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How to service the Ion Source of a Mass SpectrometerMy notes

- Ratan Mohapatra

Why a filamentA key step in mass spectrometers is the ionization of atoms in vacuum, often by electrons from a tungsten filament in the ionization source

Filaments wear out in time either as part of normal degradation or accidental events

Replacing a filament is a challenging technical experience, especially in noble gas mass spectrometers which are typically maintained at a vacuum of ~10

-10 milli bar ( or 10

-12 atm)

The fastest filament change operation I did was in a week! But some times things linger on to longer time periods

Well I must confess, replacing a filament in the mass spectrometer does teach you very important lessons of life!

PrepareServicing an Ion source typically ends in “bake out”. A noble gas mass spectrometer is baked at ~250 C for a couple of days to attain a (hot) vacuum of ~10

-8 millibar

So it is important to remove the magnet, electronics and peripheral components that cannot sustain such temperatures

It is very important to document the dismantling operation as it might lead to grave accidents both for the instrument and the user. I take photographs with my cell phone, mark each part that I dismantled, and keep a record

When the bake out is satisfactory things have to be reassembled or reconnected exactly the way it was

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Ionization Source Electrical connectionsCaution:

Improper connections are fatal as you may be supplying 8 KV to ground!

Dismantling the Ion SourceWhen dismantling the ion source, I make sure that the mass spec is exposed to air for the least possible time

For example, I would even cover of the bake out ovens in place, except for the ion source region before I break the vacuum

Due to the vacuum inside the MS it is hard to dismantle the source flange. Therefore they have 4 tapped holes where you can screw in the bolts that will push the flange away from the MS

One has to be very careful as not to hit the walls of the MS while removing the source flange

Of course I’d wear lint free gloves

I normally take a lot of pictures to help me reassemble. Just use my cell phone. It’s good to have someone around but most of the time I am not so fortunate!

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The “bold step”Pulling out the source flange very carefully… a tiny crack on the ceramic is “deadly”

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The Ionization chamber..that’s where ions are produced

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Safe housing..well the legendary “clean benches” that one boasts upon in papers rarely exist in a lab.. In the worst case, I “define” my temporary work bench take as much precautions as I can.

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Examination and planningNormally I’d change the filament as the source is clean

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Not so lucky!… well had to dismantle the source as there were carbon spots from a dirty sample.Cleaned with dilute nitric acid (not the ceramics), ethanol and acetone in a couple of cycles in an ultrasonic bath. Dried under an infra red lamp..

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ReassembleA schematic of the source was very useful in putting things back…

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All set!with some patience and concentration… the gold seal on the new generation MS was very handy

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Ready to BakeReconnect the ion source to the MS, evacuate to 10-6 milli bar by rotary and Turbo in 30 minutes. Time to cover the baking ovens

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Back to work!48 h of baking at 250 oC brought the vacuum to 10-8 milli bar, cool down and reconnect peripherals… 5 x 10-10 milli bar… time to check backgrounds and mass 40 rise… couldn’t believe the mass spec was open to air!

http://ratan.visiontech4u.ca