Preparing Samples for Stable Isotopic Measurements

Adriane here-

Recently, Andy and I have started to collaborate on a research project together. Well, the project is his, and I’ve agreed to do some lab analyses for him in exchange for being a co-author on the research paper. Being a co-author means that on a published journal article, I will have my name as one of the people who contributed to the science in the paper. My job for this project is to pick, weigh, and analyze foraminifera for stable isotope analyses. In this post, I’ll go over briefly how I do this!

Lucky for me, Andy had already picked the foraminifera he wanted to be analyzed from his sediment samples and put these into cardboard trays. Each tray is labeled so that it corresponds with the sediment sample from which it came, thus I know exactly which sample I’m working with. The first step is to take the cardboard tray and put it under the microscope. Using a paintbrush with water, I gently pick up the foraminifera specimens and place them in an aluminum tray. After I’ve filled up all 14 of my aluminum trays, I take these and weigh them on a microbalance, which is a fancy name for a scale that measures very small weights (in this case, micrograms). I want the samples to weigh between 180 to 220 micrograms, as this is the ideal mass needed to get a good measurement. After the samples are weighed, I then put them into a tall glass vial that is numbered. I have a spreadsheet on my computer where I keep track of which sample is in which vial.

The home-made device we use to pump helium into the vials and air out. We fill 10 vials at a time for about 4-5 minutes each.

After I have about 60-80 vials of weighed foraminifera, I can then begin the process of analyzing them for stable isotope measurements. In this case, we want to measure carbon and oxygen (see our ‘Isotopes‘ and ‘Carbon & Oxygen Isotopes‘ page for more details on what these data are used for). This process is a bit tedious and always makes me nervous, but it’s also kind of fun!

The acid is poured into a syringe with a needle, and then four drops of acid are inserted into each vial. It’s a very medical-like procedure for a geologic endeavor!

Analyzing foraminifera for stable isotopes means working with a mass spectrometer, a (very expensive) machine that, very simply put, measures the amount of carbon and oxygen that are within a gas. Notice that the mass spectrometer needs a gas, not a solid, to be able to take a measurement. This is where things get fun! The first step is to make sure all of the air is out of the glass vials. To work correctly, the mass spectrometer has helium constantly being pumped through it. No air is allowed into the system, as air contains oxygen, and oxygen is one of the elements we want to measure. If air gets into the mass spectrometer or into the vials, it’ll ruin the results of the analyses. To rid the vials of air, I put the vials on a contraption that continually pushes helium into the vials through one tube while letting air out of another small tube. I let the vials fill with helium for about 4 minutes each. After the vials are filled with helium, I then put acid into each vial. Four drops of 100% pure phosphoric acid is placed into each vial. This is done to turn the foraminifera, which are made of calcium carbonate, into gas (any acid placed on calcium carbonate, the material which seashells and foraminifera are made of, will cause them to dissolve). Because calcium carbonate is CaCO3, the resulting gas includes elements of both C (carbon) and O (oxygen).

Once all the vials are filled with acid, it’s then time to start the mass spectrometer! This is a very easy process considering the machine itself is complex and intimidating (well, at least to me). In short, I basically change the file names, make sure the machine knows how many samples its analyzing, and then I click the ‘Start’ button. Each sample takes ~12 minutes to analyze, so an entire run of 60 to 80 samples takes about 12 to 16 hours.

The last part of this process will be to take the results, put them into a spreadsheet, and give them to Andy. From there, Andy will have the hard but fun job of interpreting the data and writing the majority of the research paper (with help from us, when needed).

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