On Dropping Acid

It sounds like I have a bit of a problem doesn’t it? Two or three days a week I casually announce to the world that it’s time for me to drop acid.

Is that even legal?

Of course! And it’s a required part of my job. I train students to drop acid, too.

Wait. What?

OK. It’s not quite what you think. When I say ‘drop acid,’ I mean ‘inject 5-6 drops of pure phosphoric acid into an air-tight vial filled with nothing else but a carefully measured quantity of powdered rock or tooth enamel and helium.’

I liked my first misconception better. All I heard was ‘5-6 drops blah blah acid blah blah rock.’ What exactly are you trying to do?

My research involves the study of ancient environments through geochemical measurements of rocks and fossils. I do this by measuring stable isotopes of carbon and oxygen on a highfalutin, terribly expensive mass spectrometer. Isotopes of carbon provide information about the plants (or foods) that were around at the time the rock or tooth formed. Oxygen gives information about environmental factors like temperature, precipitation amount, and humidity.

But carbon and oxygen do not exist in isolation in rocks and teeth. They are incorporated into the minerals that compose the rock and tooth. (Yes, tooth enamel is a mineral, loosely referred to as bioapatite.) In both rocks and teeth, the carbon and oxygen conveniently exist together in the form of carbonate (CO32-). Carbonate bonds with calcium to form the very common mineral calcite (CaCO3), which is the primary constituent of limestone for example. In teeth, carbonate sometimes substitutes for phosphate (PO43-) in the bioapatite crystals.

What we want to do is separate the carbonate from the minerals that holds it. We also would like to make the carbonate (which is a solid) and convert it to a gas (carbon dioxide, CO2) that can be brought into the mass spectrometer and measured. Conveniently, carbon dioxide has both carbon and oxygen, so we can measure both elements at once.

Phosphoric acid (H3PO4) breaks down most carbonate-bearing minerals. It reacts directly with the carbonate to form carbon dioxide gas. Many acids will work in this way, but we prefer phosphoric acid because it tends not to add any extra carbon or oxygen to the carbon dioxide that we want to measure. It’s also strong enough to get the carbonate out of a strong mineral like bioapatite.

To do this right, I need to insure that there’s no other carbon dioxide around to contaminate the carbon dioxide that I want to measure. As it happens, there’s carbon dioxide all around us, so I need to eliminate it. The first thing I do is put some of my sample into an air-tight vial. Then, I flush all the room air out and replace it with helium. Other mass spectrometer systems might put the sample under vacuum, instead. It depends on the mass spectrometer.

Once the vial is sealed and contains nothing but helium and my sample, I inject acid. I use a regular hypodermic needle and syringe, and insert it into the vial through a silicon septum. Then I carefully count 5-6 drops of phosphoric acid. Then I do this for the next vial and so on until all the vials have acid. Then I shake them up a bit, to make sure the acid and the sample actually make contact, give it a few minutes. The reaction is pretty quick and you can usually see a little froth in the vial where the carbon dioxide is being released from the mineral.

After that, I start the analyses. A needle on the mass spectrometer pierces the same silicon septum and draws the helium and carbon dioxide out of the vial and into the mass spectrometer. Once it’s going, I walk away. I’ll check back the next day to see how the analyses have done.

So that’s what I mean when I say I’m dropping acid. Not quite as exciting as is sounded, but a very important part of the process of analyzing geological samples for isotopes of carbon and oxygen.

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