Scrutable

Interesting read.
https://physicstoday.scitation.org/do/1 ... 20221104a/

I think I need to think about how precisely this method works

In the isotope dilution method used today, scientists take highly enriched 235U and dissolve it, calibrating the liquid so they know the precise concentration. They add that solution to a similar mixture of dissolved rock, homogenizing the two. By then measuring the ratio of the two uranium isotopes in the mixture, they can learn how many atoms of natural uranium—with its known ratio of 235 to 238—are in the sample.

A mass spectrometer will give you a peak, but that doesn't tell you how much of it was in the sample. Give the sample a known amount of U235 then you have a reference peak to compare the U238 in the sample with.

Nope, still not convinced. How does adding a known enriched ratio of U235/U238 to the sample help you figure out the U235/U238 ratio in the sample more easily or more precisely than just measuring the U235/U238 ratio in the sample directly?

shpalman wrote: ↑

Tue Nov 08, 2022 11:15 am

Nope, still not convinced. How does adding a known enriched ratio of U235/U238 to the sample help you figure out the U235/U238 ratio in the sample more easily or more precisely than just measuring the U235/U238 ratio in the sample directly?

I think it's by giving a "gain" calibration (radioastronomy terminology) and determination of the background to the spectrum, so that you can calibrate the height/area under the peaks due to the sample vs that area due to the background.

Oh it's because you want the absolute number of U235 and U238 atoms in the sample, so if you add a known number of atoms of mainly U235 and a bit of U238, you can tell if there were a lot (your spike didn't make much difference) or not many (your spike made a big difference). And you can tell how much of a difference your spike made to the sample from your measurement which is actually telling you how big a difference the sample made to your spike.

See https://en.wikipedia.org/wiki/Isotope_dilution

KAJ wrote: ↑

Tue Nov 08, 2022 2:44 pm

See https://en.wikipedia.org/wiki/Isotope_dilution

I did.

IIRC that's also used for stable isotope analysis, at least for trace elements that go in the mass spec. A while since I did any, though, and the uni has a technician to do the fancy physics bits

Bird on a Fire wrote: ↑

Sat Nov 12, 2022 12:27 pm

IIRC that's also used for stable isotope analysis, at least for trace elements that go in the mass spec. A while since I did any, though, and the uni has a technician to do the fancy physics bits

Yes, that's where I've seen it used. The Wiki article I cited above says

https://en.wikipedia.org/wiki/Isotope_dilution#Applications wrote:... In addition to the use of stable isotopes, radioactive isotopes can be employed in isotope dilution...