In vacuo reduction of silver orthophosphate with graphite for high-precision oxygen isotope analysis

The reduction of silver phosphate with graphite under vacuum conditions was studied at final reaction temperatures varying from 430 to 915 degrees C to determine: (i) the CO(2) extraction yield, and (ii) the oxygen isotopic composition of CO(2). The CO(2) yield and oxygen isotopic composition were determined on a calibrated dual inlet and triple collector isotope ratio mass spectrometer. We observed the following three stages of the reduction process. (1) At temperatures below 590 degrees C only CO(2) is formed, while silver orthophosphate decays to pyrophosphate. (2) At higher temperatures, 590-830 degrees C, predominantly CO is formed from silver pyrophosphate which decays to metaphosphate; this CO was always converted into CO(2) by the glow discharge method. (3) At temperatures above 830 degrees C the noticeable sublimation of silver orthophosphate occurs. This observation was accompanied by the oxygen isotope analysis of the obtained CO(2). The measured delta(18)O value varied from -11.93 parts per thousand (at the lowest temperature) to -20.32 parts per thousand (at the highest temperature). The optimum reduction temperature range was found to be 780-830 degrees C. In this temperature range the oxygen isotopic composition of CO(2) is nearly constant and the reaction efficiency is relatively high. The determined difference between the delta(18)O value of oxygen in silver phosphate and that in CO(2) extracted from this phosphate is +0.70 parts per thousand. Copyright (c) 2010 John Wiley & Sons, Ltd.