Acknowledgment. Neutron activation analysis (NAA) was
conducted in the Ford Nuclear Reactor of the Phoenix Memorial
Laboratory at the University of Michigan. Dr. Leah Minc conducted
the analysis. We thank Stephen D. Stamatis for his help. We also
thank Dr. Alice He and Dr. William Cannella of Chevron-Texaco
Research & Technology Co. for the discussion. Support by NSF
and DOE is gratefully acknowledged.