Phase-corrected spectrum

When the phase correction has been completed, click the Accept button and a message box appears on the screen. If Yes is selected then the xf2p command is executed and the F2 phase corrected spectrum will be calculated and shown on the screen. If No is selected the new values of the PHCO and PHCl parameters are stored for future use but no phase correction is performed. 

The Forman phase correction algorithm, presented in Chap. 2, is shown in Fig. 3.6. Initially, the raw interferogram is cropped around the zero path difference (ZPD) to get a symmetric interferogram called subset. This subset is multiplied by a triangular apodization function and Fourier transformed. With the complex phase obtained from the FFT a convolution Kernel is obtained, which is used to filter the original interferogram and correct the phase. Finally the result of the last operation is Fourier transformed to get the phase corrected spectrum. This process is repeated until the convolution Kernel approximates to a Dirac delta function. 

Figure 6 shows the Fourier transformed and phase-corrected spectrum in the frequency domain. The fast Fourier transform method yields in this case 512 points with a resolution of 1 KHz. The splitting of the lines occurs due to the nuclear quadrupole interaction. 

Figure 3.11. (a) Chirped interferogram (b) phase-corrected spectrum of the source calculated from this interferogram (c) almost completely unchirped interferogram reconstructed from this spectrum. (Reproduced from [7], by permission of the Society for Applied Spectroscopy copyright 1974.)... 

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. 

The next step after apodization of the t time-domain data is Fourier transformation and phase correction. As a result of the Fourier transformations of the t2 time domain, a number of different spectra are generated. Each spectrum corresponds to the behavior of the nuclear spins during the corresponding evolution period, with one spectrum resulting from each t value. A set of spectra is thus obtained, with the rows of the matrix now containing Areal and A imaginary data points. These real and imagi-... 

If phase-sensitive spectra are not required, then magnitude-mode Pico) (or absolute-mode ) spectra may be recorded by combining the real and imaginary data points. These produce only positive signals and do not require phase correction. Since this procedure gives the best signal-to-noise ratio, it has found wide use. In heteronuclear experiments, in which the dynamic range tends to be low, the power-mode spectrum maybe preferred, since the S/N ratio is squared and a better line shape is obtained so that wider window functions can be applied. 

Phasing A process of phase correction that is carried out by a linear combination of the real and imaginary sections of a 1D spectrum to produce signals with pure absorption-mode peak shapes. 

For several technical reasons, it is not possible to acquire NMR data with perfect phase. One reason is the inability to detect XY magnetisation correctly another is the fact that we are unable to collect the data as soon as the spins are excited. These limitations mean that we have to phase correct our spectrum so that we end up with a pure absorption spectrum. What we don t want is a dispersion signal (see Spectrum 4.2). 

Take a moment to survey the spectrum and ask yourself if it is fit for purpose Of course, if you have run it yourself, then it should be fine but this may not always be so with walk-up systems. Is the line shape and resolution up to standard Has the spectrum been phased correctly Is the vertical scale well adjusted so that you can see the tops of all the peaks (except perhaps, obvious... 

FIGURE 14.5. FTIR spectra for CO adsorption on Au72Pt28/Si02 catalyst calcined at 400°C under two different sample preparation conditions (A and B). N2-purging times 0 min (the spectrum was corrected by subtracting the gas-phase CO spectrum) (A), and 1 min (top) and 52 min (bottom (the gas-phase CO is absent)) (B). 

Carries out a phase correction on your spectrum using the 0 and the T order parameters last defined during interactive phase correction (see chapter 5). 

The basic processing of ID and 2D data requires obligatory processing steps for transforming the raw data (FID) into a "readable spectrum, i.e. Fourier transformation and phase correction to produce a spectrum with absorptive lineshapes. Finally, a few additional step.s (calibration, peak picking, integration) as discussed in chapter 4 are required before the spectrum is eventually plotted. 

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. 

In reality the individual lines obtained after the Fourier transformation are composed of both absorptive A(f) and dispersive D(f) components. This non-ideality arises because of a phase shift between the phase of the radiofrequency pulses and the phase of the receiver, PHCO, and because signal detection is not started immediately after the excitation pulse but after a short delay period A. Whereas the effect of the former is the same for all lines in a spectrum and can be corrected by a zero-order phase correction PHCO, the latter depends linearly on the line frequency and can be compensated for by a first-order phase correction PHCl. Both corrections use the separately stored real and imaginary parts of the spectrum to recalculate a pure absorptive 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. 

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. 

Load the raw data of the ID H experiment measured for peracetylated glucose D NMRDATA GLUCOSE 1D H GH 002001.FID and perform a Fourier transformation. Use either the FT button in the button panel or from the Process pull-down menu choose the FT option. In the DC Correction dialog box click on the No button. Note that the calculated spectrum is incorrectly phased. Use the dual display option to compare this spectrum, showing the ring protons, with the correctly phased spectrum D NMRDATA GLUCOSE 1 D H GH 002999.l R. Exit the dual display and from the Process pull-down menu choose the Phase Correction option. The... 

For 2D data which need no phase correction the PHmod parameter in FI, available in the General parameter setup dialog box opened via the Process pull-down menu, must be set to me if a magnitude, or to ps if a power spectrum should be calculated. Use the Help tool for more informations. 

To phase either the columns or rows, click on the appropriate button in the button panel. A cross-hair cursor appears in the display field, and the Chi button is highlighted indicating that the trace for the first channel is to be selected. The spectrum display area is internally divided into three parts, corresponding to three different channels (columns or rows) that can be chosen to perform the phase correction. Select appropriate columns or rows from the 2D matrix by moving the cursor to the desired position and clicking... 

If you are satisfied with the result of this correction in F2, select suitable columns for phasing the 2D spectrum in Fl. Follow the same procedure as outlined for rows. The xflp command will appear after clicking the Accept button to initialize the phase correction in Fl. 

Note that with a non-DQ-filtered, phase sensitive COSY experiment the cross peaks are again purely absorptive while diagonal peaks irrespective of the phase correction will have both absorptive and dispersive character. Unlike most other 2D spectra, it is therefore best to phase correct a non-DQ-filtered phase sensitive COSY spectrum while examining the cross rather than the diagonal peaks. 

Load the raw data obtained for peracetylated glucose with the 2D TOCSY experiment D NMRDATA GLUCOSE 2D HH GHHTO 001001.SER and perform a 2D FT following the guidelines given above. Enter the Manual phase correction option in the Process pull-down menu and perform a phase correction in F2 and Ft according to the procedure outlined above. Try to phase all peaks to positive absorption and store the spectrum (... 001001. RR). 

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