Safety in the EM Laboratory

SAFETY IN THE ELECTRON

MICROSCOPE ROOM

by S. K. Chapman

Keywords: Electron microscopes, safety, maintenance

 MICROSCOPY AND ANALYSIS, MARCH 1989

 Steve Chapman has been involved with electron microscopes since 1964. Initially working as a service engineer in the UK his service activities eventually took him throughout Europe.  Moving into sales and marketing he worked with a number of electron microscope manufacturers where he was probably most at home in a teaching or application environment.  In 1981 he set up his own training organisation Protrain, adding a service company to his repertoire in 1983.  Running courses in many parts of the world, north and south of the equator, he has written a number of books on the operation and maintenance of scanning and transmission electron microscopes. However, Protrain is not only involved in electron microscopy but also in motor sport, where their (Go) kart racing courses are a considerable success.

 Many texts have been written on the use of the scanning and transmission electron microscopes. Our own Royal Microscopical Society has through its safety committee produced a considerable amount of useful literature on the hazards that are faced in the preparation of the specimen.  However, for every instrument there is invariably a person who is involved with day to day maintenance of that instrument.  Very few papers are written on their safety and as a guide to the safe procedures for maintaining the instrument.  In this article an outline is given of the problems that may confront the microscopist carrying out even the most basic maintenance procedures.  The hints and tips are designed to make working with the instruments a much safer occupation.

 Whilst the actual electron microscope room is probably one of the safest areas in an electron microscope laboratory, it contains a number of hazards that demand attention.  Some of these are present on all occasions.  Whilst others only present themselves when some form of maintenance or other routine procedure is being performed.

We are all aware that on opening an electron gun that has been in use, the cathode assembly will be very hot.  However, there are other hazards that should be considered prior to this point. Perhaps at this stage we should set a standard for any action toward, or performed around, the microscope.  "Never attempt any routine maintenance task without thinking of the possible hazards involved with that task!"

Most manufacturers include a "shorting bar" which falls across the cathode assembly as the gun is opened.  This bar under normal circumstances should never pass a current.  Only if things go wrong could it save your life.  In a modern microscope (post 1970) two actions need to be omitted from the conventional procedures for the high voltage to remain applied to the cathode when the system is at air.  A vacuum trip should immobilise the high voltage supply when air is let into the gun.  If you have also forgotten to switch off the high voltage, a combination of these two mistakes could lead to a dangerous situation.  We can see that a modern instrument should protect us in these two ways, however, failing to follow conventional switching procedures would leave an operator open to considerable danger.  It is true that at a high accelerating voltage most people would smell the ozone from the gun arcing before completely opening the electron gun chamber, but if you don't want to let a problem get that far the message is "do not take short cuts."

If you use an elderly microscope (pre 1970) that does not use a high frequency generator, the high voltage is generated with the assistance of a very large capacitor.  This is usually shorted to earth within the tank when the high voltage is switched off.  I have had the frightening experience of shorting out this type of gun with a screwdriver (Figure 1). Only the blinding flash indicating that the internal shorting bar had failed to discharge the capacitor; it had fallen off!  Always be sure that the gun is shorted out.  Always work as if safety devices have failed and never cut corners.

Now for that hot cathode.  Wear gloves that are thick enough to protect you from the hot metal.  Chamois leather gloves are ideal and they will not contaminate the gun chamber.  The filament has been up at about 2500⁰C so no wonder the assembly is very hot!  Another problem in this area is reaching the gun itself.  Many instruments provide steps for the operator to have access to the gun level.  If you have not had steps provided with the instrument, do not perform a balancing act on a chair or stool.  Please purchase a pair of steps as you will be changing a filament hundreds of times through the life of an instrument.

Another area of gun maintenance may require the chamber to be removed for cleaning.  Please be advised by your manufacturer on how this should be accomplished, and about the weight of the chamber.  Take great care because one mistake may mean a damaged chamber or even a damaged operator.

The electron gun is also a source of x-rays.  If the gun or condenser area is dismantled for any reason, you must ensure that it is checked for radiation leaks prior to using the instrument.  Whilst all engineers will rebuild the column with great care, only the smallest of misalignments could result in a radiation leak.  This precaution is not necessary after the opening of a gun for a filament exchange, it is only applicable after the column units have been taken apart.  Whilst on this subject, do not run the microscope with any of the column shroud panels removed.  The manufacturer may be using these to provide extra radiation protection.

The specimen chamber of the scanning electron microscope is also an area where you may inadvertently come into contact with a high voltage.  Once again this problem will only occur if a number of safety devices have failed.  Having said that, it is "quite exciting" to see a one centimetre long spark arcing across from the detector to the bottom of the final lens!  There will be up to ten thousand volts on the secondary electron detector when it is in operation.  When admitting air to the chamber the detector high voltage should be automatically switched off, if not - sparks!  There is nothing that you can do except turn off the detector if the instrument has the appropriate switch.  So take care and do not place your hand near to the detector under normal circumstances.  If there is a problem you will hear the detector arcing, and if you do, close the door and pump the instrument down.  A call for service is your next task!

Many instruments use liquid nitrogen as a coolant for an anticontamination device or an energy dispersive x-ray detector.  When handling any very cold liquid great care must be taken.  Small droplets of liquid nitrogen falling on the skin will rapidly evaporate.  The greatest danger is to the eyes, they should always be protected whilst handling the very cold liquid.  Always try to stand well above the container into which the liquid is being poured.

