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Analytical Chemistry

Picky, Picky NMR Magnets

Unless you’ve had to take care of an NMR facility, you might not have realized how many large chunks of ferromagnetic material might be moving around close to your building, and how much stray radio-frequency noise is banging around. Here’s a story on the University of Minnesota, where a research building sits right next to a light rail line, and I can easily believe that they’re having problems. A lot of folks in Cambridge and Boston can tell you stories about the trains (above and below ground) and their effects on NMR experiments.

It’s not just electromagnetic effects, of course. Good old vibration will hose things up, too, since a high-field NMR magnet needs very precise positioning of the sample and the probe for a sharp spectrum. That’s why the big magnets are always sitting on top of very expensive vibration-damping legs, and the bigger the magnet, the more impressive the technology that goes into canceling out the shakes. But radio noise is a real killer. The more machines you have, and the more nuclei you observe, the higher your chances of picking up police radios, having your observed frequencies wander into the commercial FM band (good luck there), and who knows what else.

Sometimes you can get around a specific problem by running your NMR at a bit less than its rated strength, which shifts the corresponding RF-observation windows. That seems like a shame at first (after all, you certainly paid for a 400 MHz magnet or what have you), but it’s a lot better to have a clean spectrum at 380 than it is to have unpredictable crap at 400. Good luck to the folks at Minnesota with their light-rail problems, though. At least they know where the noise is coming from!

23 comments on “Picky, Picky NMR Magnets”

  1. julien says:

    FM radiation pick-up ruined the PhD project of a friend of mine. Had to change subject completely. You would expect a manufacturer like Bruker to properly shield those fancy machines.

  2. steve says:

    When I was a post-doc at Rockefeller, the daughter of the King of Thailand (who was friendly with David Rockefeller and paid for a Thai post-doc who was in our lab) visited. Turns out she was an organic chemist. We had a new NMR she wanted to see. Before we could say anything, her videographer rushed into the room with his bag of videotapes to film her looking at the NMR. Always wondered what happened to the poor fellow when he returned to Thailand with a bag of blank tapes.

  3. Patrick says:

    The U of Minnesota is my alma mater, and while I was doing tech support in the biology buildings there (there’s an NMR center in the basement of one of them), I found a partially printed, abandoned copy of a report full of details about the expected magnetic interference and suggestions to mitigate it next to a bin (the printer had died partway through a page), and took it home for reading. (It was a public document, just not one I’d ever seen.)
    They noted the peak would be with two trains, both accelerating in opposite directions. They suggested carefully made cables (no splices for hundreds of yards, apparently abnormal for the power lines for the train) in a triangle configuration (or just a two line configuration) set up so the interference would mostly cancel out. I didn’t really understand the physics of it, but it made for fascinating reading to see it all carefully quantified.
    I don’t think the city did any of it, though…

  4. Martin says:

    Yup, 207Pb NMR on our old 500. Right in commercial FM pop radio land. 104.6 MHz

  5. Patrick says:

    I see now the article is about that exact NMR center. The big shiny building in the center of the shot, Mollecular Cellular Biology. Aahhh, home. Or what used to be home.

  6. anonymous says:

    I wonder what the folks at McGovern Institute for Brain Research at MIT have to do. A railroad track runs right through the building.

  7. Wage_Slave says:

    It does not have to be as exotic as a train line; in one version of the plans for a new building my NMR lab was positioned adjacent to a lift shaft – a nice chunk of moving vibrating metal, just vertically rather than horizontally.

  8. imarx says:

    “I wonder what the folks at McGovern Institute for Brain Research at MIT have to do. A railroad track runs right through the building.”
    Not sure about how they handle NMR, but I just saw Phillip Sharp talk and he made an offhand comment about how the engineers sunk enormous pillars 200 feet down to bedrock in order to eliminate building vibration for neuro studies.

  9. Dr. Manhattan says:

    I remember hearing a story back in the early 1980’s that New York Hospital had installed one of the first commercial patient NMRs (MRI when you speak to patients) in the basement. They supposedly had to deal with interference from Vatican Radio. Not sure if this is a true story, but I do remember there were a lot of problems in getting it to work correctly.

