UMBC Department of Chemistry and Biochemistry

NMR Safety Information

Use of the department’s NMR spectrometers is a privilege and not a right. All users must work cautiously around the high-field magnets. Failure to do so can result in costly instrument damage and serious personal injuries including death.

Each user must understand the hazards present and follow all safety practices.

High Magnetic fields

High-field NMR instruments use large superconducting magnets to generate magnetic fields. Briefly, the wires making up the electromagnet inside the superconducting magnet are cooled to liquid helium temperature (4 Kelvin). At this low temperature the electrical resistance of the wires approaches zero, and the magnet once charged can run continuously without losing power.

(Think perpetual motion.) The power continues to cycle as long as the wires are kept cold. More liquid helium must be added periodically as the helium slowly boils off. Liquid nitrogen is used to keep the liquid helium from rapidly boiling off.

To see what the inside of an NMR magnet looks like, check out the website:

http://www.jeol.com/nmr/mag_view/magnet_destruction.html

REMEMBER: The Magnet is Always On

NMR magnets have very strong, static magnetic fields. Magnet strength is normally described in terms of Gauss or Tesla units. (1 T = 10,000 G)

Earth’s magnetic field 0.6 Gauss at equator

Refrigerator magnet 100 - 150 Gauss

MRI medical scanners 0.3 - 1.5 Tesla (3 - 15,000 G)

High field NMR magnet

200 MHz 4.7 Tesla (47,000 G)

300 MHz 7.0 Tesla (70,000 G)

500 MHz 11.7 Tesla (117,000 G)

800 MHz 18.8 Tesla (181,000 G)

To put this in perspective, NMR magnets are stronger than the electromagnets used to move old cars around at the junkyard.

The strengths listed above are the strength of the magnet at its center (inside the bore where the sample is placed). The magnet field strength falls off as you move away from the magnet center. The rate at which it decreases depends on the physical size and geometry of the magnet. For example, the wider the magnet bore, the further out the magnetic field lines will extend and the stronger the magnetic field that will be felt by nearby magnetic objects.

Human bodies are not magnetic, so we can’t directly feel the strength of the magnetic field. One analogy is "You are walking on the rim of the Grand Canyon in a very heavy fog at night. You know the abyss is on your right, but you can't see quite where it is. How careful would you be?” (from Jim Frye, Varian)

Video demonstrations of the strength of high-field magnets are shown on the websites:

http://mind.ucdavis.edu/content/MRISafety.asx and http://www.radiology.uiowa.edu/mri/MRISafety-300K.rm

For most purposes, you only need to know the location where the magnetic field strength drops to 5 Gauss.

Signs, plastic chains, and/or marks on the floor mark the location of the 5 Gauss field line around each NMR magnet. The magnetic field inside the 5 Gauss region can cause damage to medical implants and pacemakers. DO NOT ENTER THE 5 GAUSS REGION IF YOU HAVE ANY MEDICAL IMPLANTS WITHOUT APPROVAL OF YOUR PHYSICAN.

The location of the 10 Gauss region is located slightly inside the 5 Gauss region. At this field strength, watches, credit cards, and other personal items can be damaged. More importantly, NO tools or metallic objects should be taken closer to the magnet than this point.

Metal objects can be attracted to the magnet causing flying metal projectiles. Ferromagnetic objects can reach speed approaching 45 mph entering the bore of the magnet. These objects can cause personal injury or death if there is anyone between them and the center of the magnet. If the objects strike the magnet they can distort magnet’s wires or internal dewars and/or become lodged inside the magnet bore. This can cause the magnet to quench.

A quench is when all the liquid helium inside the magnet suddenly boils off. This causes the magnet to lose its magnetic field and can damage the superconducting coils inside. A quench is caused when the magnet is damaged or the equilibrium inside the magnet is disturbed.

A QUENCH IS VERY DANGEROUS SINCE THE RAPIDLY EXPANDING GASES WILL DISPLACE ALL THE AIR (OXYGEN) IN THE ROOM. IF A MAGNET APPEARS TO BE IN ANY DANGER OF QUENCHING - LEAVE THE AREA IMMEDIATELY AND NOTIFY EVERYONE NEARBY OF THE DANGER.

