Measurements of the 17O excess in water with the equilibration method.

The equilibration method is the present-day standard method for measuring delta18O in water samples. The mass-to-charge ratio of 45 is measured at the same time but generally not used for further analysis. We show that an improved equilibration method can be used for precise determination of delta17O in addition to that of delta18O, and therefore can estimate 17O excess values to a precision of better than 0.1 per thousand. To control the masking effect of the 14 times more abundant 13C on mass 45, we propose to use a chemical buffer in the water samples to keep the pH value and therefore the fractionation during the equilibration process of the 13C constant. With this improved method, the precision for the delta18O value could also be slightly improved from 0.05 to 0.03 per thousand. Furthermore, we discuss the influences of the amount of water, the temperature, the CO2 gas pressures, and changes in the pH during the measuring procedure on oxygen and carbon isotopes. We noticed that measured delta45 values are a good control for delta18O measurements. This study tries to fathom the possibilities and limitations of the equilibration method for measuring 17O excess values of water samples.