NMR Spectroscopy Questions and Answers

Answer: A

Each scan runs pulse, then acquire, then wait, and the transform is applied once at the end to the accumulated sum. Transforming belongs to processing rather than to the repeated loop, which is why it comes last however many scans are taken. Getting the order right matters when setting up an experiment: the relaxation delay lies between the end of acquisition and the next pulse, so the total time per scan is the acquisition plus the delay.

Answer: B

NMR needs micrograms to milligrams where mass spectrometry works on picograms, so its sensitivity is its weak point, a direct consequence of the tiny Boltzmann excess. Against that it distinguishes isomers superbly, which mass spectrometry often cannot, it is non-destructive so the sample is recovered, and it examines an enormous range of compounds. The two techniques are complementary rather than competing, and coupling them, as in LC-NMR-MS, exploits the strengths of each.

Answer: C

The energy of a radiofrequency photon is far too small to break bonds, so the sample can be evaporated from the tube and used again, which matters for scarce natural products and for precious biological material. The energy is certainly detected, that being the whole experiment, a solvent is normally required for high resolution work, and physical contact with the magnet is beside the point. Non-destructiveness is one of the strongest practical arguments for putting NMR early in an analytical scheme.