On-line determination of δ18O values of organic substances

A new method for the automated sample preparation and on-line isotope ratio determination of δ18O values of organic compounds is presented. The principle of the production of CO as measuring gas is a sudden pyrolysis in a helium carrier stream at a temperature of 1080 °C in the absence of surplus carbon. Results obtained with this method for carbohydrates and some aromatic substances show a very satisfactory agreement with δ18O values obtained by the Rittenberg-Ponticorvo method and with known δ18O values of standards; the standard deviation observed is usually better than 0.8%. The method is in principle also suitable for nitrogen containing compounds, as it implies a gas Chromatographic separation of CO and N2. Long-chained aliphatic alcohols, aldehydes, esters, and some nitrogen containing compounds did not give satisfactory results, probably because of an insufficient CO yield.