CRT Mass Spectrograph - wait don't leave! - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 23, 2020, 02:59:24 pm
content: 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 spectrometerHere'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

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: February 23, 2020, 03:08:29 pm
content: 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.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 23, 2020, 03:14:51 pm
content: 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.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 23, 2020, 03:40:25 pm
content: 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.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 23, 2020, 04:22:55 pm
content: 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!

author: LaserSteve
author_rank: Super Contributor
timestamp: February 24, 2020, 02:50:42 pm
content: J.R. Pierce, The Guru of the optimized Electron Gun from Bell Labs, is rolling in his grave.I'll skip why the CRT will be amazingly difficult to bias correctly ,as you would need a few more electrodes and the cathode mount is "in the way", but may I suggest you build an "Omegatron", which with care, can be made with little more then a Dremel tool , diecast box, and some really good vacuum epoxy, plus a few old Cold War surplus metal Octal relays to salvage the glass to metal seals. That little 250 AMU Omegatron would be a good start and a lot easier.PMTs have ion suppression as part of their design, the compounds on the electrodes will oxidize. Your looking for a "Channeltron" or Faraday Cup. As a chemist, you'd find Channeltrons are not going to need "Special technical Magic" to make.See US Patent 3,152,280 and several open source NASA publications for details on Omegatrons.Works well at higher pressures, and since there is no getter, you will not be fighting clouds of barium oxide powder jamming up your instrument, nor fighting the Aquadag coating when it massively outgasses every time you let it up to air. With an Omegatron, a FET electrometer is all you would need for detection and there is no voltage over one hundred volts, if even that much.Not to mention the Watt or so of required tunable RF would be really easy these days to generate with a DDS board from Ebay.Some of the "Time of Flight" strategies run at 1 ATM, no filament, and no more then 1500 volts with Mosfet pulsers and are simply made of Teflon and steel tubing. The commercial large molecule versions are handheld.Dead quadrapole leak detectors are all over Ebay, and I miss the simple tower PC sized 1970s Veeco I used in a Lab.If you did it with a CRT setup you'd have to dismount the gun, remove the cathode, add the ionizer, and put it in a new housing. That is, unless you like analyzing Ba-Ca-Sr carbonates, exactly once.If you have access to "Review of Scientific Instruments, you'd find some really nice 1940s and 1960s MS designs that will get you there and consist of bent steel tubes for the housings.Steve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 24, 2020, 05:18:14 pm
content: Quote from: LaserSteve on February 24, 2020, 02:50:42 pmHarvesting the energy of people rolling in their graves is a source of sustainable energy!In all seriousness, Modyfying the electron gun was out of the questions. My hope was that injecting analytes into the electron beam would ionize them enough to let them be accelerated, but that may be a long shot. I had forgotten about coating and getter as well, that would be messy. Drilling a large hole in glass without getting waterinto the tube would be difficult at best too.I've looked at the bent tube MS's, and honestly, If i'm already building from scratch I'd rather go with a linear quadrupole.The omegatron is absolutely adorable! That's definitely something to investigate! I wonder why they went out of fashion..Thanks for the input!--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: February 24, 2020, 06:37:02 pm
content: The Omega-tron seems to need a sizable magnetic field. This can interfere with other parts of the system and not so easy to just mount to an existing system.The quadrupole mass spec seem to be easier and lower weight but still good enough for residual gas analysis.There is a limited use of slightly better resolution than keeping integer atomic numbers apart. So to really get to the range where one can distinguish different molecules of same integer part (e.g. N and CH2) it may be quite hard. So it would take a really good one to get extra information.

author: calzap
author_rank: Frequent Contributor
timestamp: February 24, 2020, 07:45:51 pm
content: Has anyone had experience drilling holes in CRT's, other tubes, or light bulbs? I'd like to know how you did it.Mike in California

author: james_s
author_rank: Super Contributor
timestamp: February 24, 2020, 08:38:04 pm
content: A significant problem is that exposing the cathode to air will quickly poison it. CRT cathodes are activated by hearing them up after drawing a hard vacuum, it's a one time process and opening the tube requires a new cathode.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 24, 2020, 09:02:46 pm
content: Omegatrons morphed into something simple looking with six big mesh grids with alternating DC bias and applied Rf that worked even better. Then the Quadrapole , Hexapole, Octopole, happened. That and better magnetics for generating high voltage swept RF made the Quad the beast of choice.Omegatrons became popular until Phillips / Sylvania / Leybold / NASA decided that making Omegatrons in a standard octal vacuum tube envelope were no longer needed. They actually were in production for a long time.This design has always appealed to me:http://lampes-et-tubes.info/sp/sp103.php?l=eBy home made tube standards it is simple and can be brazed instead of spot welded.This is one case where the details in the RGA patents are better then the ones in scientific publications, and the USPTO server is your friend.Just looked at some historical texts.J.J. Thompson built one of the first MS widgets , with a phosphor detector! Something called a "Parabola Spectrograph"That doesn't look too bad to build, just interpreting the results will probably drive you crazy due to the display format.Steve

