NMR Safety and Etiquettes
This document covers the minimal knowledge that must be shown before the university can
responsibly allow access to the NMR spectrometer:
· Personal Safety Near an NMR Spectrometer
· Instrument Safekeeping
· Locking and Shimming
· The FACES on-line reservation system, and
Etiquettes When the Schedule is Busy
--- Personal Safety Near an NMR Spectrometer
- No metal near the magnet.
NMR Magnets are always live and always at field. They are never
turned off. No ferromagnetic objects (i.e. all metal, until you know
the exceptions) should be brought near them. The obvious things are
tools, chairs, and gas cylinders. Equally important are small items that
will fly down the bore: paper clips, jewelry, wallet chains. Barrettes,
in particular, can be snatched from your hair if not secure.
- No medical implants
No persons with any medical implant should enter the NMR room without talking to the Director
first. Some pacemakers (cardiac assist devices) have wire loops that become generators.
Aneurism clips can be pulled through brain tissue.
- Leave the door open
This is a good habit around cryogens in general, but
especially an NMR. If the magnet is disturbed and loses
superconductivity, enough energy is released to boil off a
swimming-pool volume of helium in a few seconds,
displacing air. Such a "quench" happens rarely (about
once per century per magnet) and usually in response to
some abuse. But if you see the magnet “exploding
slowly” or sending out a plume or acting like a steam
whistle: follow your instinct to walk out the door.
--- Spectrometer Safety for Beginners
NMRs are expensive. They are delicate. The high cost of repairs (thousands of dollars) pales
next to the cost of shutting down research and teaching for weeks or months. Fortunately there is
no hazard to the NMR from the keyboard, and you are encouraged to explore NMR aggressively.
Anyone in the spectrometer room, even a guest, must be able to answer the question “What is
Rule 1”, or their access will be suspended.
-- Rule 1: If anything might have fallen down the bore of the magnet, close the
spectrometer and email Roger
The primary threat to the instrument is if the probe gets broken, it takes two to six months to get
it fixed and we don’t have an NMR in the interim, effectively shutting down all research
programs doing organic synthesis. If your hand isn’t shaking when you open the door, you do
not grasp the magnitude of this.
The way this has happened is a sample has broken in or above the magnet: if there is a shard of
glass in the probe, the next sample inserted will break it. The good news is the probe is trivial
to clean if you close the spectrometer and call for help -- it takes me two minutes and incurs no
expense to remove the probe and shake the glass out. However, severe damage will occur (the
detector will be broken) if a sample is inserted while a shard of glass -- or any foreign object,
large or small -- is down the bore.
Samples are most often broken when being removed from the magnet:
Climb the ladder.
Lift the sample straight up.
Most broken samples happen
when being removed without
climbing the ladder.
Close the spectrometer if a sample breaks in or
above the magnet
Samples have also been broken when dropped into the magnet without the lift air. It can fall
hard into the detector, the most fragile part of the spectrometer. (To avoid this, bounce the
sample on the lift air before letting go of it.) It is also a good habit to turn of the spinner and
wait a few seconds before ejecting a sample.
You “close the spectrometer” by leaving a note on the keyboard “Broken Sample, Do Not Use”.
Roger’s email address is posted on the magnet (r.kautz@neu.edu).
-- Locking and Shimming
Although it is possible to learn to acquire spectra by rote, you will be helpless and look foolish at
your first job, where the software is different. If you start with just a little knowledge about what
the spectrometer is doing, you’ll get a little smarter each time you use it. If you just get more
used to not knowing, the opposite happens.
Every sample perturbs the magnetic field differently, so the field has to be tuned up after each
sample change. Z0 (“Z-naught”) is the average field; Z1 and Z2 are gradients to make the field
at the ends of the sample the same as the field at the middle.
