Automatic phase correction

Fig. 5.5.5 1 D CSI datasets showing the extent of conversion during a batch reaction. The form of the feature identified as peak B is associated with a single chemical shift i.e., it is of constant form at all positions across the bed, and therefore shows that the extent of conversion is uniform throughout the bed. The low intensity horizontal streaking" effect observed in these datasets and that shown in Figure 5.5.6 are artifacts arising from the automatic phase correction applied to the data ...

With the Autom. Phase Correction option selected from the Process pull-down menu (Fig. 5.4) a fully automatic phase correction can be performed. 

The zero and first-order phase correction parameters may be modified manually either by entering numerical values in a dialog box (Numerical button), or in an interactive way, using the Zero Order and First Order buttons and their corresponding slider box. With the Automatic button a fast and rough phase correction is performed which speeds up the subsequent manual fine adjustments. This automatic phase correction is not the same as the fully automatic (and more time consuming) phase correction mentioned above. 

The constants A and B may be determined from a display of the spectrum on an oscilloscope, and entered into the computer either by teletypewriter,155 or directly, by digitization of the analog voltages from several potentiometers.160 Automatic phase-correction programs... 

Fig. 3. An example of the automatic phase correction method as described by Witjes and co-workers In (a) are five representative spectra for a small expanded region in a H NMR spectra data set containing 15 total spectra. Note the small differences in phase shift between the spectra, most notable in the central spectrum. In (b) the same NMR spectra are shown following three iterations of the phase and frequency correction. The (c) first five initial loadings (P for the NMR data set reveal that the first loading (Pi) is a good descriptor of the desired spectral line shape, while higher loadings (P2 and P3) contain the majority of the information concerning phase shifts and frequency shifts between spectra in the data set.

Computationally important properties of the Fourier analysis were discussed by Ernst at quite an early stage. For example, when combined with Hilbert transforms, the Kramers-Kronig relation between the real and imaginary components of complex data makes it possible to apply post-acquisition phase corrections. Ernst made some proposals for an automatic phase correction algorithm, but this kind of method was only routinely used after the developments made by Levitt and Freeman. - Note that compared with phase problems, the use of power... 

At the end of the 2D experiment, we will have acquired a set of N FIDs composed of quadrature data points, with N /2 points from channel A and points from channel B, acquired with sequential (alternate) sampling. How the data are processed is critical for a successful outcome. The data processing involves (a) dc (direct current) correction (performed automatically by the instrument software), (b) apodization (window multiplication) of the <2 time-domain data, (c) Fourier transformation and phase correction, (d) window multiplication of the t domain data and phase correction (unless it is a magnitude or a power-mode spectrum, in which case phase correction is not required), (e) complex Fourier transformation in Fu (f) coaddition of real and imaginary data (if phase-sensitive representation is required) to give a magnitude (M) or a power-mode (P) spectrum. Additional steps may be tilting, symmetrization, and calculation of projections. A schematic representation of the steps involved is presented in Fig. 3.5. 

Clicking with the right mouse button on the FT button opens a dialog box for activating and performing a 5th order phase correction, together with the FT. This automatically corrects non-linear phase distorsion in the spectrum, introduced by electronic filters. With the available data this correction is not necessary and its application produces no effects in the final spectrum. 

If for a series of spectra the phase correction values PHCO and PHCl are the same and if these values have been determined, either automatically or manually (see below) for one spectrum, then the phasing of the remaining spectra may simply be accomplished with the Phase Corr. button in the button panel (Fig. 4.2). Clicking with the right mouse button on this button, opens a dialog box to inspect the phase mode and to inspect/adjust the correction values PHCO and PHCl before the correction is initialized with the Execute button in this dialog box. 

With the command Phase Correction in the Process pull-down menu the button panel is switched into Phase mode (Fig. 5.4) and a semi-automatic or fully manual phase correction may be performed. 

