This Small Business Innovation Research (SBIR) Phase I project concerns the novel application of cavity-enhanced absorption spectroscopy to determine the carbon isotope ratio of carbon dioxide emitting from deep-sea hydrothermal vents. Although, this isotope ratio has recently been identified as an indicator of biological activity, the currently used analytical methods are expensive and inaccurate. The aim of this Phase I proposal is to design a portable cavity-enhanced absorption spectroscopy system to accurately determine isotope ratios of deep-sea samples in situ. Preliminary results suggest that the system can determine isotope ratios to better than 0.1 percent, which is sufficient to determine the presence of biological activity.<br/><br/>This effort will provide an accurate determination of carbon isotope ratios in an inexpensive, portable device. It will also have significant commercial impact in medical diagnostics, the petroleum industry, and environmental monitoring. Projected sales of the ratiometer in the former two areas alone are expected to exceed $65M over the next five years. Moreover, the proposed prototype can be readily modified to monitor various industrial gases, further increasing its commercial impact.