Mass Spectrometer Ionizer Filament Assembly Repair We would Like to share our experience in the local repair of ionizer assemblies for our Finnigan model 3000 Mass Speetrometer GC Peak Identifier.' The tungsten filament ribbon is readily available through Finnigan. The first requirement is a spot-welder with the necessary small electrodes for such fine work. This is a fairly expensive piece of specialized equipment that may not he readily available. We have overcome this problem by making an adaptor with a miniature set of electrodes. This adaptor can he clamped to the electrodes of astandard spot-welder. The damping action of the regular welder must be deactivated. The miniature electrodes are made from number ten capper electric wire. Adequate clamping pressure for these can he applied by a small spring or by rubber hands. See illustration for construction details.
The old filament assemhly must be cleaned after removal from the instrument and the filament support wires must he free from oxides if the weld is to take. First, clean and polish the filament holder with a slurry of acetone and 400 or 600 mesh alumina or silicon carbide grit as recommended in the cleanout procedure. The oxides must he removed from the tungsten filament support wires by applying apaste of sodium nitrite and heating with a miniature torch. Remove any excess flux; leave just a trace on the wire. For good electrical contad he surethe electrodes an the spot-welder are bright and clean. Wetting the junction where the filament wire is to be welded may help to start the weld. Six or eight watt seconds is the recommended setting for the weld. This may he varied slightly to optimize the weld. Too little current and the weld will not hold, while too much current will hum off the filament and perhaps the support wire as well. A few practice welds are in order before risking a filament assembly. The tungsten grid wire from a defunct Bayard Alpert vacuum gauge tube is the name size as the filament sunoort wire and can he used to nractiee on with no risk. New sun~ortwires can also be fashioned .. fn,m thii Rnmpgrrd wire if you happrn to break a filament Support. f o r k tunpten wireat red hcnt ns it i%hrittle nnd will break if an attempt is made to form ir rold. Silver solder will wmk well 1,) fa5trn i t to the filanlent support post. ~~~
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'Published in part inFinnigon GCIMS Users Newsletter, Vol. 3, Numher 3 (September 1917). South Dakota S t a t e University Brookings, 57006
674 / J m i of ChemicalEducation
Floyd C. Melrose Department of Chemistry H a r r y Svec College of Engineering
