DIY mass spectrometer measures potassium in dietary salt substitute

today and Applied Science we're going to talk about mass spectrometers I'm gonna show you how I built this copper tube mass spectrometer and then we're gonna use it to verify the contents of this dietary salt substitute so this project gets its start in one of my favorite places is the youtube comment section no really actually it's your comments that keep me going and in this case was the inspiration for this whole video as it turns out in 1970 Scientific American published a paper published an article describing how to build your own mass spectrometer which itself was based on an academic paper that came out in 1963 so I've gone through quite a bit of development to do this and it turned out to be quite a detective story I've worked on this thing for months and I went through a period where I thought there's no way it could work the way that the Scientific American article said it would and I thought they basically fudged the results and then I flipped back the other way and I said now it was probably my fault all along but as it turns out the truth is actually somewhere in between so this actually turned out to be quite an interesting story let's talk about how this thing works the whole business basically happens between here and here and we load our sample to be analyzed on this side and send it out into a beam where it gets bent back around and comes up this side and is detected over here and there's a really good analogy for how this works right like if we take white light and send it through a prism we get a spectrum of colors and we can measure the amount of each color that make up that white light and in this case what we're doing is taking our sample and sending it in a beam this way and we spread it out into its component masses so remember that everything is made up of the chemical elements and each one of them has its own mass so the idea with a mass spectrometer is to basically break that matter up into its atoms and just weigh how much we have of each right this is a really powerful concept because it gives us the ability to say what any chemical is so for example if we put some of this in here the chemical formula for this is C 6 H 5 and 3 and if we put this through the mass spectrometer it would tell us that we have ratios of carbon hydrogen and nitrogen with a few other assumptions and then we No oh if we ever see those that ratio of carbon hydrogen and nitrogen we know it's benzo triazole boom there you go so it's a way to give us an instant readout of what a chemical is just based on having access to a little bit of that chemical and no other information but you can see there's a pretty big problem here right if we take another chemical that has a similar formula this one is C 10 H 8 and to you'd say okay we know that ratio we could identify this but what if we had a mixture of both of these now we've got a problem because all the carbons mixed together with all the hydrogen's and all the nitrogens and we would have some new ratio but the mixture would determine what that ratio is but we don't know what the mixture is because that's what we're trying to analyze and this is actually a real problem with mass spectrometers that break everything up into the constituent atoms we just have no way of knowing what the mixture originally was and for this reason most mass spectrometers in industry are preceded by a chromatograph and so a chromatograph separates molecules based on their size and then the output of that gets put into the mass spectrometer to analyze the atoms within each molecule so it's almost like a hierarchical way of separating things into components and then analyzing each component so we're going to skip the chromatograph for today but if someone sees a scientific American article about building one in a home shop be sure and let me know in the comments and we'll make a future video about it but we'll get around the problem of having this sort of mixture ambiguity by only putting pure substances in here so if we can guarantee that we're only analyzing a pure substance then we don't have this problem with ambiguity so I keep talking about adding a sample to this thing so let's open it up and see what that actually entails I've got a little grounding strap here this is conductive copper tape and I've got a little lead here so I can quickly disconnect this and then open this up and the way it's built is basically taking a rubber stopper and loading up some needles and long sewing needles into a drill press and then shoving them through the stopper and then soldering some bits to this end of the needle so basically we get a pass through it's really the cheapest easiest way to make a vacuum pass through and at first you might be thinking well this is terrible for vacuum because the rubber is not going to be compatible with this high-end vacuum system well first of all this is not a high end vacuum system and second you might be surprised actually the record even with all this rubber exposed on the inside of my vacuum system we can still get this down to really good vacuum levels and that ended up not being a problem thankfully quick note about the vacuum system you need really good vacuum when you're doing mass spectrometry because the ions are moving relatively slowly through here so there's a lot of time for them to be interfered with by you know gas molecules that you don't want in there ideally you want literally nothing between the collector and the emitter but nothing is an impossibility so we try to do the best we can so this is just a pen engage a vacuum gauge to see how good the vacuum is in there and this is a turbo molecular pump which is really convenient for this because we have to start it up and shut it down very frequently and this is a one of the fastest start and stop you know vacuum pumps you can get and it's also very clean so the basic idea is just put everything together turn on the roughing pump on the floor there and then turn on the turbo molecular pump and the whole thing gets down to vacuum in about 10 or 15 minutes and when I say down to vacuum this thing will typically achieve 5 times 10 to the minus 4 tour which is pretty good considering there's rubber stoppers and everything in there everyone is concerned about let it you know solder copper problems and everything but we're way beyond that like we're if we have large rubber stoppers exposed that's going to be a much bigger issue than leaded copper or leaded solder and everything I made this copper tube contraption on the lathe and originally just used rubber tube fittings to connect the thing to the vacuum various vacuum pumps like little bits of rubber