ABSTRACT

The carbon-reduction technique for the oxygen isotopic analysis of sulphates has been further developed to improve its reliability for isotopic ratio measurements. Barium sulphate is reduced with carbon at 1, 000°C in a thin platinum boat as an electric resistance heating element to evolve carbon dioxide and carbon monoxide. The carbon monoxide is converted by a high-voltage (3.0kV) discharge between two parallel platinum plates in a liquid-nitrogen cooled vessel. The barium sulphate precipitated from the solution containing sulphate ions by the standard procedure can be reduced with a 98 ± 2% yield of oxygen as carbon dioxide, and analysed with the reproducibility of 0.1-0.25‰ for the oxygen isotopic ratio. No significant memory effect is observed in the analyses of barium sulphates having oxygen isotopic ratios in the range from -1 to +46‰ (SMOW).