Learning the Ropes of Pb Digestions

Coming into this summer I didn’t know exactly what to expect. I knew what I would be doing based on reading numerous papers and theses, but I admit I couldn’t fully visualize the step by step process I would be going through. After getting situated in Williamsburg, I headed to the lab in McGlothlin St. Hall to meet with my advisor Jim Kaste. Over the next couple weeks I got situated and comfortable with the equipment and procedures. I learned early on that the research I was conducting required fine motor skills and a lot of patience.

When I arrived at the lab I did a walk through of the place I would be spending a majority of my summer. I was then shown to a large cardboard box containing over 100 sediment samples from a core taken last summer from Lake Matoaka (core 1 of 2). I was tasked with deciding which samples I would analyze for lead concentration and grain size. I decided to do every cm for the top 30 cm of the core (first 30 samples) because this is where the lead concentration has the largest variability between each centimeter. I then chose to do every other cm for the next 40 centimeters (next 20 samples), which put me at 70 cm. From 70cm to 146cm (end of the core), I did every 4cm (last 19 samples), to put me at a total of 69 samples down the 146cm core.

Over the next 3 weeks or so I digested all the samples of the Matoaka core and some standards, which have a known concentration of lead, so you can compare the concentration you got and its actual value. To digest the sediment samples, I weighed out 0.75XX g of each sample into separate glass crucibles. I then ashed the samples for 6hrs and digested them with a nitric acid solution on a hot plate. Finally the digested samples are run through a funnel into plastic bottles, where they’ll sit until it’s time to run the inductively coupled plasma optical emission spectrometry (ICP-OES), which will tell us how much Pb is in each sample.

 

Example Results

This is a graph made using lead concentrations from a sediment core taken in Black Pond, NH. Pb concentration is on the x-axis and depth is on the y-axis. The top of the core is 0cm and the bottom is 30cm. Concentrations start to be elevated early in the 20th century and peaked in the mid-1960’s. The data matches up with the heavy Pb emissions  from years ore smelting, and the combustion of leaded gasoline. Legislation was passed in the 1970’s which started the decrease in Pb concentration. (Johnson et al., 1995) 

Johnson, Chris E., Siccama, C. E., Driscoll, C. T., Likens, G. E., and Moeller, R. E., 1995, Changes in Lead Biogeochemistry in Response to Decreasing Atmospheric Inputs: Ecological Applications, v. 5, p. 813-822.

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