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 minerals (calcite, dolomite, siderite) using conventional phosphoric acid digestion techniques is described. Samples are reacted in vacuo with ca. 30 mu L of anhydrous acid and the resultant sample CO, is processed for analysis via a purpose-built, manual gas clean-up line which is attached 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 isotopic data can be obtained from samples weighing as little as 30 mu g. At sample weights of <30 mu g instrumental problems are encountered which result in significantly reduced levels of accuracy and precision. Although this technique is less efficient in terms of analysis time than commercial autopreparation carbonate devices (also capable of processing ca. 30 mu g samples), the procedures described here have a distinct advantage in that they allow analysis of a range of carbonate minerals which react slowly (hours to days) at 'normal' (50-90 degrees C) reaction temperatures without a significant deterioration in precision or accuracy. This ability to analyse a range of carbonate minerals allows the potential benefits of high resolution sampling to be extended to a wider field of geochemical applications.