Dear AMMRLers,
Many many thanks to the generous colleagues who took the time to think
about my issue with field instabilities and send
comments/anecdotes/suggestions (Mark, Jerry, Martha, Jon, Razvan,
Cedric, Richard S, Bob, Johannes, Bert, Greg, Ron, Bill, Guangyu,
Matthias, Knut, Clemens, Richard H, Dimitris, Sameer).
Although a few people suggested to check vibrations, magnet damage, main
power line, grounding or unusual He boil-off rates, it is pretty clear
now that the major source of the fluctuations is indeed the local
streetcar, as initially suspected. This was confirmed this weekend as
the fluctuations started around 8am (instead of 4am) matching the Sunday
schedule. Many people have experienced a similar effect around the
world. Trams generally operate on DC current which cause
electromagnetic fluctuations (EMF). If those are on the order of a few
mG (an uncompensated 1-mG fluctuation would correspond approx to 5Hz on
a 300MHz system), it could explain the level of variation. It seems that
400m from the track is a bit far to elicit such a large effect; on the
other hand, the power lines and transformers might not follow the tracks
and be hidden closer to the lab. These EMF should be easily detected
with a flux magnetometer. We are in the process of communicating with
the public transport commission to find out more and of trying to get
our hands on a magnetometer.
Mysterious however is why our other magnets do not see a similar
effects. It was suggested that although these fluctuations are present
probably for all magnets on site, they might influence them differently
(by 1-2 orders of magnitude), depending on the level of shielding and
whether the lock system can adequately compensate for them. Right now we
are carefully testing all other magnets for field fructuations without lock.
As for solutions, there were suggestions ranging from extreme (changing
sites or replacing magnet), to tedious (actively or passively shielding
the room), to clever ( Efficient compensation of low-frequency magnetic
field disturbances in NMR with fluxgate sensors.
<
http://dx.doi.org/10.1016/j.jmr.2005.02.018> ) to plain simple
(adjusting the lock regulation parameters). We will try these one by one
(of course, in reverse order ;) and will send you a report very soon.
Many thanks again for the great help.
Best regards
Christophe
On 29.05.2015 10:17, Christophe Farès wrote:
> Dear AMMRLers,
>
> I've seen some amazingly fast and helpful crowd-problem-solving on the
> AMMRL network lately, and so I thought I would share my own issue
> hoping for a similar response ;)
>
> We have a relatively newly installed AVIII Nanobay 300MHz system (with
> an older ultrashield magnet) used in open-access for routine 1D NMR.
> The 2D experiments are however plagued with horrible T1-noise streaks
> and we identified some major field instabilities as the most probable
> source of the problem. If we run a 1D 1H continuously without lock, we
> see erratic field jumps of up to +/-30 Hz! Curious however is that
> this phenomenon does not seem to occur completely randomly. For
> instance: it seems that there are periods of 5-8 minutes every 25-35
> minutes where they are more intense. And nights between approx 11pm
> and 4am, the interferences are almost completely gone. To illustrate
> what I mean, I've attached a printout of the 2d stacks (each column
> represents 2048 1D 1H measured every 2 seconds)...
>
> This is not observed on 400, 2x 500 and 600MHz spectrometers sitting
> only a few tens of meters away.
>
> Of course, the night relative calmness and "regular" more intense
> periods made us look into public transport schedules. The closest
> streetcar stop is 400m away, but the schedule correlates quite
> wellwith this phenomenon (for instance, the first one runs at 4:30,
> intervals are 20-30 minutes).
>
> So my questions are: what could be causing these disturbances and how?
> And of course, what can we do about it?
>
> best regards
>
> Chris
>
--
Dr. Christophe Farès
Head of the NMR Department/Leiter der NMR Abteilung
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1
45470 Mülheim an der Ruhr, Germany/Deutschland
Tel: +49-208-306-2130
E-mail: fares {at} mpi-muelheim.mpg.de
Web: http://www.kofo.mpg.de/en/research/service-departements/nuclear-magnetic-resonance-spectroscopy
Received on Mon Jun 01 2015 - 21:27:03 MST