What are isotopes, and how do we measure them?

Over at Highly Allochthonous, Anne has posted a wonderful interview with a former mentor.  While it was enjoyable to read, they did manage to slip into technical isotope nerd jargon speech at one point, with the question:

I’ve got a new-fangled cavity ringdown spectrometer (CRDS) for analyzing water isotopes, and it is so much cheaper and easier to use than a traditional mass spectrometer. But I’m also limited to a just hydrogen and oxygen in water, unlike the versatility of a mass spec, so that’s a big downside. Do you care to say what you think the future of stable isotope spectrometry will be? Will the CRDS systems displace the old-school mass spec or am I buying into a passing fad?

Let me translate that for humans:

Isotopes are atom or ions with the same number of protons but different numbers of neutrons.  Because they have the same proton number, they are all the same element, so they have similar chemical behavior.  However, because they have different neutron numbers, processes which are mass dependent can treat them differently. 

Figure 1.  Isotopes of hydrogen.  Note that the heavy isotopes of hydrogen are the only isotopes which regularly use gang affiliations instead of standard chemical notation (“D” and “T”, instead of ²H and ³H)

Geology is all about deciphering the past story of the natural world.  In order to pin down the story, we like to find modern clues that are affected by just one process.  That way, if we see the clue, we know the process occurred. In reality, there is generally more than one possible process, but the nice thing about isotopes is that because they have similar chemical behavior, we can generally rule out a lot of processes that rely on chemical differences in the ancient earth, so that the isotopic signal suggest only a few (or if we are lucky, one) process was at work in the past.

Figure 2.  Isotopes of helium.  Helium, being a noble gas, is too prideful for ganster symbols.

An additional benefit of isotopes is that because they have similar chemical behaviors, their chemistry doesn’t separate them in our measurement device.  As a result, isotopic ratios can be measured much more precisely than chemical ratios can be measured.

So how do we measure them?

Traditionally, we use a mass spectrometer.  This is a device that turns the atoms (or molecules) of interest into ions by either removing or adding electrons.  The ions are then accelerated (using an electric field) through a magnet, which deflects them based on their mass (assuming a constant energy).  The individual ions (or the electrical current generated by their arrival) are then measured on collectors positioned where the magnet deflects the ions.

Figure 3.  Sketch of hydrogen isotopes being separated in a mass spectrometer.

Mass spectrometry is useful because there are lots of different ways to create ions.  With SIMS, TIMS, ICPMS, SSMS, the letters preceding MS all refer to the type of source.  Depending on the source type, isotopes of just about any element can be analysed, and the different sources allow a wide variety of types of materials to be sampled: solids, liquids, gasses, solutes in liquids, etc.  The trouble is that mass spectrometers are expensive, and generally need very secure, stable operating conditions.

Mass spectrometers have been around for 100 years.   More recently, isotope measurements have been done using optical adsorption spectrometry.  As is shown in figure 4, gas molecules (in this case hydrogen gas) contain covalent bonds, and these bonds vibrate at a specific frequency.  Photons (light) with a similar frequency are easily adsorbed by these bonds.  However, the vibrational frequency is, in part, dependent on the mass of the atoms in the bonds.  So a change in the isotope at one (or both) ends of a bond will change the frequency of the bond, and that in turn will change the frequency of the light (usually infra-red) that the bond absorbs.

Figure 4. Vibrational frequencies are dependent on isotopes.

This technique was originally used by astronomers to determine isotopic abundances of gas clouds and galaxies and other far away stuff.  However, in recent decades, advances is solid state technology has allowed for the production of cheap, flexible lasers.  These are used in a variety of optical systems which use some sort of resonator to amplify the absorption signal, and CRDS (Cavity RingDown Spectroscopy) is one such system that is commercially available.

Optical systems like this are limited, in that they can only measure polyatomic gas molecules.  They can’t measure rocks, or helium, as neither has covalent bonds in gas molecules.  They do have some advantages, though.  They are generally cheaper than mass spectrometers, and they can measure differences in molecular structure.  For example, in the molecule N₂O, ¹⁴N¹⁵N¹⁶O and ¹⁵N¹⁴N¹⁶O have identical masses, so can’t be distinguished in mass spectrometry.  But their adsorption spectra are different, so spectroscopic system can differentiate them.