tubing like this this was a super big mistake so this this PVC tubing out gas is like a beast in fact I I almost didn't believe how bad it was like even using a little section like this just you know couple centimeters exposed in there totally unacceptable it ruins the vacuum why those black Stoppers are okay and this isn't but it isn't okay and then I used this you know professional rubber vacuum hose and this was also pretty bad again I don't know why this is so much worse than those rubber blacks rubber stoppers but it was it might have just been the fact that using a hose clamp but these vacuums is just a really bad idea but again I'm using black rubber stoppers here and that's fine so anyway I ended up brazing on some custom KF fittings on the ends of this copper tube and that's how I put everything together I was originally going to use this huge glass oil diffusion pump for this project but the problem is the startup and shutdown time on this thing is just so long like over half an hour for both ways or even even more it was really killing my productivity so I'll probably come back to this in the future because it is a pretty cool device and it does work pretty well it's just slow to start up and shut down I've got a little tabletop gas burner as its power source so let's zoom in and see how this source ion source works so here's the business end of this ion source and the real key component is that tungsten filament it's basically a light bulb filament that have been extracted from tiny tungsten filament lightbulbs I've actually become a super expert in crushing light bulbs and extracting the filament carefully so that the glass bead and the filament are still intact and the way that we add a sample to this thing is put a drop of water that contains our sample dissolved in it right on the filament and let it dry either by passing a current through the filament or just blowing some hot air on it or just waiting around and eventually what we'll have is a filament that is coated with the powder basically it's just a way of getting the powder onto the filament and then we put this thing together into the mass spectrometer and turn it on and heat it up and when the filament gets to you know red heat it will actually ionize the sample for us it's actually a process that happens all by itself like you've heard of tungsten filaments boiling off electrons when it gets hot it can also produce ions the electrons smash into all those atoms and molecules that are set on the surface and it ionizes some of them and the trick is that all of these things are electrically isolated from each other in here so the filament is held at a positive voltage relative to the metal of this case so as soon as an ion is formed in there it gets dipped out of this whole assembly because there's a strong electric field that's trying to push it out but as you can see there's a slit at the top very narrow it's really only about three or four thousand isalud and in fact this is formed from razor blades so basically the way that I made this was to put razor blades between these two washers and solder everything together so that the solder holds everything in place keep in mind throughout this video I'm gonna make a lot of generalizations so when I say things like spectrometers generally work with positive ions not negative ions there's plenty of counter examples and mass spectrometers don't always break things down into component atoms a lot of times there's molecular fragments left and so on mass spectrometry is a huge field with tons and tons of different stuff going on and in a future video I'm going to talk about a more modern technique that uses lasers to do the ionization and then things really get interesting but anyway back to this ion gun so the filament is at a positive voltage we're producing positive ions the slit is at a negative potential or it's ground basically so as soon as an ion gets formed from that point on the filament it goes flinging out through the slit because there's this electric field accelerating it and it's helpful to think about this as if the beam were actually a beam of light right so the source is that well it's a light bulb filament so that's convenient and imagine that the the geometry of this will determine what the beam shape is so it's going to be shining out through that slit like a cone and the further away that filament is from the slit the more narrow the cone will be right because that's just the geometry of how this works we'd really like this thing to be perfectly aligned so that when we're shooting ions out of here they come out perfectly aligned on the axis of the stopper and it's rather difficult to get all this set up perfectly so we put these other two electrodes in here it's actually a brass plate one on each side and they're electrically isolated - and the idea there is that we can apply a little bit of voltage to these brass plates to kind of steer the beam in a way that we want to happen right so if we're if we're not perfect in aligning this filament exactly with the slit and then our system is not going to work because the beam is actually shooting out sideways we can apply a little bit of voltage here to fix things to fix the field so that we get everything aligned perfectly as it turned out these steering electrodes were not that helpful but we'll talk about the use of this a little bit later here's the detector side so we've got same rubber washer same slit on the receiver side and then there is a copper cup also attached to a needle that goes through the stopper and the cup goes to this SMA connector which goes to the preamplifier board so this is a transimpedance amplifier of gain of about a million and it's a it's actually a really nice chip but I'm not using it to its fullest capability this system is not high bandwidth but it is very high gain and the trick here is that the amplifier has to be really close to the source because the input capacitance is a big problem and this was actually an oversight that I missed for longer than I should have but if you have input capacitance that really ruins your whole trans impedance amplifier so you want this thing to be as close as possible and as little input capacitance as possible and in this case this whole thing is maybe on the order of I don't know ten puffs or something like that and that's even that is getting to be kind of a problem but we'll talk about the usage also a little bit later the power supply for the preamplifier is just two 9-volt batteries and conveniently I've got some voltage regulators here conveniently these batteries are nice because you get a bipolar supply and it's super low noise and it's isolated from everything it's actually