Here's how a mass spec works. There's an inlet for gas. Now this gas represents material that you're trying to analyze, so let's just say for now that it's some sort of vaporized water from a mixed sample. It's fed into the machine where it's ionized by electron bombardment; the newer machines now generate a plasma from the material. These charged and energized particles--you remember, they're going to be Oxygen-18 and Oxygen-16 atoms in this mixture--are then accelerated past an electromagnetic field. Now, because they're charged particles, they're going to interact with the electromagnet field and the way that they interact is going to depend on their atomic weight. Essentially, the electromagnetic field is going to bend them and the degree or the amount by which they deviate from their former path is a function of their isotopic number. Lighter isotopes are going to be deflected more because it requires a weaker electromagnetic field to separate them.

At the end of the entire process where we now have our separated lines leaving the electromagnetic field, there are these really cleverly designed ion collectors or isotopic detectors. And detectors are placed strategically to essentially register the amount of each isotope that's coming out of the electromagnetic field. This device has been around for about 50 years. One of the first applications of mass spectrometer analysis of stable isotopes was actually toward paleontological reconstruction essentially similar to what I'm going to show you today.

Since we have no 2,000 or less year old water to feed into this machine and look at it and say, well, here's how much Colorado River water and how much marine water we have in our sample. Since we don't have that, what we do is to rely on one of my favorite beasts, clams, or marine bivalves. Now, the reason why you can use these is because each animal--this is of course a skeleton--the external skeleton of the bivalve or a clam, that skeleton or shell is made up of calcium carbonate. When the animal is building its skeleton, it's using calcium ions from the seawater and it's using carbonate ions. The carbonate ions are derived ultimately from the dissolution of carbon dioxide from the atmosphere into the seawater. So, the carbonate ion is actually going to have a mixture of Oxygen-16 and Oxygen-18 atoms in it.

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