FID and Fourier Transform
Definition
The free induction decay is the time-domain signal emitted by relaxing nuclei after the RF pulse. A Fourier transform converts the FID into the conventional frequency-domain spectrum (intensity versus chemical shift).
- FID definition
- Time-domain signal from relaxing nuclei
- Trigger
- Emitted after the RF pulse
- Transform used
- Fourier transform
- Output
- Frequency-domain spectrum, intensity vs chemical shift
Common questions
Why is a Fourier transform necessary rather than reading the FID directly?+
The raw FID is a decaying oscillation in time that is not directly interpretable, so the transform decomposes it into the frequency components that correspond to the distinct chemical shifts chemists actually read on an NMR spectrum.
What does the shape of the FID tell an operator before transformation?+
Its decay rate reflects relaxation times and signal-to-noise, so a rapidly decaying or noisy FID warns of a weak or poorly shimmed sample before the transform is even applied.
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