1. Chemical and Biological Sciences, Pacific Northwest National Laboratory, 999 Battelle Boulevard, P8-08, Richland, Washington 99350, Stable-Isotope Ratio Facility for Environmental Research, Department of Biology, University of Utah, 257 South 1400 East, Salt Lake City, Utah 84112-0840, and Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850