A timed electron impact ion source for time-of-flight mass spectrometry

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ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

A timed electron impact ion source for time-of-flight mass spectrometry

Post by ChristofferBraestrup »

This doesn't totally fit any category but 'vacuum technology' encompasses pretty much anything in a vacuum I guess. Feel free to move.

I am currently constructing an ion source for my homebrew time-of-flight (TOF) mass spectrometer.
It is an idea investigated by Burle in the 1960's. They commercialized the channel electron multiplier (channeltron) and I guess were looking for applications.

Time-of-flight mass spectrometry: Accelerating an ion and timing it between two detectors. As energy is set, speed, and therefore flight time is proportional to mass.

In practice, the trick here is to use a channeltron as ion source; if the anode cap is removed, the electron bunch is shot out and can ionize residual gases.

The timing signal is extracted from the channeltron similar to using the "dynode" signal from a PMT as output.

A quartz window allows light in, and generates photoelectrons upon hitting the channeltron cone. a variable intensity light source is needed, the intensity set so electron pulses are clearly separated.

The ions are then guided through a drift tube towards another channeltron, operated normally as a detector.

This is the basic idea. In reality only 3 components in vacuo; two channeltrons and a neg bias'd drift tube inbetween. Data acquisition could be an oscilloscope, triggering on the start pulse, or a nim system, with a time to pulse height converter and MCA. I have tested the TOF electronics in a small youtube video:

https://www.youtube.com/watch?v=VG3VIH1 ... ogiraffery

Attached is some pictures of the setup. The channeltron is a modified Galileo Electro Optics 4770 with the anode cap and mount removed.


See attached photos.

One thing that I'm unsure of: Should the output voltage of the channeltron (here, close to 0V) be held at a negative voltage, to ensure the electrons shooting out is still attracted to the (grounded) chamber wall?

The setup as pictured, yields no pulse output with Vdet=-2800V in a vacuum of 4*10^-7 torr. These channeltrons normally needs a little higher voltage, I have a new 7kV 1mA NIM PSU on its way!
Thanks for the look, I hope to have more info soon.
Attachments
feedthrough wiring of channeltron
feedthrough wiring of channeltron
assembled source
assembled source
components laid out
components laid out
specific wiring of source channeltron
specific wiring of source channeltron
Ion source mechanism; electron impact
Ion source mechanism; electron impact
General setup, simplified
General setup, simplified
ChristofferBraestrup
Posts: 131
Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: A timed electron impact ion source for time-of-flight mass spectrometry

Post by ChristofferBraestrup »

Similarly no electron output with the cone at ground potential and the output at +HV. I was worried the negative potential cone would deflect electrons out towards the chamber walls, so this change was attempted.

The channeltron draws significant current, however, more than expected, and I seem to remember channeltrons failing towards lower resistance.

I think the reason channeltrons (especially Ebay ones) has a shoddy rep is that people will replace their old MS detector, put the old one into the new box and put it away. This then gets mistaken for new or NOS and sold.

For reference, the russian ones I bought, VEU-6 works fine, and are very sensitive to photoelectrons. these have un-removable end caps though.
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