Nuclear Magnetic Resonance (NMR) facilities present unique hazards
not found in most laboratory environments. The NMR facilities maintain
superconducting magnets which have a sustained field that is always
present. A substantial inventory of cryogenic liquid is also present
within the units.
Facility design and installation:
Design and installation of a new NMR facility requires a number of
safety considerations and it is essential to work closely with the
manufacturers to identify issues associated with installation and
operation. Temperature, structural support and magnetic field isolation
should be considered in the design of and NMR facility. Successful
operation of an NMR requires very tight temperature control. The higher
frequency the NMR, the tighter the temperature control must be. An
estimated +/- 1 to 2 degree Fahrenheit temperature control for a 300
MHZ NMR is suggested.
NMRs must also be installed in locations where magnetic fields do
not create hazards for building occupants outside of the controlled
The weight of an instrument is in the order of several tons which
requires that it be placed in an area with substantial structural
support. If the structural support includes steel beams or steel reinforced
concrete, these ferromagnetic materials may have an effect on the
magnetic field. The device should not be located near sources of RF
such as heavy motors or relays.
(1) Tight environmental temperature control is required
(2) Structural support for heavy equipment and vibration control is
(3) Magnetic field isolation/control is required
Magnetic fields can generate large attractive forces on ferromagnetic
(metal) objects. Such objects include most tools, gas cylinders, pocketknives,
key rings, and most electronics. Any such object that gets too close
to the magnet will be accelerated towards the magnet with great force.
Metal belt buckles, steel tipped shoes, medical implants and any other
metal on the person may be strongly attracted when close to the magnet.
A best case scenario is simply lost time and expense of removing
the object from the magnet. Larger objects (floor polisher for example)
are troublesome and can seriously damage the magnet.
Worst case is injury of the user or bystanders that could occur in
two ways. First, an object pulled with great force towards the magnet
could strike someone. Second, the object striking the magnet could
cause the magnet to quench (i.e., become resistive). This vaporizes
the magnets cryogenic cooling gases (helium, nitrogen), which will
displace air in the laboratory. In this instance everyone must immediately
leave the laboratory to avoid the potential for asphyxiation. Once
energized the field of the superconducting magnet of the spectrometer
is always present. These magnetic fields propagate horizontally and
vertically. These fields extend outside the magnet; therefore, no
movable metal objects should be allowed within the danger area of
The generally accepted safe field is 5 gauss. It is good practice
in NMR laboratories to indicate the 5 G magnetic field line (e.g.
tape on the floor around the magnet). Magnetic fields may affect certain
heart pacemakers. Demand-type pacemakers may be switched to basic
rate pacing. Persons fitted with pacemakers should not be permitted
in the area. The stray field of the NMR magnet is well characterized
by the vendor. Metal and instrumentation should all be outside this
line. The control unit for the NMR where operators are located should
also be outside the 5 Gauss threshold. It is necessary to use steel
tools for maintenance and repair of the consoles, but such work should
only be done by the NMR staff (or engineers from the manufacturers)
and users are not allowed near the magnet during such work. Where
possible, non-magnetic tools should be used on the magnets themselves.
These risks are minimized by preventing access to the NMR rooms by
anyone other than the NMR staff and trained users. Anyone else needing
to enter the NMR rooms should only do so in the presence of one of
the NMR staff. Combination locks should be reset regularly to prevent
their codes becoming known by others. The NMR vendor should be required
to inspect the space and approve your NMR laboratory plans prior to
delivery and setup. Since the magnetic strengths of NMR magnet vary
substantially depending on their application, the 5 G line will vary
from one field strength to another. In addition, new magnetic systems
that are actively shielded have the 5 G line much closer to the exterior
shell, and in some cases, inside the exterior shell. The point is
that no two NMR laboratories are the same.
For information only and not to be considered exact specifications,
the University of California Santa Barbara website gave the following
information for the 5 Gauss perimeter (both horizontal and vertical):
||5 Gauss Threshold
||~ 1.5 Meters (5 feet)
||2.2 – 2.8 Meters (7.2 – 9.2 feet)
||2.8 – 3.6 Meters (9.2 – 11.8 feet)
As in any laboratory, unauthorized personnel should not be permitted
into the NMR laboratory. This should be clearly identified outside
the laboratory. In addition, a sign should be posted outside the laboratory
that indicates a magnetic field is present within the laboratory.
