Adaptation of the Unterzaucher procedure for determination of oxygen-18 in organic substances

Abstract

A method for the preparation of carbon dioxide from complex organic material for oxygen isotopic analysis is described. A commercial elemental analyzer has been modified so that oxygen contained in the organic material is quantitatively converted to carbon dioxide by the Schuetze-Unterzaucher technique, chromatographically purified, and transferred to a sample container for subsequent analysis by isotope ratio mass spectrometry. The organic sample is pyrolyzed, and the products of pyrolysis are equilibrated with elemental carbon at 1060 /sup 0/C to produce CO, and the CO is oxidized to CO/sub 2/ by I/sub 2/O/sub 5/. The details of these processes are considered, and a quantitative model is developed to allow correction for contamination of the carbon dioxide oxygen pool by an oxygen blank, oxygen from previous samples (memory), an oxygen from iodine pentoxide. Procedures for determination of the parameters used in the mathematical correction and routine application of the model to isotopic analysis are outlined. At natural abundance, the standard deviation for determination of the fractional abundance of oxygen-18 in a sample of organic material is 2 x 10/sup -7/ (equivalent to 0.1%). The detection limit for /sup 18/O as a tracer in biological materials is better than 1 atom excess/10/sup 6/more » atoms total O. Analyses of independently established standards show that results obtained by the mathematical correction procedure are accurate and allow determination of abundances of /sup 18/O in the sucrose standards prepared by Hardcastle and Friedman.« less