author: LaserSteve
author_rank: Super Contributor
timestamp: February 24, 2020, 09:25:37 pm
content: Usually the tube walls are so thin and the COE 88 LEAD or Soft glass so fragile that we use the standard glassblowing technique of melting a spot and blowing a bubble, then knocking off the bubble and fusing on new glass.Its possible to make a filter to trap the moisture out of your breath.Steve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 24, 2020, 10:14:36 pm
content: Quote from: Kleinstein on February 24, 2020, 06:37:02 pmThat is where ionization techniques come into play. I can't imagine seeing the same ionization form generating both something like N*- (I have a hard time believing that is ever seen) as well as CH2, which is a molecular fragment.Either way, if I ever commit to building an MS, I will absolutely couple my sample in through some sort of chromatography. Maybe just a room temp. GC, similar to the one I've built previously.I'll have to keep my eyes open for an omegatron tube, though I imagine they'd be quite the collectible. If one built one themselves, mounted on a metal tube socket in a standard KF40 vacuum flange I guess it could be possible to build the tube fro scratch. The real tube looks to have elements of gold plated foil, likely nickel or something similar. If I can live with seeing copper everywhere I guess copper foil could be used, too.

author: LaserSteve
author_rank: Super Contributor
timestamp: February 25, 2020, 04:08:48 pm
content: This just got interesting:See if you can find a copy of:"Development of Mass Spectrometers from Thompson and Aston to Present"By Munzenberg...Then look at this:http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0103-97331999000300002The above URL is:Brazilian Journal of PhysicsPrint version ISSN 0103-9733On-line version ISSN 1678-4448Braz. J. Phys. vol.29 n.3 São Paulo Sept. 1999https://doi.org/10.1590/S0103-97331999000300002100 years of ion beams: Willy Wien's canal raysTurns out the WIEN filter rods DO NOT have to be in the vacuum... May result in "Fuzzyness" but they do NOT have to be in the vacuum.Steve

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 25, 2020, 08:56:32 pm
content: Thanks! I'll check it out! Atmospheric pressure quadrupole is an interesting concept! but I mean, it's not uncommon for the ions to be generated at atmospheric pressure and then sucked through a pinhole into the high vacuum region, so it makes conceptual sense.Have I missed some theory, or isn't the "omegatron" just a tiny cyclotron? In that case they are still used! Ion Cyclotron Resonance - MS is totally a thing!I wonder if one could make a pocket sized conventional Dee-type cyclotron as a mass analyzer.--Chris

author: Kleinstein
author_rank: Super Contributor
timestamp: February 25, 2020, 09:23:32 pm
content: A cyclotron tends to need quite some magnet, e.g. similar to the magnet used for the permanent magnet NMR.I would not call that pocket size. The question is more if it is light enough to be in the desk top category.A point with the omegatron is that it usually includes the electron beam to generate the ions.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 25, 2020, 09:35:15 pm
content: One of Craddock's first cyclotrons, this cutie, was operated at fairly low fields, i believe.You're right the modern ICR-MS's uses a superconducting magnet, and a fairly high RF freq., but I believe the omegatrons was all the way down in the 1-2 MHz, and with just a small ceramic magnet on each side.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 26, 2020, 05:23:26 pm
content: The datasheet for the Phillips tube gives a magnetic field of some 0.4 T as an example - it does not need superconductors, but still a sizable magnet.The relatively flat form factor of the old one from Craddock suggest that it is made to work inside a sizable magnet with a field back to front (or the other way).The vacuum / tube part can be relatively small and portable, but the magnet may need wheels.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: February 26, 2020, 06:02:04 pm
content: The old non-superconducting research magnets commonly used for CW-NMR and EPR, as well as small cyclotrons are indeed huge, and usually water-cooled.The omegatrons however, apparantly not so. I found a design with ferrous magnets something like 3cm in diameter!I believe one of the main visions for omegatron MS's was upper atmosphere research, so it was designed with rocket/balloon payload-ism in mind.