are other procedures which can allow peak-picking interpolation between data points and the one used by the author is a simple equation to fit the maximum intensity and one point either side to a parabola and compute the position of its maximum. Bruker peak-pick table positions for instance are not separated by a multiple of the digital resolution and it would seem that they use the same or an analogous procedure. 

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The new instrument introduced for inspection of multi-layer structures from polymeric and composite metals and materials in air-space industry and this is acoustic flaw detector AD-64M. The principle of its operation based on impedance and free vibration methods with further spectral processing of the obtained signal. 

J. L. Campbell, W. Maenhaut, E. Bombelka, E. Clayton, X Malmqvist, J. A. Maxwell, J. Pallon, and J. Vandenhaute. Nucl Instr. Meth. Phys. Res. Bl4,204, 1986. Comparison of five PDCE spectral processing techniques. 

Automatic spectral processing, spectral analysis, spectral prediction and structural confirmation elucidation have been reviewed. and two-... 

Figure 6.2 Common sampling configurations for spectral process analyzer systems.

All the spectral processing operations, change of scale from cm-1 to pm were made through the Omnic software of our spectrometer. 

Figure 15.7 Examples of spectral processing. Acquisition parameters listed in Table 15.1 were used to measure a solution of tempol. The background spectrum was collected using water, which shows a feature due to a cavity defect.

Spectral Processing in the Auditory Cortex Mitchell L. Sutter... 

Spectral Processing In Cochlear Implants Colette M. McKay... 

A novel spectral processing approach based on information theory and the matrix pencil method was proposed by Lin et al.55 that enhances sensitivity and restores the baseline and phase properties, and hence is particularly useful, for example, for delayed acquisition experiments such as SPEDA spectra. 

The contribution of water absorption in solution spectra can be more than 99% of the total signal. The technological sophistication and high sensitivity of modem FTIR instruments make it possible to extract protein spectra from solution data however extreme care must be exercised to insure accuracy. Frequently FTIR data processing is not done with sufficient rigor, and hence flawed analyses have appeared in print. As a part of this work, a careful exploration of the various aspects of data collection, spectral processing, and structural analysis was performed in order to develop a reproducible and reliable protocol. The complete protocol is presented in the following sections. 

Fig. 7.3 (a) 2D Brillouin zone of graphene showing characteristic points K and T and Dirac cones located at the six comers (K points), (b) Second-order double resonance scheme for the D peak (close to F) (c) Raman spectral process for the D peak (involving two neighboring K points of the Brillouin zone K and K ). El is the incident laser energy. (After Ref. [46, 48])... 

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... 

Delaglio F, Grzesiek S, Vuister GW, Zhu G, FTeifer J, Bax A (1995) NMRPipe a multidimensional spectral processing system based on UNIX pipes. J Biomol NMR 6 277-293... 

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