I am trying to determine d15N on samples of cellulose (cellulose filter paper). The filter paper has been ground in a coffee grinder, but is still fibrous (that is, while no longer recognizable as paper, it is not a powder either). I've been trying to
push the method, using what is probably too much material per cap (20-23 mg, which translates to 9-10 mg C). The amount of N (total in capsule) varies a lot, from 15 ug to 250 ug. The goal of the study is to determine the 15N signature of N incorporated by
microorganisms colonizing and feeding on the cellulose during deployment in an environmental setting. The samples are wrapped in 5x9 tins. After folding, the capsules were rolled by the gloved fingers maneuver to get tight balls. I've jacked up the oxygen
pulse to exceed the stoichiometric need for oxygen (also more than enough according to Paul Brooks' formula for O2 pulse based on carbon and tin weight).
The samples were run on a Europa (Sercon) ANCA, using 1000 C combustion temperature, and a chromium oxide/copper oxide/silver wool-packed combustion tube. The results seem varied--some samples show a nice peak shape, while others have have a subtle tail,
with a bump up in the ratio of Beam2 to Beam1 in the latter part of the peak, where it would normally return to baseline. For samples having this characteristic, dAir tends to be negative, and for those with low N, very negative. If I use a narrower integration
window, leaving off the tail and the bump, the data looks a lot better--reps are tight, the tailing samples become a lot more positive while those without tailing are little changed. So how to interpret? My thinking is that the tailing/bumps are evidence
of carbon monoxide/incomplete combustion, but why are some this way while others are not? I'm wondering if the material in the tailing samples is not as dry as that in the better looking ones. The samples have been stored in a desiccator.
Anyone want to weigh in?
Harry Read, PhD
Department of Soil Science
University of Wisconsin-Madison
1525 Observatory Drive
Madison, Wisconsin 53706