Phase Correction and Spectral Presentation

If the phase-sensitive detectors are adjusted to give a phase angle (Eq. 3.8) ( j — 4 r ) = 0, the real part of the FT spectrum corresponds to pure absorption at the pulse frequency, but off-resonance lines display phase angles proportional to their off-resonance frequency as a consequence of limited rf power and nonzero pulse width (Eq. 2.55). However, acquisition of data as complex numbers from the two phase-sensitive detectors and subsequent processing with a complex Fourier transform permit us to obtain a spectrum that represents a pure absorption mode. 

Because the real and imaginary parts of the spectrum always have exactly a 90° phase relative to each other at each resonance frequency, it is easy for the 

The process of phasing the spectrum consists of choosing the coefficients of real and imaginary parts to optimize the appearance of the overall spectrum, either by manual adjustment or by application of one of several algorithms. 

For some purposes in 2D NMR it is sufficient to obtain only the absolute value mode, that is, the square root of the sums of the squares of absorption and dispersion. The absolute value mode is independent of the phase of the signals, hence is simpler to compute and avoids the need for interactive phasing. However, as a composite of absorption and dispersion, it includes the long wings characteristic of the dispersion mode (see Fig. 2.10) and thus has substantially greater overlap of lines than pure absorption. 

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