a really nice way to do a front-end like this and then the output from this million gain trans impedance amplifier goes to this cable and then over to the bench and we'll talk about that next this is the cable from the preamplifier going - this low-pass filter and then we also apply another 40 DB of gain so in other times a hundred so the total gain here is one hundred million and then the output from this filter goes into the scope the scope is configured in XY mode so the vertical component of the trace is coming from our detector and amplifier here and the x position of the trace is coming from our control system so I mentioned that we were going to be shooting this beam of electrons through the system and we're going to be detecting it and showing the detection results here but how do we actually spread this into a spectrum like I've been talking about using a prism in this and that but we need some way of actually shifting the spectrum back and forth to see the different components so one way we could do this is to use something like a CCD or like an image sensor like you know in a typical light based spectrometer you shine your white light in and use a prism or diffraction grating and then you use like an image sensor to capture all the different wavelengths at one time but that's very difficult to do here because the signal is just so darn small we're gonna be talking about sub nano amps signals and having an array detector that's able to amplify nano amps signals from a huge sector of sensors is actually pretty difficult so one way that we can fix this problem is to just have one detector and then rotate the prism back and forth that would be kind of equivalent to applying a changing magnetic field to move this path of ions back and forth we basically sweep through different magnetic field strengths and stronger magnetic fields curve the particle beam more tightly than weaker magnetic fields and this is a totally valid way to do it - but the problem with that is we need a huge electromagnet and we need to control it very carefully we might have problems with hysteresis and so on so there's actually another trick let's take a look at the schematic and we can see what we're gonna do instead instead of changing the magnetic field strength which would kind of be equivalent to rotating the prism in a light based spectrometer what we're gonna do instead is change the incoming ion beam speed we're basically gonna be changing voltage back and forth and I know we're kind of pushing the analogy far here but this is sort of equivalent to reg shifting the entire beam of light that were going that we're sending into our prison so if we were to redshift all of the light going into our prison by a known amount that would indeed shift the output spectrum and if we knew what that shift was then we would be able to figure out you know what we're doing and in reality this is pretty easy basically we start off with a DC power supply set to a fixed value that's kind of our our you know base acceleration voltage and then we put an AC supply in series with it so the AC voltage is just added on to the DC voltage and this AC voltage represents our continuous reg shifting that's going back and forth and it's this AC signal that that drives the exposition on the oscilloscope here's a better look at the internals of the control circuit the original schematic specified a potentiometer and a filament transformer but instead I'm using this very act over here and the very act powers this transformer and in this transformer powers the filament and no real particular reason to do it I just had these parts on hand the big meter over here describes how much current is going through the filament and that's on the order of about three tenths of an amp and then the small meter the micro amp meter is wired in series with the filament and the high voltage supply it's kind of fun to have a current meter that's not connected physically to anything right like the ions are jumping off the surface of this tungsten filament and the meter will show that ion stream like the actual current involved with that ion stream and they're leaving the filament and going down the tube of the spectrometer it's kind of fun to have a current meter that's just not really physically connected to anything that is how it works one point that's different from the Scientific American article they claim that the emission current should be about 1 micro amp I never got this thing to work with one micro amp of ion emission I needed at least five or ten to see a signal at the other end one theme that keeps coming up in my testing is that they're really stretching how well this thing works in terms of the numbers and so getting away with one microgram of emission current means that your signal is going to be probably less than a nano amp and it's really tough or at all and it's really tough to sort of extract that out so to make your chance a little bit better just give it an extra order of magnitude of transmitted current another major problem of using this thing is that the amount of mass that goes in there is limited right like we put this thing together putting some fluid on the tungsten filament waiting for it to dry then assembling the whole machine starting it up and trying to get a signal and after the filament is hot for about five or ten minutes most of the usable mass has been boiled away either not all of it gets ionized actually a lot of it just ends up spraying the inside of that ion gun chamber but the point is that you only get five or ten minutes of use out of this thing before it uses up all of its sample then you have to wait for the whole system to cool down and bring it back up to room pressure add more sample to it put it all back together vacuum it back down only to get another five or ten minutes of time to get this thing working so it's a very challenging and sometimes exasperated to get going so that makes it even more satisfying when I finally got a signal out of it so let's take a look at that so finally after months of poking at this thing I got a signal and the signal looks just like the one from the Scientific American article and the one from the dudeney paper so it must be correct done upload to youtube but not so fast this is where things actually get really interesting I think there's a couple problems with this result and since the result is the same in the original paper the Scientific American article and my own results I think there's a sort of a systemic problem in here that's interesting to look at so first let's unpack what this signal is showing us there's two obvious Peaks here and allegedly what this is is the two isotopes of potassium so the