Assessment of carbonate-phosphoric acid analytical technique performed using GasBench II in continuous flow isotope ratio mass spectrometry

Abstract

The commonly used online automated analytical method (in particular Thermo GasBench II) to determine stable carbon and oxygen isotope ratio composition of calcium carbonate in continuous flow isotope ratio mass spectrometry is evaluated. We have analyzed the isotopic composition of three in-house calcium carbonate standards as unknown samples for assessment of speed, accuracy and precision of isotopic measurements. Isotopic analyses of CO 2 extracted from carbonate-phosphoric acid reaction were performed, where temperature of reaction and the time allowed for the reaction was varied in the range 25–90 °C and 3–101 min (and 12 h), respectively. Our results suggest that for isotopic analyses, carbonate-acid reaction may be conducted at any temperature in the range 50–90 °C for an hour; 50–70 °C seems to be a more restricted range. The results are reproduced accurately if the raw isotopic values are normalized against international standards also analyzed under the same experimental conditions as that of the samples. For isotopically homogeneous calcium carbonate samples, isotopic analyses may also be performed after 15 min of carbonate-acid reaction at 70 °C, which is significantly less time than what is routinely used. Care must be taken to ensure complete dissolution of carbonate sample as incomplete reaction or partial dissolution of isotopically heterogeneous grains would result in respective isotopic variability of CO 2 gas produced during carbonate-acid reaction.