Benchtop NMR

Off-topic conversations and chit-chat.

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Trying to determine if looking at benchtop NMR for metabolomic studies on fruit juice adulteration is a worthwhile endeavor. Bruker, who only makes large models, has an extensive database that I believe is built on a 400mHz model which costs a lot, uses cryotanks, and is described as a handful in terms of maintenance. Wondering if a 60mHz or 80mHz benchtop would produce data that would align.

I do have a call in with them but while I'm waiting thought I'd take a chance and ask here.

If anyone has experience here would appreciate any insights
The only one we had when I was in college was a Varian 60mhz floor model about 2'x2' on casters about 3'tall.

I think 60mhz can only do Proton NMR, the 400mhz will do C13 also but those normally require liquid Hydrogen or Helium for cryo, very expensive. If you can get by with only Proton then maybe a benchtop model would work.
The past is there to guide us into the future, not to dwell in.
Sensitivity and resolution of any 80 MHz console are no match for a 400 MHz NMR.

So the main question is: What molecular weights are you looking into?
As a rule of thumb, smaller molecules are easier to observe.

Next question would be pulse sequences: Are you looking into 1D-1H NMR or some 2D spectra as well?

Another question is measurement time: You can get S/N down quite a bit with a sufficient number of scans, so if you can live with measurements over night, you have more options.

You also have to consider relaxation times: If you have aromatic systems, those can become quite significant.

We have a 400 MHz NMR available btw.
Someone at work has one, although I've not seen any data from it. It belongs to an organic synthesis group, and I think they for "quickie" protons while running reactions.

We actually have 400mhz, 500mhz, and 700mhz in the building(along with a home built solid state magic angle NMR with a 500mhz Oxford magnet). The 700mhz is pretty much exclusively reserved for the biochemists and only made available to everyone else when one of the others is down, but between teaching labs and research groups the 400 and 500 can get plenty busy.

FWIW, I learned NMR on an Anasazi Instruments 90mhz FTIR that was built around a Varian permanent magnet(I think from the 1970s)-the magnet was roughly the size of a washing machine. We almost exclusively used neat liquids and VERY concentrated solutions-I don't recall whether or not the instrument had a separate lock channel, but if it did we didn't use it. We'd instead put TMS in every sample, and bought solvents with 1% TMS already added.

We could do carbon and even some limited 2D work on that instrument(and I did it) although carbon in solution would sometimes require a crazy number of scans. I even used Cr(ACAC) a few times to reduce the experiment times to a few hours long. Even that could cause issues due to the lack of a lock signal as you could get peak broadening due to field drift in that time. Even on a fairly clean spectrum, splitting was "tight" and certain types of molecules(esp. alkanes) would give something that was better described as a "distorted singlet" than a true doublet or triplet. I recall running NOESY, I think, a few times, and the software(which was written by someone with a very dry sense of humor) would prompt "Enter T2 Relaxation Delay. If you have not performed a T2 relaxation delay experiment, what follows is a waste of time."

It met our needs well as a teaching/light research duty instrument. For heavy research use, we had a contact at the University of Kentucky that would run samples on a high field instrument if we needed them.

One last thing-on the 90mhz instrument, it was second nature to put a sample in and check to make sure it was spinning by looking down the magnet bore. Of course, that's really needed, but it also brings in spectral artifacts like spinning sidebands. You can't really look down the bore of a typical superconducting magnet, and when I started using them and would try to spin samples I would find that it had been so long since it had been done on ours that the spinning pneumatics wouldn't work. I talked to our staff NMR spectroscopist, who wasn't overly concerned about the fact that they wouldn't spin since "that causes other problems in your spectrum" and "isn't really necessary."
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