Today’s busy labs are packed full of new technologies and electronic equipment; space is at a premium and this busy setting, combined with the growth – and noise – produced by surrounding cities, causes a significant problem for the performance of electron microscopes. The ideal location for electron microscopes should be free from vibration and external magnetic fields, and only then will the operator be able to avoid any movement of the electron beam and get the best possible image quality.
A crucial part of achieving this perfect situation is a scrupulous pre-installation site survey by either an independent service provider or the equipment supplier, to determine the existence of any external factors likely to exert an adverse effect on the quality of the imaging.
The image on the right is with cancelling on; the image on the left, with it off.
Typically, this will include measurement of acoustic levels, magnetic fields and floor vibrations in X, Y and Z directions, and direct comparison with the environmental specifications of the equipment to be installed. Where necessary, action can then be taken to mitigate any unwanted interferences, for example, by installing a magnetic field cancelling system.
What causes interference?
Electron microscopes are affected not only by conditions within the room where they are installed, but also by the location of the building itself. While there are several possible sources of interference, the most common causes are vibration through the floor or the air – acoustic vibration – and magnetic fields. Anything that moves or rattles – whether regular or random – can potentially create vibrations, including other electronic equipment, air conditioning systems, people simply walking around the laboratory, doors opening and closing, even traffic in the street, nearby railways or ocean waves. External factors too can have a significant effect; magnetic fields generated by trains and electric trams can extend for hundreds of metres, and even the location of the car park in comparison to the microscopy room can make a difference!
Power cables, distribution boards and earth leakage
Power cables can be a common cause of interference for electron microscopes. The live wire in a power cable carries electric current to an appliance, while the neutral cable completes the circuit, to form a path for the current to flow back to the mains. These two wires should effectively cancel each other out but, as the currents in the wires get larger, the difference between the wires grows and this leads to large magnetic fields. Similarly, when power comes directly into a building or lab and is directed by distribution boards to various circuits, this creates large magnetic fields. In some instances, large leaks down the earth wire also produce huge fields, often identified by simply turning off different areas of the power supply.
Although lights are frequently turned off in microscopy rooms during imaging, lighting can still cause interference. In general, LEDs tend to be the best to use, whereas fluorescent lights should be avoided. Changing the lights is often the simplest solution to overcoming the problem.
Trains produce interference in multiple orders of magnitude higher than can be tolerated by electron microscopes, depending on how far away the tracks are. This is not only from vibrations as trains pass by, but also from magnetic fields running along the power lines, which can affect instruments hundreds of metres away. In different countries – and even regions within countries – trains run at varying frequencies and voltages, producing an array of magnetic fields.
Often, people simply don’t realise how close they are to a lift or elevator. As lifts move up and down, they cause interference gradients that constantly change over different heights, causing images to wander and jump around the screen. It is even possible for site survey readings to show you what floor a lift is on.
Proximity to roads or car parks can cause interference from vehicle movements, especially from heavier vehicles, such as trucks, or the regular passing of buses.
Although it may seem obvious that magnets have magnetic fields, large systems built around magnets – such as NMR platforms – are designed to keep as much field inside as possible, and therefore the extent of their effect should, in theory, be minimal. In reality, this is not always the case and sometimes it is better to move the microscope and magnet further apart.
While there are undoubtedly challenges in setting up and maintaining a stable microscopy environment – especially as the world becomes a busier place – measures can be put in place to ensure these are mitigated. The key is to think ahead and engage in discussions with microscopy suppliers, site surveyors and, in the case of a new build, architects at the earliest possible opportunity. Having an experienced site surveyor with the right equipment is vital to measure both vibration and magnetic field interference from different locations and heights, identify common and unusual causes of interference, and understand the magnitude of their effects. They can then advise on the best way to counteract these problems by implementing measures to reduce or eliminate interference, whether that’s relocating the microscope, taking out the root cause, or installing a cancelling system. This will allow electron microscopy to be installed in the most stable environment possible, whether it is in an existing facility or a newly set-up laboratory, ensuring the highest quality imaging is maintained.
Written by Robert Christy, Senior Customer Support Scientist, Spicer Consulting