Ciro Santilli once visited the chemistry department of an university, and the chemists were obsessed with NMR. They had small benchtop NMR machines. They had larger machines. They had a room full of huge machines. They had them in corridors and on desk tops. Chemists really love that stuff. More precisely, these are used for NMR spectroscopy.
Basically measures the concentration of certain isotopes in a region of space.
Video 1. Introduction to NMR by Allery Chemistry. Source.
  • only works with an odd number of nucleons
  • apply strong magnetic field, this separates the energy of up and down spins. Most spins align with field.
  • send radio waves into sample to make nucleons go to upper energy level. We can see that the energy difference is small since we are talking about radio waves, low frequency.
  • when nucleon goes back down, it re-emits radio waves, and we detect that. TODO: how do we not get that confused with the input wave, which is presumably at the same frequency? It appears to send pulses, and then wait for the response.
Video 2. How to Prepare and Run a NMR Sample by University of Bath (2017) Source. This is a more direct howto, cool to see. Uses a Bruker Corporation 300. They have a robotic arm add-on. Shows spectrum on computer screen at the end. Shame no molecule identification after that!
Video 3. Proton Nuclear Magnetic Resonance by Royal Society Of Chemistry (2008) Source. Says clearly that NMR is the most important way to identify organic compounds.
Video 4. Introductory NMR & MRI: Video 01 by Magritek (2009) Source. Precession and Resonance. Precession has a natural frequency for any angle of the wheel.
Video 5. Introductory NMR & MRI: Video 02 by Magritek (2009) Source. The influence of temperature on spin statistics. At 300K, the number of up and down spins are very similar. As you reduce temperature, we get more and more on lower energy state.
Video 6. Introductory NMR & MRI: Video 03 by Magritek (2009) Source. The influence of temperature on spin statistics. At 300K, the number of up and down spins are very similar. As you reduce temperature, we get more and more on lower energy state.
The equation is simple: frequency is proportional to field strength!
Used to identify organic compounds.
Seems to be based on the effects that electrons around the nuclei (shielding electrons) have on the outcome of NMR.
So it is a bit unklike MRI where you are interested in the position of certain nuclei in space (of course, these being atoms, you can't see their positions in space).
Video 1. What's Nuclear Magnetic Resonance by Bruker Corporation (2020) Source. Good 3D animatinos showing the structure of the NMR machine. We understand that it is very bulky largely due to the cryogenic system. It then talks a bit about organic compound identification by talking about ethanol, i.e. this is NMR spectroscopy, but it is a bit too much to follow closely. Basically the electron configuration alters the nuclear response somehow, and allows identifying functional groups.
Using NMR to image inside peoples bodies!
Video 1. How MRI Works Part 1 by thePIRL (2018) Source.

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