The FID is a complex data set comprising the signals from the detectors along both the x and y axes:
S = Sx + i Sy
(i represents the square root of -1)
The Fourier transform also produces a complex spectrum. The real component of the spectrum is the absorption spectrum, which shows peaks at the Larmor frequencies for the various spins. The width of the peak is determined by the effective T2 value: T2*.
w = ( π T2* )-1
The width of the peak (w) is characterized by measuring its full width at a height half way between the baseline and the peak. (FWHM = full width at half maximum).
The imaginary component of the spectrum is the dispersion spectrum, which is the derivative of the absorption spectrum.
This behavior is shown in the graph below. Enter a value for T2* between 0.01 and 1 sec. Select with the absorption or dispersion signal or both and observe the shapes of the peaks.
If the acquisition of the FID begins with the bulk magnetization perfectly aligned along the y axis at t = 0, then the Sx signal (a cosine function) begins at a maximum and the Sy signal (a sine function) begins at zero. The result is that the Fourier transform produces the pure absorption signal as the real component of the spectrum and the pure dispersion signal as the imaginary component of the spectrum, as described above.
If the bulk magnetization does not begin perfectly aligned along the y axis at t = 0, then the absorption and dispersion spectra are mixed.
The exercise below illustrates this effect. The simulation at the left shows the bulk magnetization as viewed along the z axis, which allows one to easily observe the phase angle (the angle off the y axis) for the bulk magnetization at t = 0. There are numerous experimental issues that can cause this angle to differ from zero. The time required for a pulse is one source of a non-zero phase angle.
Answer the following questions by carefully observing the simulation and adjusting the phase correction.
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