The EDS detector also becomes a hazard in itself.  The position of the detector, particularly on a transmission electron microscope, makes it a prime cause of cracked heads.  Try to protect your operators by padding the corners of the detector support. Filling the detector on a scanning electron microscope may require moving behind the instrument.  Take care not to tread on any cables or the pumping lines.

The rotary pump seems to be an innocuous part of the electron microscope laboratory but it contains hidden dangers!  The fumes from any hot oil are said to be carcinogenic (Ref 2), contaminated vapours even more so.  Therefore it is most important that the rotary pump is either fitted with a good quality oil mist filter, or its fumes are ducted to the outside world.  Keep a constant check on the state of a filter.  If it becomes discoloured or moist with oil, change it!  If you can smell oil when pumping out a vessel, check the filter and the state of the oil.  Do not forget that ALL the rotary pumps in your laboratory should be treated in this way.  In particular scanning electron microscopists should pay attention to the pump on their sputter coater. This pump is constantly pumping against the gases given off by specimens and their adhesives.  It will certainly be the most contaminated of your rotary pumps.  Even a good filter may be inefficient when the pump oil is severely contaminated, so be sure to have the pump oils changed at least every year.  More often if they are heavily used.  Fresh oils are either colourless or a faint yellow colour; when contaminated they become dark yellow to brown.  Do not get the contaminated oil on your hands if you carry out your own pump maintenance, as it may also cause dermatitis.  If this dirty task is left with your engineer make sure that it IS being carried out.  If you are not being charged for the oil it is probable that it is not being changed!  It is your safety that is at stake, find out.

A most recent problem for me is the sensitising of this electron microscope operator by cleaning agents.  Even that most user-friendly material, like Duraglit, will start to affect you if you use it year after year.  Twenty years of cleaning electron microscopes and now the skin peels off my finger tips when I have been using my favourite cleaning media.  What will affect you?  Treat all the cleaning agents as a hazard and you will not be far wrong.  Watch out for the vapours from acetone, and ether, or any other solvent that you may use.  This is a particular problem if you are using a solvent as a leak-detecting agent.  In this case the solvent is sprayed at possible leak sites, an indication of it been drawn into the vacuum system being a dramatic rise in pressure on a Penning gauge.  This is an area that you should be aware of when the service engineer is looking for leaks also.  It does not take long for the air in most electron microscope rooms to become saturated with the vapour.  Keep the doors open and visit the engineer from time to time.

The ultrasonic cleaner is a great asset in any laboratory but it too is a hazard.  We all hate the constant high pitched noise emitted from the unit, but the real problem comes with holding or handling of components in the cleaner.  Do not take components from, or hold components in, the cleaner whilst it is switched on. Damage may be caused to the wrists.  Do not ask me what goes wrong but it makes the wrists very painful if you ignore this advice.  Keep all fluids covered whilst you are using them for cleaning.  The heat generated by the cleaning action will cause a high level of vapourisation.  Keep the ultrasonic cleaner in a fume cupboard if at all possible.

The annual service visit, or that panic emergency call, does not mean that you have a chance to dive off to the library for a day or so.  Stay with your service engineer.  There are many dangers involved with the service of an electron microscope.  Anyone removing panels from an instrument has the possibility of coming into contact with an electrical circuit.  Do not leave a service engineer with an instrument without someone within shouting distance.  Safety in YOUR laboratory is YOUR responsibility!

Should you fall into the category of being totally your own service engineer may I offer a few additional words of warning.  You may conduct your mechanical maintenance with a high degree of skill, thinking well ahead of what the next action may be.

Good, then you should be safe.  However, how often do you venture into the electrics and electronics?  You may well feel capable of changing fuses, do you make sure that the instrument is switched off?  Do you remove any metal objects from your hands and wrists?  They could be the cause of you getting an electric shock!  Do you make sure that the rotary pump is switched off before commencing maintenance (belt burns hurt) and do you take great care when working round a hot diffusion pump?  Watch out when handling the water connections.  Switching off the microscope first is a good starting point.  One of the most dangerous areas with many microscopes is the room light switching circuit.  Make sure you know exactly which area is switching off when you switch off the microscope.  Some installations allow the mains for the room light to remain connected even when the microscope is switched off; does yours?  If you are receiving guidance over the phone double-check the action recommended.  If you are guided into the electrics and electronics make sure that you know EXACTLY what action you are to take.  When using a multimeter make sure that the settings are EXACTLY those the engineer has suggested; AC, DC, volts or ohms.  Make sure the connections that you make are EXACTLY those suggested.  If in doubt ASK AGAIN.  Finally, is there always someone within shouting distance when YOU are acting as service engineer?  Well there should be!  Please remember that advice over the phone is invariably free.  Do not be afraid to ask and if you have any doubt about tackling a problem, DON'T.

Safety in the electron microscope room, as I have already said, is probably considered to be the lesser of your laboratories evils.  Do not fall into the trap of becoming careless, because that is when the electron microscope is at its most dangerous!

 References:

Chapman, S.K., Maintaining & Monitoring The Transmission Electron Microscope, Oxford University

Press, 1986 pp 37-39.

Meek, G.A., Practical Electron Microscopy For Biologists, John Wiley and Sons, 1977, pp 4546