element or the compound that we're looking at is potassium chloride that's what this you know no salt dietary salt substitute is and potassium happens to have two common isotopes so similar to carbon-14 dating you know how that works you you know carbon normally has six protons and six neutrons but sometimes it has an extra two neutrons carbon-14 and that one happens to be radioactive so over time the carbon-14 diminishes in value just because it's radiating away these neutrons are becoming carbon-12 and if you measure the ratio of carbon 14 to carbon-12 you can estimate the date of this thing so essentially what we're doing with the potassium is potassium dating this substance but in our case the potassium the heavier variant of potassium that has these extra neutrons is stable so you can't really date it using this technique and in fact all the potassium samples in the world will have this ratio because it's stable it it'll almost never change so the two most common kinds of potassium are potassium thirty-nine and potassium forty-one and the ratio should be approximately fifteen to one so the first problem with this graph is that the peak Heights are not in the ratio of 15 to one it's really it's hard to see because the gridlines aren't quite so visible in the dudeney paper but it looks to me like it's less than ten to one and kind of maybe closer to six to one so that's way off and we might be able to explain that away by saying well you know it's a non-linearity in the amplifier or maybe there was like mass fractioning going on or the thing boiled off the lighter variant first and then there was more of the heavy one or this or that something but I don't buy it I think there's a big problem in there but an even bigger bigger problem than that is that the acceleration voltages are wrong so they're correct for potassium in general but the separation between the peaks is too high to be realistic so let's dig quickly into the math and I'll show you why I think this is the case so I mentioned that the accelerating voltage was overall correct for potassium and here's what I mean by that since we're dealing with particle physics we have a very simple formula that describes how a charged particle is going to move in a magnetic field and here it is so the radius of curvature is equal to one over the magnetic field times the square root of two times the mass of the particle times the electric field that the particle was accelerated in divided by the charge and we can just plug in the numbers we know the magnetic field because you can measure it directly we know the radius because it's literally the curvature of the copper pipe it's hard to change that and we know what the fundamental charges we're gonna make one assumption here is that all of the particles that this mass spectrometer produces have single charge like one on ization one missing electron and if we rearrange the formula to get the mass out we end up with this we can plug in the numbers and all these numbers are constant so we can just put them all together and what we end up with is this constant divided by the accelerating voltage equals the mass in kilograms so if we go back to the oscilloscope trace the center voltage here is approximately 110 volts but I've got a 10 to one voltage divider on the x-axis so that top peak is about 1.6 volts above zero but really that's 16 volts above so a total of 126 volts and if we plug in the numbers 126 volts into the formula we end up with this really tiny number in kilograms and if we convert that to atomic mass units we get 39 perfect that's exactly what potassium should be now of course I've massaged the numbers a little bit so this one comes out perfect right like the magnetic field strength varies a lot from the center to the edge and you can kind of tweak it a little bit there was a little bit of tweaking going on but that's not going to affect what I say next now the other peak the smaller peak is about 0.6 volts on the x-axis below the center voltage that means 6 volts lower than 110 or 104 and if we use the same formula we're not massaging the numbers between same formula 104 volts in we end up with 47 atomic mass units it's way off that should be 41 if we were correct it would be 41 so I kind of went back and forth and tried to find a problem in a system or maybe I didn't measure something correctly but I feel like with all the constants held or with all the values even the measured ones held really the Delta should be correct I can't see a reason why the center voltage would be correct and then the Delta would be wrong but here's the thing if we go backwards and plug 41 atomic mass units is this to figure out what voltage it should be it comes out to be about a hundred twenty volts or about six volts down from 126 or 0.6 on this x-axis on the oscilloscope plot and look at that that's you notice how asymmetric this peak is right there it is 0.6 down that is actually the the isotope of potassium sitting there and then it gets even better if you look at the dudeney papers and the Scientific American papers their Peaks are really asymmetric - so I am almost certain that the isotope is hiding in there and the reason that we can't see it as a separate peak is because the Machine just doesn't have the resolution to separate them although it's close so if that asymmetric peak is really both isotopes of potassium what is that other peak that's sitting there I don't know I don't think it's real like I don't think it's actually an isotope or a particle in the system that's being counted I think it's an artifact of the measurement and lending a little bit more weight to this theory look at the shape of the small peak and the shape of the big peak they're almost the same in that they both have this kind of asymmetric thing going on which I find highly suspicious and in playing around with this thing for you know now it's been days or hours or whatever I get the feeling that it's some kind of like a mirroring type thing like the system is producing a mirror of that large peak and that's why it's there there's something that's basically reflecting the signal whether it's in the electrical system but I haven't figured out what it is some kind of weird hysteresis thing where when it's scanning forwards it it has an offset to that it doesn't have when it's scanning backwards in voltage or something I don't know but you guys are pretty smart and I'd like to hear what you think in the comments and I think we're gonna be talking about spectrometers a little bit more next time we're gonna get into a different kind of spectrometer that uses lasers and I kind of hinted at that and I hope you found this interesting and I will see you next time bye

