Measuring oxygen yields of a thermal conversion/elemental analyzer‐isotope ratio mass spectrometer for organic and inorganic materials through injection of CO

The thermal conversion/elemental analyzer‐isotope ratio mass spectrometer (TC/EA‐IRMS) is widely used to measure the δ¹⁸O value of various substances. A premise for accurate δ¹⁸O measurement is that the oxygen in the sample can be converted into carbon monoxide (CO) quantitatively or at least proportionally. Therefore, a precise method to determine the oxygen yield of TC/EA‐IRMS measurements is needed. Most studies have used the CO peak area obtained from a known amount of a solid reference material (for example, benzoic acid) to calibrate the oxygen yield of the sample. Although it was assumed that the oxygen yield of the solid reference material is 100%, no direct evidence has been provided. As CO is the analyte gas for δ¹⁸O measurement by IRMS, in this study, we use a six‐port valve to inject CO gas into the TC/EA. The CO is carried to the IRMS by the He carrier gas and the CO peak area is measured by the IRMS. The CO peak area thus obtained from a known amount of the injected CO is used to calibrate the oxygen yield of the sample. The oxygen yields of commonly used organic and inorganic reference materials such as benzoic acid (C₆H₅COOH), silver phosphate (Ag₃PO₄), calcium carbonate (CaCO₃) and silicon dioxide (SiO₂) are investigated at different reactor temperatures and sample sizes. We obtained excellent linear correlation between the peak area for the injected CO and its oxygen atom amount. C₆H₅COOH has the highest oxygen yield, followed by Ag₃PO₄, CaCO₃ and SiO₂. The oxygen yields of TC/EA‐IRMS are less than 100% for both organic and inorganic substances, but the yields are relatively stable at the specified reactor temperature and for a given quantity of sample. Copyright © 2014 John Wiley & Sons, Ltd.