Determination of clumped isotopes in carbonate using isotope ratio mass spectrometer: Effects of extraction potential and long-term stability

In this study, we developed an instrument tuning method for analyzing clumped isotope on carbonates using isotope ratio mass spectrometer (IRMS). The ion source extraction potential is found governing accuracy and precision for CO2 clumped isotope analysis, with 90% extraction as an optimum parameter. Under this condition, we obtained a relatively stable linearity of mass spectrometer, as shown by little variation in slopes (0.0075-0.0077) for three heated CO2 gas lines (plot of Delta(47) VS. delta(47)) observed over a period of one year. By contrast, the intercepts of the heated gas lines displayed a larger variation, ranging from -0.79 parts per thousand to -0.89 parts per thousand, which may be partially contributed by some artificial factor, e.g., variable cooling rates of heated CO2 gases. Nevertheless, repeated measurements on NBS 19 demonstrate long-term reproducible Delta(47) values, comparable with previously published results. The stable linearity of the IRMS results ensures that clumped isotopes can be analyzed precisely without frequently constructing a reference frame through heated CO2 gases. Our results provide a basis for optimizing IRMS performance for clumped isotope analysis. (C) 2014 Elsevier B.V. All rights reserved.