Superconducting magnets have contributed immensely to the scope of applications of NMR technique. However, working in the vicinity of strong magnetic fields poses several hazards which cannot be overlooked and additional safety precautions are necessary in the NMR laboratory.
The article highlights hazards associated with the NMR laboratory and suggests safety measures. The associated hazards are due to:
- Strong magnetic fields
- Use of cryogenic liquids
- Magnet quenching
- Accidents resulting from NMR tubes and improper sample handling
Strong Magnetic fields
Magnets have inherent property of attracting ferromagnetic objects in their vicinity. Superconducting magnets generate very strong fields (20 Tesla or more at frequencies above 800 MHz). This translates to field strengths of around 200,000 gauss or higher. Stray fields are present in all the three dimensions around such superconducting magnets and have strong tendency to attract metallic objects towards them. From basic physics we know that the closer the object to the magnet greater is the force of attraction. A metallic object will initially start to move slowly but in no time will gain enough acceleration and move like a fierce projectile towards the magnet. As superconducting magnets do not require power to sustain their magnetic fields it must be remembered that in emergencies superconducting magnets cannot be switched off and they remain on. The following precautions are a must for persons working in NMR laboratories:
- Do not carry any metallic objects beyond safe zone which is clearly demarcated on the floor in laboratories housing superconducting magnets
- Persons including visitors with cardiac pacemakers, bio- stimulators and neuro- stimulators should not be allowed inside the laboratory. This includes persons having metallic bone or joint implants. Serious injury or even death can result due to malfunction of such devices in presence of such strong magnetic fields
- Watches, coins, jewellery items, magnetic strip bank cards should be left outside the laboratory.
- Ferrous tools such as screwdrivers, wrenches, tweezers should not be taken inside as these can get drawn to the magnet at rapid speeds and can cause injuries. Care should also be taken to remove spatulas from lab coat pockets
- Never bring compressed gas cylinders inside NMR laboratories. Due to their big mass they can result in crushing of limbs or persons between them and magnets due to the large attractive forces
- Use non-magnetic dewars when transferring cryogens to the magnet
Ferrous objects can puncture the magnet body due to the strong attraction resulting in damage to magnet and its coils and can lead to quenching of the magnet
Superconductivity requires very low temperatures. The coil is made of several hundred kilometers of wire made of superconducting material housed in dewar of liquid helium. The evaporative loss of liquid helium is minimized by an outer concentric dewar filled with liquid nitrogen.
Liquid helium has a temperature of -269°C and liquid nitrogen of -196°C. Such cryogens pose dangers such as asphyxiation, frostbite and chemical explosions.
- Handle cryogens wearing protective gloves, goggles and closed toe safety shoes
- Repeated replenishment is necessary. Loose fitting dewar caps can lead to condensation of oxygen from atmosphere. When oxygen levels accumulate rapid combustion or explosion can occur if materials coming in contact are combustible
- Very carefully and slowly transfer cryogenic liquids so as to avoid splashing
Magnet quenching takes place when a magnet suddenly loses its superconductivity. Due to failure of the superconducting coil or the cryogenic system the coil becomes resistive and generates heat which leads to explosive expansion of helium gas from the magnet. The rapid escape of helium displaces the air in the room and this can lead to asphyxiation in the confined space of the laboratory. In such a situation leave the lab immediately and instruct others present to evacuate as well.
Depletion of oxygen levels below 18% in the environment starts showing oxygen deficiency affects and oxygen levels falling below 6% can result in sudden death. It is advisable to install oxygen level monitors in the lab with sound alarms when oxygen levels fall below unsafe levels.
Improper handling of NMR tubes and samples
NMR tubes have thin walls and get broken easily. Majority of accidents take place during the process of capping and insertion into the magnet.
- Instead of using fingers use your palm to hold the tube close to the spinner and insert with minimum force
- Use minimal force to cap the tube
- Do not use tubes which are cracked, bear scratches or are damaged in any other manner
- Wrap label tape carefully around the top end of the tube or better still affix on top of the cap. Never fix labels or flaps of paper on tubes as these can impair spinning inside the turbine
- Remember to press the air lift button for insertion of the sample tube otherwise there can be damage to tube or the machine.
Subsequent articles will be devoted to scope of applications of NMR technique.
- Broken samples can result in injuries or lead to dangerous spills of toxic chemicals
- Samples also require solvents for dissolution .Read the safety instructions of such solvents beforehand and take the required handling precautions.
- NMR magnets are tall and often step ladders are provided to facilitate sample loading. Climb the ladders carefully taking care not to lose your balance.