CARBON AND OXYGEN-ISOTOPE RATIO ANALYSIS OF SMALL CARBONATE SAMPLES BY CONVENTIONAL PHOSPHORIC-ACID DIGESTION - SAMPLE PREPARATION AND CALIBRATION

A modified method for the carbon and oxygen isotope analysis of 'small samples' (<100 mu g) of common, naturally occurring carbonate mineral s (calcite, dolomite, siderite) using conventional phosphoric acid dig estion techniques is described. Samples are reacted in vacuo with ca. 30 mu L of anhydrous acid and the resultant sample CO, is processed fo r analysis via a purpose-built, manual gas clean-up line which is atta ched directly to the inlet of a stable isotope ratio mass spectrometer . Comparisons between isotopic values obtained from well characterized calcite, dolomite and siderite using both conventional (requiring 2-3 mg) and 'small sample' analytical techniques show that reliable isoto pic data can be obtained from samples weighing as little as 30 mu g. A t sample weights of <30 mu g instrumental problems are encountered whi ch result in significantly reduced levels of accuracy and precision. A lthough this technique is less efficient in terms of analysis time tha n commercial autopreparation carbonate devices (also capable of proces sing ca. 30 mu g samples), the procedures described here have a distin ct advantage in that they allow analysis of a range of carbonate miner als which react slowly (hours to days) at 'normal' (50-90 degrees C) r eaction temperatures without a significant deterioration in precision or accuracy. This ability to analyse a range of carbonate minerals all ows the potential benefits of high resolution sampling to be extended to a wider field of geochemical applications.