My home built mass spectrometer

What can a mass spectrometer do?

A mass spectrometer separates molecules of different mass and presents the result as a graph.

  • The high peak is nitrogen at mass 28
  • The smaller peak is water at mass 18

To the left of this are two badly separated peaks, probably:

  •  OH at mass 17 and oxygen atoms at mass 16.
  • These are fragments of water created within the spectrometer.

What makes it work?

The Ion Source

  • A heated filament supplies electrons, that are accelerated towards a collector by an electric field.

  • On their way towards the collector, they collide with gas molecules within the vacuum system. Electrons are stripped from the gas molecules, creating positively charged ions.

  • These ions are drawn and accelerated out into the mass spectrometer by a negative electric field. 

The Magnetic Sector

  • The ions coming from the source passes a narrow slit, and then into the magnetic field, in the form of a 90 degree sector. 

  • The ions are deflected in the magnetic field, according to their mass and the strength of the electric field.

  • By sweeping the strength of the electric field, ions of different mass are able to pass through the exit slit.

  • They are focused by the sector magnet at the exit slit.

The Ion Detector

  • The ions passing the exit slit are collected by an electrode, formed as a faraday cup.

  • A very sensitive current amplifier converts the tiny current from the faraday cup to a voltage, that corresponds to the abundance of ions of a certain mass.

  • This is presented as an XY-graph, with the mass on the X-axis and the abundance on the Y-axis.

Why did I build it?

  • I wanted to be able to leak check my electrical feed throughs, before I incorporated them into my Crooke´s tubes and other evacuated discharge tubes that I make.

  • Leak testing with helium is an established and very effective method, that can be done with a mass spectrometer.

  • It could be built as a permanent addition to my vacuum system.
  • Not many DIY mass spectrometers have been built, so it was a challenge.

  • I already had the vacuum system, and I had some ideas how it could be done.

I was inspired by a few other sucessful amateur efforts on the internet: