by Stacy Carolin
Time to finally deal with the beastie: mass spectrometer, aka "George." Dr. Jess Adkins, my extra adopted advisor/professor/mentor/friend, is the head of the Caltech geochemistry lab into which I was invited, which includes two (!!) ThermoScientific NEPTUNE Multicollector ICP-MS (Inductively Coupled Plasma Mass Spectrometers). The older NEPTUNE that I work with is named George, which was given by postdoc extraordinaire Guillaume Paris referencing George Milton in Of Mice and Men.
Before giving George samples to eat, I think it's important to talk a little about how a mass spectrometer actually works, which will hopefully augment the limited information we all learned from CSI. Most simply, a mass spectrometer measures the mass-to-charge ratio of charged particles. The steps are:
1. Liquid sample is vaporized
2. The components of the sample (uranium and thorium) are ionized (this means given electric charge) by a plasma
3. The charged particles ("ions") are separated by their mass-to-charge ratio by electromagnetic fields (magnet)
4. The different mass ions are detected
5. The signal is processed
Ok. Step one. The liquid sample is first vaporized using a nebulizer, a little plastic guy that uses compressed gas to turn the liquid into an aerosol. The aerosol then flows through a curved heated spray chamber and "membrane desolvator module," which catches larger solvent droplets (oxides and hydrides, aka bad guys we don't want), and allows the smallest aerosol vapors to continue to the ICP-MS.
Next, the sample is ionized using an inductively coupled plasma (ICP) (ooo, cool! plasma!!) which is produced using an induction coil, argon gas flow, and an electric spark. The plasma is sustained in a torch and consists mostly of argon atoms with a small fraction of free electrons and argon ions (argon atoms that lost an electron, so positively charged). And its temperature is on the order of >5,000 K! As the nebulized sample enters through the ICP, it evaporates and any solids that were dissolved in the liquid aerosol vaporize and then break down into individual atoms ("atomization"). Then the plasma ionizes these atoms (steals off an electron to make the the atom a "positive" ion).
Ok, now we have a bunch of charged atoms with different masses ready to continue on through the mass spectrometer. I am running our uranium samples first, so we have U-238 (most common in sample), U-236 (from the spike), U-235 (not nearly as common as U-238, but small percentage occur naturally), and U-234 (radiogenic daughter of U-238, tiny amount). So the "mass-to-charge" ratio of these, assuming each has a +1 charge after passing through the plasma, equals 238, 236, 235, and 234 (note that they are all different values, but very close!). Following the plasma ionization, they are sent through a curved magnet, and based on electromagnetic laws including kinetic energy, centripetal force, and magnetic field strength, we can determine exactly where to place the collector cups for each ion mass using the equation:
This is what makes the multi-collector ICP-MS so amazing. It is able to measure several different masses all at the same time using multiple cups. This fact is imperative for our age calculations, which require ratios of several different atoms to be known.
The final step is detection. U-238, U-236, and U-235 are more abundant, and can be detected using Faraday cups, which are metal conductive cups that catch charged particles in a vacuum and then produce a current. The measured current can be used to determine the number of ions that hit the cup (in our case 1 volt = 62.5 million charged atom counts per second). U-234 is not great enough in our sample to be able to produce enough current, so it is instead measured on a "secondary electron multiplier" (SEM) which is basically an amplifier that produces an avalanche of electrons from a single ion, which can then be measured in counts per second.
Alright, now that we understand a little about how this all works, I can get started bringing the samples to the mass spectrometer. I add about 0.4-1.0 mL of 5% nitric acid to our tiny solid uranium samples, then transfer the liquid samples into "autosampler" vials to be loaded into the ICP-MS.
Next I spend about half a day preparing the ICP-MS for a "run" by completing several tests that would take way too long to discuss. Just trust that I did them all right ;) I then prepare a sequence in the editor program in the NEPTUNE software, and once everything is in place, hit GO! Yay! Now we just let George do his thing, checking on him frequently, and hopefully no errors come up. Each sample takes about 1 hour for measurement, and then the same will have to be done to set up a run of thorium tomorrow. But once the run is complete we will finally have DATA!!, which can actually be analyzed for stalagmite AGES! Finally! Get pumped :)