Replacing a filament on an isotope ratio mass spectrometer

A mass spectrometer is a machine capable of ionizing chemical species and separating them based on their respective mass-to-charge ratios, under the influence of a magnetic field. A filament is that part of the mass spectrometer (housed inside the "ion-source") which, via electric heating, furnishes electrons which ultimately ionize the introduced chemical species (such as say, species of carbon dioxide). Commonly, practitioners of mass spectrometry might find that there is no signal being produced in the source which, apart from a broken heart, leads to downtime for sample analysis (an orange instead of a green emission light will indicate this). A faulty filament due to "burn-out" is probably a good first suspect for a complete lack of signal, and therefore, replacing a filament might be a nifty trick to have up your sleeve, even if you are a semi-serious user of the instrument. Furthermore, I assure you that after some practice, this will become a rather routine operation that can be done quickly and effectively. Obvious disclaimer: I hope that you are replacing your filament responsibly and carefully: even if it is your 3rd/4th time, I'd recommend supervision of some sort.* In this blog post (since we had to do this in our lab last week), for the sake of documentation, I detail the steps in changing a filament in a Thermo Fisher Delta V Plus Isotope Ratio Mass Spectrometer (IRMS). Of course, I am sure that different techniques from different labs and small tips and tricks work as effectively (or more) than what I have detailed here, but these set of steps work really well with our setup for the filament replacement operation.

Broadly, this operation consists of five steps:

  1. Create space for the source on a workbench

  2. Remove the source from the mass spectrometer

  3. Remove the faulty filament

  4. Replace the faulty filament with a new one

  5. Put the source back into the mass spectrometer

Step 1: Create space for the source on a workbench

  • Clear up space on a workbench (away from the edges), close to your IRMS where you can place the source and ultimately, replace the filament.
  • Cover the space with a large sheet(s) of aluminum foil atop which the source can be placed. 
  • Make sure you have a pair of latex gloves on for the entire procedure (especially while working with the source).
  • Breathe and make sure that you are alert - you're dealing with expensive equipment!

Step 2: Remove the source from the mass spectrometer

Removing the source from inside the IRMS.

Removing the metallic claws which hold the outer casing of the ion source.

  • First and foremost, ensure that on the ISODAT software, the ion source is OFF. Essential!
  • Now, you want to make sure that all your valves are closed in your IRMS framework (ISODAT tip: ctrl+click allows you to close all valves on the dual inlet) prior to powering down the pumps.
  • After these valves are closed, turn off the main and inlet pump switches in front of your mass spectrometer. 
  • Brace yourself for the absolutely unnerving sound of the turbo pumps spinning down (for reference) - it usually takes about 5-10 minutes. Let it sit for an additional 5-10 minutes after powering down.
  • Now, unfasten the two screws (hex keys are your friends) at the top and bottom of the casing door.
  • Next, disconnect the wired connectors that are attached to the board (which sits on top of the source).
  • Finally, while holding onto the board and outer casing of the source flange such that it is held at a constant angle (perpendicular to your palm), undo the screws holding the metallic claws using a nut driver. Make sure that you loosen each one by small amounts (and move onto the diagonally opposite one) instead of loosening one of them entirely.
  • As the last screw is almost removed, firm up your grip on the source, and slowly remove it while holding it at its base with both hands.
  • Note the configuration and orientation of the guide pin with reference to the orientation of the source as you slowly remove it from the mass spectrometer.
  • Place the source on top of the aluminum foil with the black screws of the board at the bottom and the focusing quad (or smaller side up) at the top.

Step 3: Remove the faulty filament

Front view of the source (facing the filament).

Backside view of the source (behind the filament).

Loosen the screw holding the part so you can remove the magnet.