Channel: Applied Science

Views: 349,974

Keywords: mass spectrometer, mass spectrometry, applied science, ion, ion beam, potassium, mass spec, ben krasnow, krasnow, ion physics, particle physics, physics, molecule

Id: nIKhUizkXxA

Length: 26min 44sec (1604 seconds)

Published: Fri Nov 29 2019

Reddit Comments

[u/prosteDeni]

This dude Is insanely awesome. He made his own solár panels, spectroscopy, electron microscope and many more things

[u/Spinster_Tchotchkes]

If our assignment was to pick one person to go back in time to try and accelerate advancements in technology with whatever limited tools and materials are available where he lands, this is our guy.

[u/sivoboceze]

Any chance we could do a DIY NMR? It would save me about 300 grand, but I imagine the niobium superconductor and steady liquid helium supply is going to be a little difficult to get our hands on.

[u/theBuddhaofGaming]

Now DIY an EPR spectrometer.

[u/DangerousBill]

I remember reading that article in Scientific American, and thinking, "I could totally build one of those things." But alas, I was living on a post-doc salary at the time, and let that dream go.

One thing that is easily in range of a home experimenter is an ion mobility spectrometer, useful for measuring things like pesticides, chemical weapons, and some gases that are hard to measure elseways, like ammonia.

A friend who had been involved in the military development of the IMS, built one in his home workshop out of steel washers, the americium source from a smoke detector, a faraday cup, and a D cell, plus the requisite electronics. The electronics were not critical, because the transit time of a typical ion was around 30 milliseconds, not microseconds as in an MS.

[u/jarhwasd]

I cant believe I didn't know of this guy before now. He just gained a subscriber.

[u/Philias2]

First thought on seeing the title was "that's probably Ben Krasnow" and sure enough. This guy is awesome.

[u/porridgeGuzzler]

Smart guy! Most commercial mass specs use very mild ionization techniques so we can view “molecular ions”, that is really the most useful result.

[u/JTKatt]

I watch this guy. Lots of engineering with a strong understanding of how the chemistry impacts the engineering. He restored a electron microscope. That's Cool