The phase correction parameters PHCO and PHCl are always based on the initial spectrum obtained directly after a Fourier transformation operation. Phasing is performed with respect to a reference point which is marked on the spectrum by the spectrum cursor. When the Phase mode is first entered, the program automatically places the spectrum cursor on the tallest point in the spectrum, but you are free to move this spectrum cursor anywhere in the spectrum using the Maximum Cursor or the Perpendicular Cursor option. 

Attention With spectra measured on spectrometers equipped with digital filters (DMX, DRX spectrometers), the automatically performed phase correction (DMX Phase Corr.) will be applied twice when the newly created FID is Fourier transformed again. This will introduce the baseline roll characteristic for the data of these type of spectrometers. A first order phase correction must then be performed manually by setting the PHCl value close to -22000 for the data available in the NMR data base. 

Correction, Window Function (Exponential LB = 1.0 Hz) and FT. In the frequency domain select Phase Correction (6th Order), Peak Picking (positive Peaks only X Range whole Spectrum) of the whole region. Save Spectrum (set Processing Number Increment = 1) and Plot Spectrum (set the plot parameters according to your preferences). Execute the automatic processing and if you are satisfied with the result, store this job for processing 1D C raw data as C.JOB. 

Step 3 The Zero Order correction is automatically selected, the phase of the reference signal is adjusted using the horizontal slider button in a similar manner to phasing a ID spectrum. Clicking on the First Order button switches to the first-order phase correction which is performed in an analogous way. When the spectrum is correctly phased the phase correction is started using the Accept button. 

For manual phase correction, the value of the intercept and the slope of the phase correction or equivalent parameters such as the phases at the two ends are entered by the operator into the computer after the FT is performed and the resulting spectrum inspected for absorption mode lines everywhere. Alternatively, one can run a single line spectrum at various frequency offsets and empirically determine the phase correction for each location in the spectral window. Either of these alternatives must be performed with a model sample having narrow symmetric lines before a sample having broad and possibly asymmetric lines is looked at. As the phase shift is largely an instrument determined parameter, once the phase correction has been determined, it should remain the same for all spectra taken under the same conditions except for minor adjustments. The correction can be stored in the computer and applied automatically. Metallic samples are exceptions to this rule because the electrical conductivity and thus the skin... 

Figure 10.46. The Raman spectrum of sulphur in the spectral range 100-280 cm calculated by Fourier transformation of the interferogram top, no apodization (boxcar) bottom, apodization function Norton-Beer weak. In both cases, a zerofilling factor of 2 and the power spectrum for phase correction were chosen. Further parameter used Store page selected frequencies for file first 9394 and last 5894 Limit data page limit resolution to 4 cm, limit phase resolution to 32 cm, direction both, data points both Peak search page mode absolute largest value, symmetry of the interferogram automatic.

The chemical shift, amplitude, phase, and width (or damping factor) are estimated for each peak. In the frequency domain, it is common to phase the spectrum by applying constant and linear phase corrections, determined either automatically or manually. 

Determine the discharge device for the discharge of a three-phase 6.6 kV, 50 Hz, 1000 kVAr, yconnected capacitor bank, connected in units of 10 x 100 kVAr each, through an automatic p.f. correction relay, having a closing cycle of 10 seconds. Data available from the capacitor manufacturer,... 

As you are no doubt aware, integrals are one of the key parameters in the interpretation of proton spectra and are pivotal in quantification. They measure the area under a peak and this is directly proportional to the number of protons (in the case of proton NMR) in that environment. Most software will automatically try to identify the peaks in your spectrum and integrate them for you. If you need to do it yourself, then it is a fairly trivial matter of defining the start and end point of the integrals of interest. The only complication is that you may need to tweak the slope and bias of the integral. This should be unnecessary if you have got the phase and baseline of your spectrum correct. If you find that you need to adjust slope and bias, we suggest that you go back and try to sort out baseline and phase a bit better. 

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