Electronics > Projects, Designs, and Technical Stuff
CRT Mass Spectrograph - wait don't leave!
ChristofferB:
Ok so this kinda came to me in a dream, but it'd be nice to get some feedback in the concept stage.
Using an oscilloscope cathode ray tube as a visual mass spectrometer
Here's why I think it could work:
- Tube already has provisions for generating and accelerating charged particles
- Tube has electrostatic deflection plates which could be utilized as a mass analyzer (deflection must be proportional to m/z)
- Fluorescent phosphor should light up no matter what ions hit it, right?
The plan (concept, more than plan, really):
- Drill a pinhole in the electron gun assembly of the CRT flask, vacuum escapes. Insert thin fused quarz capillary into electron gun assembly, seal with UHV-epoxy. This could be a GC column supplied with minute quantities of volatile molecules to be analyzed in a stream of He/N2/Ar
- Drill a larger hole in the tube further towards the screen, mount a standard aluminium KF25 vacuum flange with copious amounts of high vacuum epoxy and/or cement. Flex tubing to turbo pump and vacuum system
-Put scope in XY mode crank the deflection in one direction until a string of spots is observed.
Now, even hydrogen is 2000 times heavier than an electron, so there will be needed a LOT of voltage on the deflection system, but thiscould be messed around with. Magnetic deflection from the outside also a possibility.
Ionization might be a problem, but the electron gun is kindof similar to an electron ionization source from a mass spectrometer. Electrons are still likely to accelerate, but will be deflected so much they'll probably crash into the deflector.
What do you think? Is this even semi-realistic?
--Chris
Conrad Hoffman:
It doesn't sound completely crazy. I used to use a quadrupole gas analyzer and I remember it also having some magnets, but that was a long time ago. Also used and rebuilt a Veeco helium leak checker, MS-90 I think. These things weren't rocket surgery and could be serviced with shade-tree auto mechanic skills or a bit more. With a moderate scientific background, you should be able to build something that demonstrates the concept or can even make useful measurements.
ChristofferB:
I'm glad to hear it!
I've looked at the MS leak testers/ residual gas analyzers, they're pretty nice for a tiny MS.
I guess I could calculate the charge on the deflectors needed to deflect a given ion a given amount, but I'll have to dig up my notes.
Kleinstein:
In theory it may work, bit I am afraid the intensity on the screen may be rather low, as the ion current one gets with simple ionization is relatively low. With a high ion current one could also get damage to the screen. Chances are one gets better sensitivity picking up the ions directly as a current.
So I would more like start with some vacuum HW and add the electrodes, ionizer and so on.
ChristofferB:
That's true but that's a completely separate project. Building a quadrupole mass analyzer would also be interesting but the elegant idea herer would be minimal changes to the ion optics.
However if one wanted an electrical output from the CRT:
If one made a plate with a small (<1mm^2ish) hole at some point on the deflection path and coupled a PMT to it - then the deflection could be swept giving a true mass spectrometer.
Honestly my biggest practical fear is the implications of drilling a large hole in the PMT without compromizing functionality- or ability to withstand vacuum!
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