[Ben Krasnow] Builds A Mass Spectrometer

One of the features that made Scientific American magazine great was a column called “The Amateur Scientist.” Every month, readers were treated to experiments that could be done at home, or some scientific apparatus that could be built on the cheap. Luckily, [Ben Krasnow]’s fans remember the series and urged him to tackle a build from it: a DIY mass spectrometer. (Video, embedded below the break.)

[Ben] just released the video below showing early experiments with a copper tube contraption that was five months in the making; it turns out that analytical particle physics isn’t as easy as it sounds. The idea behind mas spectrometry is to ionize a sample, accelerate the ions as they pass through a magnetic field, and measure the deflection of the particles as a function of their mass-to-charge ratio. But as [Ben] discovered, the details of turning a simple principle into a working instrument are extremely non-trivial.

His rig uses filaments extracted from carefully crushed incandescent lamps to ionize samples of potassium iodide chloride; applied to the filament and dried, the salt solution is ionized when the filament is heated. The stream of ions is accelerated by a high-voltage field and streamed through a narrow slit formed by two razor blades. A detector sits orthogonal to the emitter across a powerful magnetic field, with a high-gain trans-impedance amplifier connected. With old analog meters and big variacs, the whole thing has a great mad scientist vibe to it that reminds us a bit of his one-component interferometer setup.

[Ben]’s data from the potassium sample agreed with expected results, and the instrument is almost sensitive enough to discern the difference between two different isotopes of potassium. He promises upgrades to the mass spec in the future, including perhaps laser ionization of the samples. We’re looking forward to that.

Thanks to the mysterious [M] for the tip.

29 thoughts on “[Ben Krasnow] Builds A Mass Spectrometer

  1. Awesome build! Potassium-40, being radioactive, could be a good comparison measurement and possibly simpler to do, even if starting from scratch.

    + points of making lightly radioactive plasma!

    1. Yup. Though if you want a rig to test a lot of solid samples you’d do better getting some thin tungsten wire and making yourself a little spiral bucket to drop a small chunk into…. or you could do it with flashbulbs and discover what an amazing amount of substances you wouldn’t expect have large amounts of magnesium in…

    2. Yeah, I had to download to watch on a computer with sound… then download the second video regarding Ben posted since the first post was incomplete. Interesting. Yes unless he comes up with a way to feed the interface like with a GC/LC/ICP mass spectrometer.

  2. I’m working through the video, but does anyone know which transimpedance amplifier chip he’s using?

    I need to amplify a similar small signal in my lab, and a factor of a million is turning out to be very difficult and I’m looking for alternatives.

    1. We did a gain of a million for seismographs with just a pair of 741’s of 1000 each (1970’s). What bandwidth do you need? For seismic it is very low and we could include low pass filtering in the feedback. LM308 is good but much better and still cheap is available now.

      1. It should just be a straight tube tube with potential difference between source and target. Ionized bits will be accelerated at rates proportional to their mass. It takes a fast ADC, but the ST32F family has 3 ADCs at 1 or 2MHz that can be interleaved. Do a high pulse rate and add them up for low noise.

        Old ones I saw had long tubes, like 8 feet. But maybe to deal with bandwidth of the electronics at the time. Back then the signal went to a high speed scope with a Polaroid camera with the 3000 speed film. The camera exposure could be as long as hundreds or thousands of pulses. Today, I wonder if it can be a lot smaller.

        I guess the critical design constraint is the travel time difference between two of the lightest masses you need to detect.

        1. TOF (time of flight) is by far the easiest to implement. And you definitely don’t need a reflectrom ‘bend’ in order to make it work. Quadrupoles are simple in principle but require very good PSUs and RF generators, which are spendy. The length of the tube is proportional to resolution, and not a limit of ADC TDC of the day. I agree 1Mhz will probably get you adequate resolution at least to prove this. Most TOF have 2-4 Ghz digitization. Detector can be as simple as a farraday cup (soda can and a wire). The apparatus here is a magnetic sector which wasn’t really started. Most TOF are coupled to beam-line instruments using ESI or MALDI based ionization, although EI, which is what is described here is great. Biggest limitation to sector instruments is classically mass range, which is <500 m/z TOF can hit megadalton or better depending on ionization source, flight tube and vacuum efficiency

          1. A buddy back around 1980 bought a TOF + GC from Boeing Surplus and it took up a garage. He wanted to do dioxin screening and add a micro to get digital data. Ohio Scientific or the like – I had used one with 12 bit ADC’s to automate a combination fluorescence spectrophotomerter/lifetime instrument. I don’t know if he ever got it going or modernized. I have often thought it would be nice project to make a DIY using what we have easily available today.

  3. I think the extraneous peak is from a fragment. It has been a long time, but I thought a mass spec can get atoms, and molecule fragments and it is that collection o data that can allow unique identifications. Maybe I’m mixing up with a mass spec with GC on the front end.

    1. That’d be the case for complex organics, but I think the reason he went with KCl is specifically so he’s only got two ionized species, and one is going to bend the other way so it’ll be invisible.

      1. I think they are all ionized from heat by loosing an electron, not like a solution. And he accelerates with an electric field, so only + or only – ions are sent, depending on polarity?

  4. You can use some types of chromatography using very crude apparatus you don’t even need specialized silica packing material for columns you can use paper. Try putting a simple drop of your solution on a coffee filter it will separate out as it seeps across the coffee filter. Try this with some colored ink and you’ll see how it works.

    Ps. I enjoyed discovering your videos. I grew up building scientific American amateur science projects everything from the linear Excelerator to the cyclotron.

    -Daniel Preston
    http://Www.luminatiVTOL.aero

  5. The smaller peak is a cluster, a multiply charged cluster like K3Cl2+ or similar, the build is impressive and the simplicity too. You should read some papers about ionization and ion production.

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