Optimization of oxygen-18 ratio measurements by equilibration - Isotope ratio mass spectrometry using various water...

In the acknowledge method for the determination of oxygen isotope ratio from water, the oxygen-18 ratio (δ¹⁸O) quantification is performed on a relatively high amount of liquid solution. Recently, we proved that the oxygen-18 determinations can be made directly on a pasty matrix, using so called in situ measurements. In this paper, the accuracy of in situ measurements was demonstrated by the correspondence between δ¹⁸O values quantified in situ and those performed on the water, previously extracted from the pasty matrix itself. The use of matrices with various humidity (17-100% of water) allowed the construction of a model able to estimate the optimum amount of matrix requested for in situ equilibration according to its humidity. This study reveals that only 6 µL of liquid (water) is enough to obtain an accurate value and that, in the experimental configuration used, the minimum water content is 20% to obtain reliable and reproducible results.