Direct determination of the oxygen 18 stable isotope ratio of in situ water contained in pasty matrices

Abstract

Traditionally, the oxygen 18 stable isotope measurement of water using a CO₂ equilibration system is performed on a liquid solution that could be a limitation for various applications. This paper demonstrates that δ¹⁸O measurements of in situ water contained in pasty matrices (dried fruits) can be performed directly on a matrix without previous water extraction using a commercial system of isotope ratio mass spectrometry (IRMS) under similar equilibration conditions used for liquids (6 h at 40 °C). The main difference is that instead of pouring a liquid in the equilibration vial, a pasty matrix is spread on the vial walls using a spatula. Water standard analysis is used for equilibration gas calibration. The increase in the amount of pasty matrix in the vial leads to an increase in δ¹⁸O values, until a plateau is obtained, indicating the optimum sample mass that is found to be 200 mg for a paste containing 35% of humidity. The results are obtained with a repeatability of 0.27‰. A set of 37 prunes is used to illustrate the method application for the discrimination between 2 different kinds of products: the δ¹⁸O value allows a clear discrimination between the prunes re-hydrated (δ¹⁸O_VSMOW range: −0.78 to +3.77‰) and non-re-hydrated (δ¹⁸O_VSMOW range: +8.97 to 14.86‰).