  • Although the ion source looks very complicated, remain calm! Don't panic and make sure that you don't disrupt any of the leads while placing it on the foil. 
  • Familiarize yourself with the asymmetry of the source and rotate it such that the filament side faces you.
  • Locate the magnet situated behind a holder, right on top of the of the filament.
  • Using a small flathead screwdriver, preferably from a set of screwdrivers dedicated to source maintenance and repair, slowly unscrew the part that holds in the magnet.
  • Now, pivot the part upwards, such that the magnet can be removed, and tighten the screw such that the part is positioned upwards and away from the filament.
  • Keeping the direction and orientation intact, remove the magnet and place it beside the source on the aluminum foil, in the same direction as it was facing while housed inside. This is an important step because when you put the magnet back into place, the poles have to be in the correct orientation. 
  • Next, gently loosen the bottom two screws, beneath the loops i.e. the "legs" of the filament, such that when you move them with your hands (GLOVES ON!), they can be removed out of the casing. 
  • Finally, completely remove the top two screws that hold the filament in place (one just beneath the screw holding the magnet holder in place and its counterpart on the other side). Keep these two screws carefully on one side of the aluminum sheet. Do not lose these! For utmost safety, you could place them in a small, empty beaker beside the source.
  • With these two screws out and the bottom two loosened, you should be able to slowly work the faulty filament free, and remove it. While doing so, make sure that accidentally none of the other leads in the source outside of the filament casing touch each other.
  • You can use a multimeter on the two leads of the filament and confirm that something has burnt out (i.e. there is no continuity between them). Additionally, if you have one lying around, you can take a look at the tungsten wire through a microscope and observe whether it has truly "burnt out" (see pictures).
  • You are more than halfway through now!

Step 4. Replace the faulty filament with a new filament

Continuity is broken in the faulty filament and you can see that its tungsten wire is broken (top two images) whereas the new filament (below) displays a resistance of 0.3Ω and contains an intact tungsten wire.

  • Remove the new filament from its case and check its continuity with a multimeter to make sure that it isn't a dud (it should read 0.3-0.5 Ω). 
  • Now, slowly insert the filament "legs" inside the slots and try to position the head such that it aligns with the top two slots.
  • Gently tighten the bottom two screws and make sure that they make contact with the filament legs.
  • Now retrieve the two top screws and fasten them into place at the top of the filament. Impact screwdrivers can be very useful here. 
  • Make sure that the new filament is seated firmly in its spot and that there are no leads that are touching each other. 
  • With the same N-S orientation as it had before, place the magnet back into its place, atop the filament and slowly tighten the screw which holds it in place. 

Step 5. Put the source back into the mass spectrometer

Make sure none of the leads are touching when you put the source back inside!

Make sure none of the leads are touching when you put the source back inside!

  • You are almost finished - all you need to do is make sure the source goes back inside the mass spectrometer safe and sound! Before doing that, make sure that no leads are touching each other and make sure that the filament is firmly in its place.
  • Now, with both hands, put the source back into the mass spectrometer. You need to be slow, but firm. You might hear the top portion of the source bend slightly - but that's OK! The springs are meant to be compressed. 
  • Make sure that the orientation is exactly like it was before - this is easy to do if you look at the circuit boards and connectors at the base of the source.
  • Ensure that the pin on the source goes into the housing slot during this procedure.
  • Holding it in place with one hand, with the other hand, slowly screw the metallic claws back into their places. 
  • Clip the connectors back into place.
  • Screw the outer door shut.
  • Power the pumps back on and wait for the vacuum to get down to their usual levels (1-2e-6 range). The source light will turn to green in front of the mass spectrometer when it is safe to turn the source on.
  • Now, turn the source on and wait for the big moment!
  • Is the emission light green? Yes?? Well, you have successfully replaced a filament! Ideally, you want a box-to-trap ratio of 0.75:0.75 in the signal. No? Hmmm. Give it some time, but if you see no signal on your software, it is likely that some leads were in contact, and you will have to remove the source again and check those leads. Otherwise, there is always the possibility that the new filament is not as hot (can happen with bad batches of filaments) - so you're going to have to get back into it!

A big round of thanks to Allison Lawman and Anthony Krupa for help during this particular operation pictured above!