Tungsten filaments are an integral part of ionization sources of many mass spectrometers. They produce electrons that ionize a sample. The ions are measured by measuring electric potentials or counting ions. I thought of sharing my notes on a filament changing exercise in Noble Gas Mass Spectrometry, although many of the steps are common to others.

1. Text
How to service the Ion Source of a Mass Spectrometer
My notes
- Ratan Mohapatra

2. Why a filament
A 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!

3. Prepare
Servicing 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

4. Text
Ionization Source Electrical connections
Caution:
Improper connections are fatal as you may be supplying 8 KV to ground!

5. Dismantling the Ion Source
When 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!

6. Text
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.

9. Text
Examination and planning
Normally I’d change the filament as the source is clean

10. Text
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..

11. Text
Reassemble
A schematic of the source was very useful in putting things back…

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

13. Text
Ready to Bake
Reconnect 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

14. Text
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!

15. http://ratan.visiontech4u.ca