No metal objects are to be brought close to the magnet. Keep all tools and equipment outside the 5 Gauss area. See the instrument manager before you use any non-routine equipment with any of the NMR instruments.

Assume all metal objects are ferromagnetic and will be attracted to the magnet unless verified otherwise by the instrument manager.

Attractive force increases as the distance from the center of the magnet is decreased. The increase is inversely proportional and depends on the mass, shape, and composition of the object. In generally, the larger the metal object, the further away from the magnet, it should be kept.

However, small metal objects, such as paper clips and staples, can also cause problems. If they make it into the bore of the magnetic, they can cause problems with shimming and possibly damage the probe.

Cryogenic gases

The liquid helium and liquid nitrogen used in the NMR magnets are extremely cold. Helium liquefies at 4 K (-269 °C), and nitrogen liquefies at 77 K.

Be careful and stay out of the NMR room when the magnet is being filled with cryogenic gases. Prolonged contact with liquid nitrogen or even brief contact with liquid helium will cause frostbite.

If the magnet quenches, the liquid helium and nitrogen inside the magnet may quickly boil off. Due to their large expansion ratios (nitrogen 695:1, helium 760:1), these gases can quickly displace all the oxygen in the NMR room and cause asphyxiation. The temperature of the room will also drop increasing the risk of hypothermia.

Other hazards

NMR tubes have thin walls and are easily broken. Be careful when inserting tube into spinner turbine. If you break a tube, clean up any broken glass and chemicals. Dirty sample tubes and dirty spinners can contaminate or damage probe. Use a Kimwipe to wipe off the sample tube and the spinner.

Carefully align the sample tube using the depth gauges before inserting the tube into the magnet. A misaligned tube can break inside the magnet bore and damage the NMR probe. Also make sure the sample is floating on the lift air before you let go of it.

Inserting sample often requires using a stool or ladder. Be careful not to lose your balance and fall.

The air lift can eject sample tube out of bore quickly. Do not look directly down the bore of the magnet. Wear eye protection. Use special caution using manual air lift on the 200 MHz NMR.

Too high of pulse power can damage probes. This is most likely to occur during decoupling experiments. Do not change the decoupling power levels on the 200 and 300 MHz NMR instruments. Only use the standard or default pulse power levels and pulse lengths for each of the NMR probes (current values can be found in the instrument logbook) unless you have been trained on how to safety calibrate pulse lengths.

UMBC, Department of Chemistry and Biochemistry

NMR Safety Agreement

I understand that I am responsible for working cautiously and carefully around the NMR instruments for my own personal safety, the safety of others, and the safety of the equipment.

I will immediately notify others working near the NMR instruments of any dangerous situation, and I will contact the instrument manager and/or NMR faculty advisors (listed on lab door) as soon as possible.

I will enter the 5 Gauss region of the magnetic field only when absolutely necessary and only for instrument operations for which I have been trained (e.g. sample changes, probe tuning).

I will not bring any items, besides my sample tube, into the 5 Gauss region of the magnet without the specific permission of the instrument manager.

I will replace any safety chains moved immediately when finished using the instrument.

I will follow all training and usage procedures regarding the NMR instruments, and I will request additional training from the instrument manager whenever I have questions or concerns.

I will present the instrument manager with a written experimental plan for approval prior to performing any experiments that require non-routine equipment or pulse sequences to be used.

I understand that it is my responsibility to follow these safety rules and that any damage to the instrument caused by failing to do so will be the fiscal responsibility of my research supervisor and myself.

Failure to follow these rules will result in immediate suspension of NMR privileges, which will only be restored after discussion between the instrument manager and the research supervisor.

User Name: ____________________________________________________________

User Signature: ___________________________________________________________

Date: __________________________

Research Supervisor Name: __________________________________________________

Research Supervisor Signature: ________________________________________________

Date: __________________________