Thursday, September 20, 2012

Gloves and Goggles, etc.

by Stacy Carolin
 Lab suit on. Lab hat on. Lab shoes on. Goggles on. Gloves on. Music on. :)

Alright, we made it into the super clean room! Finally! But wait, why do we need to be all dressed up in here? Because we are attempting to measure TRACE elements in those stalagmite powder samples we created from drilling. TRACE means that there are very very VERY few of these guys in the sample (for example, U-238, the by-far most abundant element out of the three we're looking for, usually has a concentration in our stalagmites of only about 0.1 to 1 parts per million, aka 0.0001%!) So, if while I am processing these samples to prepare them for the mass spectrometer they are contaminated by an unknown amount of air particles in the room (i.e. tiny pieces of dust which contain some thorium happen to fall into them) the entire batch will have to be thrown out. Yikes! So to prevent this I am working in a clean room, where air is constantly being recirculated and filtered, there are absolutely NO metals so no opportunity for corrosion, floors and workstations are constantly cleaned, and any scientists inside the room must be completely covered with special garments so they don't shed fibers or dead skin cells.

Inevitably, a tiny amount of uranium and thorium atoms that are floating around as part of "escapee" dust or somehow present in not-perfectly-cleaned reusable lab vials are going to slightly contaminate our samples. To correct for this, I am going to add six "blanks" to the batch. I do the EXACT same chemistry to these "blanks" as I do to our individual samples, except that they have no stalagmite powder in them to begin with. That way, we know the average number of uranium and thorium atoms found in these "blanks" must have come from any overall room contamination, so we can subtract that value from our sample values before calculating the age. The larger the variability of contamination in our "blanks," the greater the age error will be in our calculations, so it is extremely important that I work super carefully in a very clean environment while I process all of these samples (real and blank).

Ok yay! Time to start prepping! FIRST, I need to determine the exact weight of powder I have for each. To do this I weigh each one 3 times on a digital lab balance. SECOND, I pour the sample powder into a labeled 22 mL teflon beaker, then add about 15 drops of concentrated nitric acid until all of the calcite powder is dissolved. THIRD, I do what's called "spiking" our samples, which is adding a known amount of U-236 and Th-229 "spike" into each of our now liquid samples. U-236 and Th-229 are atoms that do not naturally occur, so any that the mass spectrometer measures we will know came from our "spike" and not from the stalagmite. And since I know exactly how many I put in (based on the weight and known concentration of the "spike"), the "spike" is what will allow us to convert whatever the mass spectrometer spits out for those atoms we're interested in (U-238, U-234, and Th-230) into actual values.

FOURTH (and last for today) I add 20 drops of perchloric acid to each of our samples, then put them on a hot plate to "dry down" (turn back to solid). Perchloric acid is a very potent oxidizing agent of organics, and is therefore added and then heated in order to remove organic material such as bat guano from our samples.

Hooray! First day of chemistry done! Just to recap, today we (1) "spiked" our samples, which is a step needed in order to complete our age calculations later on, and (2) dissolved and heated our samples in concentrated oxidizing acids in order to remove any organic material waste. Our goal is to eventually reduce these samples down to JUST uranium and thorium atoms floating around in acid, so we will continue the chemical "purifying" process tomorrow. Phew! :)