A comparison has been made of oxygen isotope analyses of natural and
synthetic phosphates using three methods in current use and Ag3PO4 as
the analyte. Of these methods, conventional fluotination using BrF5
provides the most precise and accurate measurements and these analyses
serve as the basis for comparison. Fluorination liberates 100% of the
oxygen in Ag3PO4 and the isotopic composition of this oxygen can be
readily normalized to accepted oxygen isotope ratios of international
reference standards. The widely used method of high-temperature reaction
with graphite in isolated silica tubes is also precise but requires
calibration for scale compression resulting from a combination of
factors including incomplete extraction of oxygen, reaction temperature,
possible oxygen exchange with the silica tube and/or differences in the
grain size of the graphite used. The recently developed method based on
high-temperature carbon reduction and continuous flow mass spectrometric
analysis of CO is relatively fast, requires little sample and provides
100% yields for oxygen. At the present time, this method is less
precise than the other methods examined and requires calibration against
standards on a run to run basis. Five phosphate reference standards with
delta(18)O values ranging from -5.2 parts per thousand to 34.0 parts per
thousand were prepared and packaged for distribution to active workers
in the field. Analyses of these standards will allow normalization and
calibration of results obtained using any available method of oxygen
isotope analysis of phosphate. (C) 2002 Elsevier Science B.V. All rights
reserved.