  10. Wile E. Coyote, Genius says:

    These all sound like good reasons to move chemistry out of the big cities and into rural areas, where you can buy up a bunch of land cheaply and keep all those problems at a distance. Why again is everyone in Cambridge with this extraordinarily expensive and sensitive equipment?

  11. anon says:

    “I wonder what the folks at McGovern Institute for Brain Research at MIT have to do. A railroad track runs right through the building.”
    There’s only ~two trains per day on that line. I think the bigger concern would be the subway Red Line, which the building is also nearly atop.

  12. GladToMoveToProcess says:

    My late wife ran an ultra-high resolution nmr and could see the spectra degrade when people in the adjoining lab pulled out the drawers of their file cabinet. She finally persuaded them to move the cabinet.

  13. UMassPolySci says:

    When the Silvio Conte polymer science facility was first built on the UMass-Amherst campus, the highest-powered NMR they had (500MHz) was setup on the fourth floor of the building. All users understood that the small blip at ~4.5 ppm resulted from building vibrations.

  14. Rhenium says:

    What a beautiful NMR room.
    There is one at Northwestern that reminds me of a cathedral as you look down upon it from the X-ray facility.

  15. oldnuke says:

    @Rhenium: Ah, the Church of Our Lady of the Blessed Spin.
    Every time a student’s phone sends a text, they’ve discovered a new structure.

  16. gippgig says:

    If you ride a subway or light rail line take a magnetic compass on board and watch what happens!

  17. Eric Webb says:

    I am completely ignorant to all things NMR, but wouldn’t it be possible to shield the rooms against RF radiation with a grounded, conductive mesh like what certain acronymic governmental agencies use for TEMPEST shielding?

  18. Dave says:

    Oh, yeah, you don’t have to worry about just outside magnetic fields. Sometimes, the building itself will produce stray magnetic fields. In a previous office building that we had moved the group into, quite a few users with CRT based monitors were complaining that their monitors were failing, because the image kept bouncing up and down.
    Well, it turns out not to have been a problem with a bunch of monitors suddenly failing all at the same time. I noticed that all of the complaining users were located in adjacent offices along one hallway. Thinking this was suspicious, I whipped together a small audio amplifier driven by a coil, which would pickup a varying magnetic field. When I moved the coil over a section of the floor, it hummed like crazy, at 60 Hz (the power line frequency). It seems that the building contractor had buried some conduit in the concrete floor. This conduit ran, as I traced it with my magnetic sensing equipment, from the main breaker panel, to a subcircuit breaker panel on the other side of the building. The circuit was rated for 200 Amps, and that was producing enough of a stray, varying magnetic field that it caused the CRT monitor images to bounce about two or three scan lines. It was especially annoying, since the vertical scan rate was close, but not exactly, 60 Hz, which caused a beat frequency, making the image bounce up and down a few times per second.
    Now, I was asked about the human aspect of that, and I ran screaming away from that question. In any case, we all moved out of that building a few weeks later.
    So, watch what’s in that concrete floor under your feet.

  19. AVS-600 says:

    @10: Because that’s where all the people who use the expensive equipment and all the people who maintain the expensive equipment seem to want to live.

  20. Jim says:

    Back in my miss-spent youth I maintained ETEC scanning electron microscopes and e-beam lithography equipment. Both were extremely sensitive to both magnetic fields and vibration. A site survey with gaussmeters was always performed before installation.

  21. Rhenium says:

    @15 Old Nuke
    Our proton that art in nuclei,
    hallowed be thy spin.

  22. Orv says:

    There was (and is) some consternation at University of Washington about the subway line that’s going to run directly under campus. However, it’s about 80 feet down, and dirt does attenuate radio waves pretty well.
    I remember seeing a room at Michigan Tech that was completely enclosed in transformer steel to block outside magnetic fields — the magnetic equivalent of a Faraday cage, I suppose.

  23. Anonymous says:

    @10: Because those of us who aren’t have to live in some god-awful place like Champaign, Illinois…

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