Correcting the Phase
NMR signals can appear as absorption signals (symmetric, narrow) or dispersion signals (antisymmetric, broad). The two differ by a phase angle of 90°. Most spectra require phase correction so that all signals appear in absorption mode, and iNMR contains an array of automatic algorithms that do a good job in this respect. To achieve even better results you may want to refine the phase manually.
To Correct the Phase Manually:
- Choose Process > Phase Correction.
- If the phase controls are disabled, it means that your spectrum is real. You can make it complex with the command Process > Make Complex.
- Choose the peak with the most accurate phase and put a mark on it with
⌘-click(on the Mac) or Alt Gr-click (on Windows);
you have chosen what is improperly called the pivot point.
- If you are correcting the vertical dimension, create a horizontal mark instead, with the combination option-click.
- If no peak is in absorption mode, first slowly move the big slider at the left of the panel to improve the situation, then create the mark.
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Move the large slider on the right side of the panel (first-order correction) until
the other peaks also appear correctly phased.
- If the right slider reaches the end of its range, move the central slider toward the “coarse” position.
- If, instead, the phase changes too rapidly, move the central slider toward the “fine” position.
- Every time you move the central slider, in either direction, the thumb of the right slider returns to the middle position.
- As soon as you identify a peak in pure absorption (perfectly symmetric), move the mark onto that peak.
Repeat steps 2 and 3.
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As the correction improves, you should amplify the spectrum with the plus key.
Many users try to judge the phase from the shape of the peaks. This is the hard way. The easiest and most reliable approach is to watch the baseline.
When the phase is correct, the two baseline branches at the sides of a peak meet perfectly. They may merge into a single line or form a V exactly at the base of the signal. A phase error immediately destroys this symmetry and the two branches no longer meet.
In practice, I rarely look at the peak itself. I look at the baseline.
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As the correction improves, you should amplify the spectrum with the plus key.
If more than one mark is present, or if you enter the correction values numerically, the pivot point is ignored.