Can a Cathode Ray Tube be Used to Build a DIY Mass Spectrometer?

[thegeekguy]

I'm wondering if it would be plausible to use a cathode ray tube (crt) style monitor to build my own mass spectrometer. Unfortunately, commercial models are prohibitively expensive, but if I had plans or at least a good idea of how to build one, I have the nagging feeling I could.
My idea is as follows:
The tube's electron gun would ionize the gas being analyzed
The ions would be pulled toward the anode at the front of the tube, and be deflected by one of the two deflection coils normally used for positioning the electron beam. The ions would hit the phosphor, where a photomultiplier would be positioned on the other side, connected to an oscilloscope. The deflection coil would be energized with a sawtooth wave to deflect the ions across the photomultiplier and send a repeating waveform graph of the ions mass from the photomultiplier to the oscilloscope where the substance's mass spec could be read.
This is of course assuming I could generate sufficient vacuum.
I'm wondering if the premise of this even makes sense. I have only a cursory knowledge of chemistry and physics, I'm still only at the high school level, so please forgive me if this is utter nonsense.
-Mark M

 

[maimonides]

Hello Mark,
I fear there´s only the Radio Yerevan answer: In principle yes, but ...
a few hints:
- the vacuum system alone will probably exceed your budget
- you need openings for to attach the vacuum system and an inlet for your samples. Cutting the glass of a CRT is not so easy, and then you need to attach flanges or so to it ...
- the magnetic field you can generate with the coils in a CRT is far to low

So sorry, no.

Maybe you can find a more realistic project
maimonides

 

[thegeekguy]

I have access to a vacuum pump, so I was planning on removing the electron gun and high voltage supply completely, and mount them inside of a thick walled pipe that could take the stress of a vacuum without failing, to avoid many of the glass and air tightness woes. As for the magnet and phosphor, I have found a photomultiplier tube with a phosphor already deposited on its surface. I also could build a powerful electromagnet to steer the beam. I'm not terribly worried about the mechanical issues of the spectrometer, I just haven't seen anything that goes into great depth as far as theory. I was wondering if my assumptions were correct. Does anyone know of anyplace I could find specifics about how these work? I understand about accelerating the ions toward the detector, but how powerful would the magnet need to be? And how high of a vacuum?
-Thanks!

 

[alxm]

I have access to a vacuum pump

What kind of vacuum pump? What maimonides is alluding to, is that an ordinary vacuum pump isn't good enough.
For the high vacuums required for MS, you need a turbomolecular pump.

 

[thegeekguy]

I believe it is turbo-molecular. It is used for an electron microscope in a physics lab, so I believe it is sufficient.

 

[alxm]

I believe it is turbo-molecular. It is used for an electron microscope in a physics lab, so I believe it is sufficient.

If you knew what you were doing, you'd know whether it was or not. Not all electron microscopes use them. Also, who in their right mind would let a high school student utilize a very delicate pump that costs thousands of dollars for his DIY project?

To be honest, this whole proposal is insane, unrealistic and (if you'd actually go so far as trying) quite dangerous.

 

[hodges]

I remember reading an article in an old amateur science magazine about someone actually doing this. I recall they were able to see that potassium salts are composed of two isotopes due to two different amounts of deflection.

I seem to recall this was from a CD full of old articles from either Popular Science or Scientific American magazine. The article in the question was from the 1950s I believe. You might browse online and see if you can find this collection. I seem to remember someone buying the CD but I don't remember where.

hodges