Stable isotope ratio of the lighter elements, C, H, N, O, and S, offers an outstanding means to trace various geological processes and biological cycles. Isotope Ratio Mass Spectroscopy (IRMS) is commonly used because of its high sensitivity, but requires huge Mass Spectroscopy system which is one of difficulty for constructing compact system to apply on online measurement in field. We report here an alternative approach for measuring isotope ratios based on ultra-sensitive absorption spectroscopy of rapidly swept continuous-wave cavity ring-down technique (RS－CW－CRDS). Cavity ring-down spectroscopy (CRDS) is a very convenient technique which can obtain absolute concentration in gas media by measuring the decay time of out coming signal from optical cavity. As sample gas media, we have adopted carbon dioxide since it is widely used in field of biologically, geologically, and medically as tracer.
