Comparison of mass spectrometry and spectroscopy methods applied to measurements of water triple oxygen isotopic compositions
High-precision measurements of oxygen isotopic compositions in water are of great importance for understanding regional and global hydrological cycles and historical hydroclimate change. Several methods have been developed to determine the triple oxygen isotopic proxy (17O-excess) in water. However, these methods are not applicable because of poor precision or poisonous compound utilisation. Recently, a method based on CO2-H2O equilibration followed by an isotopic exchange between CO2 and O2 catalysed by hot platinum, was developed to measure water 17O-excess with high precision (less than 10 per meg, 1 per meg=10−6). It has also been demonstrated that this method has the capability to obtain sufficiently precise water 17O-excess values, by performing more injections per sample and/or giving longer integration time per injection. Here, we describe the water 17O-excess measurement method using cavity ring-down spectroscopy (Picarro L2140-i) and mass spectrometry (MAT 253) devices, the latter of which comprises CO2-H2O equilibrium and CO2-O2 equilibration systems that produce pretreated O2 for measurement. The excellent agreement between the oxygen isotopic compositions of international standard waters, derived from both methods, and the recommended or previously reported values, confirms the reliability and accuracy of the two methods. Each method has advantages and disadvantages in terms of efficiency, sensitivity to environmental disturbance, sample quantity demand, and so on.