The vendor often gives these signs upon request at no cost. Most vendors
also have signs that indicate people with pacemakers or metal medical
implants are not permitted in the laboratory. It is always good practice
to educate non-users (other colleagues, administrative support, custodial
services, etc) in the general area of the laboratory what is inside
the laboratory so in the event of an emergency they posses basic information
for their own safety or to inform others (emergency personal) of the
(1) Limit access to laboratory to only qualified personnel
(2) People with medical implants should not be permitted in the laboratory
(3) No metal objects permitted within the danger of the magnetic field
(4) Post sign outside laboratory “Unauthorized personal not
(5) Post sign outside laboratory “High Magnetic Field”
(6) Educate others that work in the general area about the NMR
(7) Be aware of any metal object worn on your body
(8) Do not bring magnetic recording material near magnet (ATM, disks,
(9) Minimize time within the 5 G line
Cryogenic Liquid Nitrogen/Helium Fills
Once operational the magnetic field does not turn off. To sustain
the superconducting field cryogenic gases are used. The inner cooling
gas is liquid helium with outer dewar containing liquid nitrogen.
The main risks are burns when handling cryogens and asphyxiation if
a magnet quenches. These risks are minimized by only allowing experienced
individuals to fill the magnets with liquid nitrogen and liquid helium.
Gloves, eye protection, and closed shoes must be worn during transfer.
The expansion ratio of the gases can be used to determine the volume
of helium or nitrogen gas that would be released if all the liquid
in the NMR were to vaporize. This allows calculation of risk in case
of a quench and necessary emergency exhaust ventilation requirements.
At least two staff should be present during refilling and appropriate
safety clothing must be worn (gloves and eye protection). Refills
must be continuously attended. It is particularly important that the
person filling the magnet, once trained, should do so on a regular
basis so as to be familiar with the required routine. Magnet quenches
(the rapid release of gaseous cryogens from the cryostat into the
room) should trigger an alarm thereby preventing any risk of asphyxiation
due to the large volume expansion.
In the event of a quench, personnel should evacuate the area (a quench
warranting evacuation would be obvious by the noise of the escaping
gas and clouds of vapor).
Access to the NMR rooms must be strictly limited to the NMR staff
during refills and any major maintenance. During other periods access
must be limited to a known set of users via combination locks. The
magnet cryostats continuously expel a small quantity of gaseous He
and N2 into the air. This does not present a hazard since during everyday
use the air is constantly changed in the NMR rooms by the ventilation
system. Any drop in the oxygen content of the air can be detected
by oxygen monitoring. A site-specific operating procedure should be
developed by the NMR staff for topping up of liquid cryogens. Even
though magnetic quenching usually is very obvious, it is prudent to
install an oxygen sensor in the laboratory that alarms when oxygen
levels are approaching an unsafe level. Since the possibility of a
quench is higher when filling the magnet, and since the transfer involves
manual operations, there is a remote possibility that an operator
could be rendered unconscious around the time of a quench. Fills should
only be done by a single operator when the fill cannot be deferred,
and exceptional caution should then be used. Cryogen tanks on wheels
must be secured around the magnet if used for filling operations.
(1) Gloves, eye protection, and closed shoes worn during transfer
(2) Secure tanks during transfer
(3) Never leave the room until transfer is complete
(4) The laboratory should be evacuated in the event of a quench
(5) It is prudent to invest in an oxygen sensor for your laboratory
(6) Always have more than one person in the area during fill
(7) Create a simple plan for personnel in unlikely event of quench
Additional safety considerations
Anyone working with the NMR should receive proper training. Training
from the vendor is recommended for the person who will primarily be
responsible for the NMR.
The magnet/dewar has a high center of gravity and could tip over if
struck by a large object. In addition to serious injuries to persons
near the magnet, the sudden release of nitrogen and helium gases from
the dewar will displace breathable oxygen in the room. Support ropes,
bolting to the floor, or other methods should be used to stabilize
Do not exceed the boiling or freezing points of the test sample. A
sample subjected to a temperature change can build up excessive pressure
which can break the tube. Broken glass, projectiles and hot or toxic
chemicals can cause injury. To avoid this hazard, establish the freezing
and boiling points of a sample before doing a variable temperature
experiment, and never rapidly heat or cool a sample. Always wear safety
glasses near the magnet when performing variable temperature experiments.
Be very careful with sample tubes as they are fragile and break easily.
The top of the sample tube can break off when the probe is removed.
The sample should be ejected before removing the probe from the magnet.
Use extreme caution when removing the probe if the sample cannot be
Do not operate NMR spectrometers in the presence of flammable gases
or fumes. Flammable gases or fumes create the risk of injury or death
from inhalation, fire and explosion.
Do not look down the upper barrel of an NMR spectrometer if a probe
is in place. Pneumatic ejection of a sample from the probe due to
pressure buildup could cause injury.
Electrical and radio frequency risks are similar to those encountered
in the use/maintenance of other laboratory equipment and are minimized
by restricting any modification/maintenance of the equipment to the
NMR staff (or very experienced users) in consultation with the manufacturer.
Only carbon dioxide fire extinguishers should be used to avoid equipment
damage and exceptional care is needed to ensure that fire extinguishers
are not used near the magnet cryostat. In case of serious flooding,
or in other situations where there is risk of electrocution, the equipment
circuit breakers should be turned off.
(adapted from PennState Environmental Health and Safety Website