July 18, 2008

Grist for the Mill

After you follow my how-to video and get some lovely tooth sections, the next step in the increasingly long process of running strontium isotope samples is to drill out enamel. The enamel that forms earliest in life is closest to the junction with the dentine, as a tooth grows in both directions - enamel laid up into the crown, and dentine laid down into the roots. The dentist working with the geochemist here at UNC feels that the best enamel for strontium analysis to find migrants is located about 50 micrometers from the dento-enamel junction. That's 1,000 times smaller than a millimeter. In order to get enough strontium for analysis in the mass spec, I have to drill out between 5-10 mg of enamel. With sample weight this low and precision at the DEJ essential, it means using a micromill - a microscope/drill attached to a computer. Using the software installed, you can draw a line on the sample where you want the drill to go. So I can map the area 50 µm from the DEJ, tell the drill to make any number of passes, and give it the appropriate depth.

On Tuesday, I managed to affix my samples (the middle section of 5 teeth) to glass microscope slides with resin, and I affixed the slides to the removable micromill platform. On Wednesday, I loaded the platform into the mill and learned how to use it from a geology grad student. The interface is pretty easy, and in no time I was drawing lines and watching the mill at work. The grad student, however, had told me the settings she used on the mill - she'd set it to make one pass at a depth of perhaps 15 µm. It took me 4 hours of milling one sample to get enough material (10mg) appropriate for strontium analysis. And then it dawned on me - the grad student had mentioned that for her analyses, she needed 5-10 µg of material. I need 1,000 times that! So on Thursday, I set the micromill to 3 passes at 250 µm depth each - and got much better results. Since each tooth section is precisely 3mm, I can go much deeper to get all the enamel I need. Which will be important when I'm drilling teeth that are incredibly worn and lacking a lot of enamel.

This whole week, then, amounted to learning how to do a bunch of stuff, from affixing samples to slides (resin really doesn't work too well, and I ended up having to re-stick the teeth with Duco cement, the osteologist's best friend), to working a micromill, to realizing that µm was not 10 times smaller than mm. Today I get to pour nitric acid on my samples and dry them out, and next week I'll learn about "columns" ... whatever that means. In the meantime, here's a short video of the automated goodness of the micromill.


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