-- Locking
Lock is short for “Field Frequency Lock”. It continuously adjust Z0 for you during data
acquisition, as trains go by or elevators go up and down. Without locking, the lines of your
spectrum move during acquisition and would be a broad smear instead of a sharp line.
Locking is done in two steps. The first step is to adjust Z0. Then you turn the lock on and adjust
the other parameters.
Z0 adjusts the static magnetic field. (Changing Z0 across its entire range would changes your
sample’s frequency by about 10 ppm). You will adjust Z0 to tune your sample’s solvent line to
the spectrometer frequency - which I will call a tuning fork inside the electronics cabinet. The
correct Z0 will change significantly for different solvents; typical values are given on a chart by
the spectrometer.
The lock display shows the difference frequency between the solvent line and the tuning fork:
Z0 too high or too low
Z0 correct
The more humps you see, the further out of tune your sample is. Adjust Z0 to get fewer humps
… until there are no humps (a step function).
After you turn the lock on, adjust lockpower to the value on the chart next to the spectrometer.
Although you may increase the lock power to see the humps while adjusting Z0, if you leave the
lock power too high it can suppress the solvent line instead of stimulate it. This won’t hurt the
spectrometer or the sample, but the lock level will jump around and you won’t be able to shim.
--- Shimming
"Shimming" means making the magnetic field the same at the top, the middle, and the bottom of
the sample.
In contrast to Z0, which adjusted the magnetic field uniformly across the sample, Z1 and Z2
change the ends relative to the middle. Z1 is a linear correction, Z2 is quadratic (literally Z2),
You adjust Z1 and Z2 in whatever direction increases the lock level.
NMR Peak When Poorly Shimmed
Well Shimmed
The lock level is the height of the solvent line. Because the area of the solvent line is constant,
if you make the line narrower it gets taller.
The highest lock level you can obtain may be quite different for different samples -- you might
max out at 60% for one sample and 90% for another. (And it of course also depends on the lock
gain.) The true test of your shims is whether the peaks in your spectrum are sharp. Critical
samples (multi-day acquisitions) get a final polish shimming on the spectrum, instead of the lock.
Beginners should not mess with Z3 - Z5. In fact, because the spectrometer will have whatever
shims the last user set, it’s a very good idea to load a good standard shimset (type bestshim)
before you lock and shim your sample.
- Common Problems
If the lock level is jumping around, it is usually because the lock power is too high.
If you optimize Z1 then optimize Z2, you may still have a lousy shim. Switch between Z1 and
Z2 several times as you are shimming. Or walk Z2 through several steps, optimizing Z1 at each
step, and keep the combination that gives the highest lock level.
-- Other Important Concerns
Learning NMR requires significant time at the console. There are a few things which you should
get in the habit of doing right away, while your spectrum is acquiring, to increase the time
available to explore, and to avoid taking incorrect or invalid spectra.
--- Annotate spectrum
As soon as you type ga to acquire a spectrum, type gettext and annotate it. Most chemists
print out NMR spectra and paste them in their notebook (or e-notebook). The spectrum must
say: what the sample is, the date, your name, and tell where the data is stored. (If not, your
notebook is incomplete and you shouldn't get paid.)
ELK-21-35
10 mg in CDCl3 (+TMS)
(second TLC spot; not HPLC-purified)
filename
(optional notes about your sample)
Ezra Kravitz, 13 October 2012
NMR400:hanson
Name, Date
Spectrometer: login
If the work you are doing isn’t worth annotating, it’s not worth doing. I would suggest that if
you haven’t annotated your spectrum and someone else needs the spectrometer, you should
forfeit your time.
- Set Reference
Setting the zero ppm mark to TMS is the only kosher way to set the ppm scale. Calibrating the
ppm scale to solvent peaks is acceptably close for casual work, if samples are a dilute solution.
But WILL BE IN ERROR for mixed solvents or concentrated (> 10 mM) solutions.
- While the spectrum is acquiring you can also
Set integral resets;
Choose line broadening (noise suppression)