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We are in process of designing an NMR experiment for our physics lab. The equipment has been set and we obtained resonance frequency for some samples. The setup is a bit rudimentary with a very small frequency range available (16 to 22 MHz). Now I know we can do a bunch of things with a NMR signal with regards to sample detection and analyzing the nature of the compound but that would take it more towards chemistry or physical chemistry. I want to keep it more towards physics. So, I have two queries in this regards:

  1. When we obtained the signal on an oscilloscope (picture attached), in the XY mode, the x axis port comes from the magnetic field and it appears that it is the strength of the oscillating magnetic field.enter image description hereThe Y axis in the oscilloscope comes from the NMR probe. But I can't seem to find what the NMR probe actually measures. The output must be voltage as a function of that signal but I cant understand what is it measuring. In an ESR (electron spin resonance) setup for example I have seen that when the resonance frequency is set the loss of energy from the oscillating field causes the impedance of the field to change and that is what is being measured. Is it the same for NMR?

  2. In an ESR if you use DPPH (DiPhenyl Picryl Hydrazyl) as a sample and measure the gyromagnetic ratio of the electron, since there is only one unpaired electron in the entire molecule the g-factor of the electron comes out to be very close to that of a free electron. I was wondering, is it possible to find a compound where there is only a free proton (primarily) and if that compound could be used to measure the approximate g-factor of a free proton just like in the case of ESR of DPPH? And if yes, then which material is recommended?

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  • $\begingroup$ What is the sample? What time per division? What magnetic field? Is this pulse NMR? $\endgroup$
    – user137289
    Commented Dec 28, 2016 at 13:55
  • $\begingroup$ I can procure almost anything. Currently I have water, glycerine, Teflon, Polystyrene. Magnetic field can be taken at least upto 1 T. It is a continuous wave NMR. $\endgroup$
    – Shaz
    Commented Dec 28, 2016 at 13:57
  • $\begingroup$ Why is the lineshape of this secret sample not symmetrical? $\endgroup$
    – user137289
    Commented Dec 28, 2016 at 14:06
  • $\begingroup$ Because the graph is a snapshot from the oscilloscope where the y axis comes from the NMR probe and the x axis has the fluctuating magnetic field. At least I think that is the reason. I think you get a symmetric graph with frequency on the x axis. $\endgroup$
    – Shaz
    Commented Dec 28, 2016 at 15:55
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    $\begingroup$ Are we missing an important part of your picture? If the horizontal axis is the magnetic field strength, then you must be sweeping the field (i.e., not constant). Or are you sweeping the RF frequency on the horizontal axis? The y-axis is then going to be a response, and the squiggle you see at the middle of the screen is when the system hits resonance. without knowing the make and model of your NMR it's hard to help. $\endgroup$
    – Bill N
    Commented Dec 28, 2016 at 16:07

2 Answers 2

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1) In NMR you excite the nucelons in the matter with the magnetic field, causing them to emit e-m waves. It is the em-wave amplitude on your y-axis. Assuming you set up is standard and there isn't a problem somewhere.
It's very similar to ESR except the question of what is being excited (clue's in the respective names)

2) Theoretically hydrogen, which ionises to a proton plasma. I have no practical experience on this though, I want that to be clear. As I'm sure you know Hydrogen is explosive

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  • $\begingroup$ Thanks for the answer. I guess that would explain why the signal is decaying in the picture I posted. $\endgroup$
    – Shaz
    Commented Dec 27, 2016 at 5:44
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So, part of the answer was already given by KMLCarter. The y axis has the energy absorbed by the atoms.

The answer for second part is that any material in which only Hydrogen nuclei are present will have a gyromagnetic ratio close that of a free proton. For example the gyromagnetic ratio of a free proton is 42.5774806 MHz/T (ref: Wikipedia) and the gyromagnetic ratio of glycerine or water is quite close to that value. There is always a slight change in the values due to the additional field from surrounding atoms.

So here is what I have decided to do with the equipment. A reference material was already provided with a known gyromagnetic ratio and it will be use to calibrate the magnetic field of the equipment. Once that is done, water or glycerine will be used to measure the approximate gyromagnetic ratio of a proton. Once could try to do the same for another material to measure it for the neutron as well. Then I am thinking of using a material with a nuclear spin of 3/2. That will give more states of energy. But it still remains to see if I that can be done within limited range of frequency that the equipment has.

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