Element Analysis in Vegetation: The Sample Prep Process

Although my last post focused on the frustrations of broken instruments and the pace of research, I have, in reality, covered a lot of ground this summer with respect to method development. My research project has given me experience in two different types of research that I plan to pursue further in graduate school: environmental field sampling, and analytical chemistry. I’ve learned new methods and techniques, and have even developed some of my own. Here is a basic walk-through of my research this summer, from field to data.

Element Selection: There are a lot of potentially harmful elements out there. 118, in fact. Most elements or compounds you can imagine can be toxic or benign, depending on how much of each is present in the environment. To give myself a starting point, my advisor, Dr. Kaste, suggested testing a sample of coal to find out exactly what was in it. I sent the coal off to be analyzed by two methods, ICP-mass spectrometry and neutron activation analysis. I then compared the levels of common toxic elements in the coal to typical crustal levels, and found that the following were present in elevated concentrations in coal: arsenic, selenium, cadmium and copper. Mercury was below the limit of detection of my instrument, but it is known to be enriched in coal.

Field Sampling: For my field work, I identified sites along the railroad tracks with nearby or overhanging vegetation. I wanted to narrow my study to one tree to account for uptake differences between species, and went with the most prevalent: loblolly pine. I collected samples from trees within ten meters of the tracks, and measured the height of each sample above the ground.

Sample Processing: I used a huge sample processing mill at the Keck Lab to grind my needles to a fine powder, which has been passed through a .5 mm screen. The high surface area of the particles is ideal for digestion.

Digestion Optimization: For much of the summer, I worked on refining and optimizing a new digestion method for vegetation that uses UV light in the place of perchloric acid, a highly hazardous oxidizing agent which is used in most plant matter digestion. I aimed to substitute UV oxidation for perchloric acid, and develop a less acid-intensive digestion procedure. I found out quickly that placing pine needles directly in the UV digester was asking for a lot of froth and mess, and so I added a pre-digestion step. A twelve-hour pre-digestion in 70% nitric acid, followed by a 6 hour UV oxidation, was successful at breaking down the majority of the samples. After the process I filtered the contents of the tubes, and compared the filtrate mass with the initial sample mass.

I have since found that a 12-hour digestion leads to a clearer solution. Together, these two processes represent a significant time investment, and so I am in the process of comparing my data to a less time-intensive microwave digestion method.

Next Time: I have so far tested samples and certified reference materials for selenium, arsenic and lead, as well as manganese, calcium and zinc. These last three elements are essential to plant functioning, and I have been using them to verify my digestion methods with the certified reference material. In my next post, I will delve into the analytical instrumentation behind my forthcoming data.