An online method combining a Gasbench II with continuous flow isotope ratio mass spectrometry to determine the content and isotopic compositions of minor amounts of carbonate in silicate rocks

Abstract

An online method using continuous flow isotope ratio mass spectrometry (CF-IRMS) interfaced with a Gasbench II device was established to analyze carbon and oxygen isotopic compositions and to estimate the content of minor amounts of carbonate in silicate rocks. The mixtures of standard materials and high-purity quartz are firstly used to calibrate different quantities of carbonate in silicates. The results suggest that the accuracy and precision of the online analysis are both better than those obtained using an offline method. There is a positive correlation between the carbonate weight and the Mass44 ion beam intensity (or peak area). When the weight of carbonate in the mixtures is greater than 70 microg (equal to approximately 1800 mV Mass44 ion beam intensity), the delta(13)C and delta(18)O values of samples usually have accuracy and precision of +/-0.1 per thousand and +/-0.2 per thousand (1sigma), respectively. If the weight is less than 70 microg, some limitations (e.g., not perfectly linear) are encountered that significantly reduce the accuracy and precision. The measured delta(18)O values are systematically lower than the true values by -0.3 to -0.7 per thousand; the lower the carbonate content, the lower the measured delta(18)O value. For samples with lower carbonate content, the required phosphoric acid doses are higher and more oxygen isotope exchanges with the water in the phosphoric acid. To guarantee accurate results with high precision, multiple analyses of in-house standards and an artificial MERCK sample with delta(13)C values from -35.58 to 1.61 per thousand and delta(18)O from 6.04 to 18.96 per thousand were analyzed simultaneously with the unknown sample. This enables correction of the measured raw data for the natural sample based on multiple-point normalization. The results indicate that the method can be successfully applied to a range of natural rocks.