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By
Luc Harmsen
Anaspec
South Africa
Tel:+27 (0) 11 476 3455
Fax:+27 (0) 11 476 7290
E-mail: anaspec@icon.co.za
I would like to point out the dangers of cleaning microscope parts in solvents. If you clean your own E.M., then usually you are also aware of the "freshness" and source of the chemicals/solvents in your lab. In our travels we often find that we cannot get the solvents we would like or need and have to make do with a lot of strange variations. We have also found contaminated solvents which can cause all sorts of problems. Usually you can visibly see the residue. It appears as white stains. It's the invisible residues that catch us some times.
The basic rules we follow is as follows:
Clean only if necessary. If you find that the test specimen is proving that the E.M. is not up to spec, only then clean the column bits. If the gun assembly is still fairly clean, do not polish. You will only add dirt.
Use a polishing paste that you know works and what it takes to remove it. This can only be tested by experience unfortunately. Pastes we use vary from Wenol™, Pol™ metal polish, Pikal Metal Polish and high quality diamond pastes, all of which work in different circumstances. As we are normally charging by the hour, the customers don't appreciate us trying to polish away for hours on end. So we find the diamond pastes work better on the gun parts as they remove the tungsten very quickly.
We then commonly use ethanol as a solvent for these pastes, as acetone is not always available. One of the better (but unfortunately more expensive) solvents we do use is Arklone™ . But it is also not always readily available either.
The secrets we have learned are simple and work well. Polish the parts well with cotton buds and the diamond paste. Once it is clean, immerse them immediately in the ethanol/solvent. This ensures that the paste does not harden on the parts. Use an ultrasonic cleaner to clean the parts for at least 10 minutes cycle time. Remove the solvent and fill the beaker again with new solvent and give an ultrasonic exposure again. If Arklone is available, then use this as the final solvent.
When taking the parts out of the solvent, "polish" again with clean "solvent dunked" cotton buds. Normally you find that there is still quite a lot of "dirt" that comes off. Always use clean solvent. And beware of fibers that can be left after polishing.
Check the parts under a light microscope for any particles, residues or fibers.
Once the E.M. is re-assembled allow it to pump down over night, at least, before checking for a beam.
With the gun assembly, we would again suggest you do not clean if it is not necessary, as a light coating of tungsten does not affect performance , at least not significantly. For the higher voltage TEMs this would have to be monitored very closely.
We have had good success ultrasonically cleaning with the Protrain formula of 10% Silvo, 10% ammonia and water. Rinsing thoroughly afterword in water then some alcohol. Again I will point out that we do not always have all the fancy chemicals most labs have available to them, but if this method works for us, I am sure the added steps and specialized chemicals listed can only improve cleaning by a smaller margin.
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