The online 18O/16O analysis: development and application.

Abstract

Oxygen is a globally important element, in inorganic compounds it is involved in key structural compounds of the earth, and it is providing functional groups for most organic compounds. Oxygen isotope ratio analyses on these compounds provide information on the oxygen sources and (formation) reactions in which the compound was involved. Therefore generally applicable analytical methods for the delta18O determination are highly desirable. After a description of online high temperature "carbon reduction" methods for the conversion of oxygen in the compounds mentioned into CO and discussion of problems involved, especially in analysis of inorganic substances like carbonates and silicates, selected examples for the application of these carbon reduction methods will be presented. The well-known correlation of delta18O values of cellulose (in tree rings) to that of (leaf) water is used since the 1970's as climate indicator. The observed enrichment in 18O in cellulose by about 27% in comparison to leaf water is attributed to an oxygen equilibrium isotope effect between carbonyl groups and water. But not all carbohydrates show this "general"18O enrichment relative to water. By means of a recently published model a non-statistical oxygen isotope distribution in carbohydrates is postulated. Oxygen isotope discrimination has been recognized long ago as an important principle of authenticity evaluation in food and beverages. Though many basic mechanisms of isotope discrimination in nature are known, the corresponding application of data is often based only on empirical observations in this field. The fact that oxygen in natural organic plant material is derived from three sources (CO2, H2O, O2) with distinct differences in and that their incorporation by different reactions implies oxygen isotope effects of different sizes, is therefore of special importance, and it is attempted by means of individual isotopic increments of functional groups to predict global delta18O values of natural and nature-identical compounds. Vice versa delta18O values can also give hints on the elucidation of biosynthetic pathways.