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

The equilibration method is the present-day standard method for measuring delta O-18 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 delta O-17 in addition to that of delta O-18, and therefore can estimate O-17 excess values to a precision of better than 0.1 parts per thousand. To control the masking effect of the 14 times more abundant C-13 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 C-13 constant. With this improved method, the precision for the delta O-18 value could also be slightly improved from 0.05 to 0.03 parts 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 delta(45) values are a good control for delta O-18 measurements. This study tries to fathom the possibilities and limitations of the equilibration method for measuring O-17 excess values of water samples.