Optimization of instrument setup and post-run corrections for oxygen and hydrogen stable isotope measurements of water by isotope ratio infrared spectroscopy (IRIS)

Light stable isotope analyses of hydrogen (H-2/H-1) and oxygen (O-18/O-16) of water are used in many terrestrial and marine aquatic studies. The advantage of using stable isotope ratios is that water molecules serve as ubiquitous and already present natural tracers. Within recent years, these analyses have been revolutionized by the development of new isotope ratio laser spectroscopy (IRIS) systems that are cheaper, more robust, and mobile compared with traditional isotope ratio mass spectrometry (IRMS). Although easier to operate, laser systems also need thorough calibration with international reference materials, and raw data need correction for analytical effects (i.e., memory and drift). This study presents modifications to the hardware for liquid water injection, an optimized sequence layout and a simple post-run correction procedure. These protocols will maximize precision, accuracy, and sample throughput via an efficient memory correction. The number of injections per unknown sample can be reduced to 4 or less. This procedure meets the demands of faster throughput with reduced costs per analysis. Procedures presented here are based on real analyses. They were also verified by an international proficiency test and traditional IRMS techniques.