| Abstract
| - The equilibration method is the present-day standardmethod for measuring δ18O in water samples. The mass-to-charge ratio of 45 is measured at the same time butgenerally not used for further analysis. We show that animproved equilibration method can be used for precisedetermination of δ17O in addition to that of δ18O, andtherefore can estimate 17O excess values to a precisionof better than 0.1‰. To control the masking effect of the14 times more abundant 13C on mass 45, we propose touse a chemical buffer in the water samples to keep thepH value and therefore the fractionation during theequilibration process of the 13C constant. With thisimproved method, the precision for the δ18O value couldalso be slightly improved from 0.05 to 0.03‰. Furthermore, we discuss the influences of the amount of water,the temperature, the CO2 gas pressures, and changes inthe pH during the measuring procedure on oxygen andcarbon isotopes. We noticed that measured δ45 values area good control for δ18O measurements. This study triesto fathom the possibilities and limitations of the equilibration method for measuring 17O excess values of watersamples.
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