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I'm interested in measuring the stable isotope ratios of some bird feathers. I'm aware that a mass spectrometer is required to measure stable isotope ratios. Is it possible to measure stable isotope ratios at home, using your own 'DIY' spectrometer?

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    $\begingroup$ Mass-spectrometer does not have any similarity with normal optical spectrometers, It consist of hi-performance vacuum pump, strong magnet and complicated vacuum chamber with ... additions. It is often big and requires some skill to operate. So you probably should go to nearest lab with said spectrometer. Measuring mass-spectrum is usually quite fast, so the cost should not be an issue. $\endgroup$ – permeakra Jun 9 '13 at 18:38
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I am assuming that you wish to measure isotopes of carbon. It is unknown to me how you will extract sample from a bird feather that is suitable for measurement in a spectrometer, and does not taint the isotope ratios. The spectrometers that can distinguish between isotopes are generally quite sensitive and complicated. So the literal answer to your question is "NO, you can not do it at home with a DIY spectrometer."

But, you might be able to purchase a spectrometer. You might be able to differentiate carbon isotopes in carbonyl compounds with infrared spectroscopy. (But don't count on it, unless you are somehow making a gaseous product such as $\ce{CO}$ or $\ce{CO2}$.)

The usual way of measuring carbon isotopes is via mass spectrometry. You can probably get a crude measurement with a machine that costs $20,000 (on eBay) but then you must get it calibrated and keep it supplied with carrier gas, etc. In short, it will bankrupt you.

User permeakra has the best idea: send a sample to a lab. But, I am not sure of any lab will allow you to send samples through their spectrometer, not knowing anything about your reputation or the volume of business that they might expect to get from you.

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On 2019-11-29 Ben Krasnow uploaded a YouTube video DIY mass spectrometer measures potassium in dietary salt substitute showing an outstanding construction of a mass-spec apparatus in his home shop based on the articles by Dewdney [1] and Stong [2] with some variations and improvements, like altered electrical scheme and increased emission current.

The main downsides were limited operation time (10–15 min), tedious assembly/disassembly and vacuuming procedures required for each run, low resolution and possible signal artifacts. However, for the tested probe of potassium chloride, Ben was able to demonstrate the presence of both stable isotopes $\ce{^{39}K}$ and $\ce{^{41}K}$ in the approximate expected ratio of $14:1$.

Screenlist

From the original article [1]:

A mass spectrometer has been designed around simple constituents: the vacuum system is made from copper plumbing parts; electrical leads into the vacuum are via darning needles pushed through rubber stoppers; the filament of the thermal ionization source is from a small light bulb; object and image slits are made from bits of razor blades sandwiched between brass washers. The magnetic field is supplied by a permanent magnet fitted with homemade cylin­drical pole pieces (of such a diameter to give second-order direction focusing). A modest vacuum of $\pu{E-4 mmHg}$ is required. When the instrument is focused (by adjusting the position of the pole pieces) and aligned (by rotating greased rubber stoppers) a resolving power of 50 can be obtained.

Complete mass spectrometer
Fig. 2. Complete mass spectrometer. (1) 90° elbow, (2) source tee, (3) collector tee, (4) to (9) connections leading to vacuum pump, (10) external part of source, (11) external part of collector, (12) cover on miniature tube socket.

Cross section
Fig. 3. Cross section of the essentials of the mass spectrometer. The numbered parts are those visible in Fig. 2.

References

  1. Dewdney, J. W. Poor Man’s Mass Spectrometer. American Journal of Physics 1963, 31 (12), 932–937. https://doi.org/10.1119/1.1969211.
  2. Stong, C. L. The Amateur Scientist. Sci Am 1970, 223 (1), 120–128. https://doi.org/10.1038/scientificamerican0770-120.
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    $\begingroup$ The small $\Delta{}t$ between the publication of the video, and your post on ChemSE is remarkable. My interpretation: You know many of ChemSE's nuggets in and out, and seeing the video gave you a «wait a sec, this matches well to one of the 32k questions ...» moment. Tip to the hat and +1 for your detailed answer. $\endgroup$ – Buttonwood Nov 30 '19 at 8:57
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    $\begingroup$ This is an outstanding answer $\endgroup$ – Faissaloo Nov 30 '19 at 11:56
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    $\begingroup$ @faissaloo Thank you, but it's really Ben Krasnow who deserves all the credit. I subscribed to his channel about five years ago and the content was always highly educational and of top notch quality without any exceptions. Both the levels of preparation and the amount of the background work in his videos is incredible, and yet he manages to make hardcore science accessible to the public. And he drives a DeLorean:) $\endgroup$ – andselisk Nov 30 '19 at 12:03
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Not by a homebuilt mass spectrometer, you can't. At least not with a reasonable amount of effort.

A mass spectrometer is a very complicated and sensitive charged-particle optics instrument. A useful mass spectrometer for materials that are not a gas or vapor at near room temperature is far more complicated and expensive than that. A mass spectrometer sensitive enough for distinguishing isotopes in molecules of significant size is even harder. You must be an expert with ultrahigh vacuum, high-speed and ultra-sensitivity electronics, precision high-voltage electronics, particle physics, mechanical fabrication...

It's akin to building your own electron microscope: far from impossible, and hobbyists have done it themselves, but these are people whose main passion and area of knowledge was ultrahigh vacuum and particle physics, not chemistry. I gave up on my diy mass spectrometer project when I concluded that I was not equal to the electronic design.

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