Challenges Associated with Earth's field NMR

Question

The ACS recently hosted a virtual conference and there was an interesting talk on NMR experiments using the Earth's magnetic field. What surprised me was the number of molecules studied with Earth's field NMR so far is around 22. Another interesting aspect was that NMR at low magnetic field strength is richer in the number of lines as compared to the extremely strong magnets used today. I think spectroscopists might be happier if they get a rich spectrum (good for finger printing).

This site has many NMR researchers, my question is why Earth's field NMR never caught up? The number of molecules studied so far is very low. Is it theoretically very challenging or the analytical sensitivity is hopelessly low?

Answer 1

1. Earth’s field is <1 gauss, so >10,000x lower than even benchtop NMR, which today is always >1 T. At such low fields there is essentially no spectral dispersion from chemical shift, so we give up the most important analytical information provided by NMR.
2. At such low fields sensitivity is extremely low, requiring a large sample volume of order 1 L, at least for inductive detection.
3. It is difficult to preserve the high homogeneity of the Earth’s field in normal laboratory and industrial environments.
4. Rare-earth benchtop magnets at 1-3 T do not cost much more than an earth’s field instrument, and they provide much more analytical power.

[disclosure: I’m in the benchtop NMR biz qmagnetics.com]

Answer 2

One question I'd ask in return is, do you expect ultralow-field NMR to replace high-field magnets? You'd likely agree that this is impossible, given how dependent other fields of research are on high-field NMR. So it's not plausible for universities or other institutions to get rid of their high-field setups and replace them with ultralow-field NMR.

Once we accept that, then the question becomes: what is the benefit of adding ultralow-field setups to an existing high-field NMR facility? Given that the low-field spectra are difficult to interpret (after all we're talking about magnetic fields several orders of magnitude smaller than what we're used to, and possibly with multiple nuclei overlapping with each other), I don't see why people would be in a rush to get their hands on low-field setups, when they can get their 500 MHz spectra and go off happy. On top of that, there's an obvious barrier to entry in terms of technical knowledge required to build and operate such a spectrometer.

I certainly don't mean to discredit the existing research that has been done on it, of course. It is not bad, or useless, to perform research on this. However, realistically speaking, and as of the time of writing, I don't see the incentive for this to be "the next big thing"... yet.

The current trend in "lower"-field NMR is probably benchtop NMR spectroscopy, and (apart from arguments like portability) that's almost certainly only because it still gives you pretty decent spectra (e.g.., it's suitable for undergraduate chemists to look at and apply the $n+1$ rule, etc.). If benchtop spectrometers were 1 kHz I doubt anybody would be buying them.