Stable isotope ratios using cavity ring-down spectroscopy:: Determination of 13C/12C for carbon dioxide in human breath

We have constructed a cavity ring-down spectrometer employing a near-IR external cavity diode laser capable of measuring C-13/C-12 isotopic ratios in CO2 in human breath. The system, which has a demonstrated minimum detectable absorption loss of 3.2 x 10(-11) cm(-1) Hz(-1/2), determines the isotopic ratio of (COO)-C-13-O-16-O-16/(COO)-C-12-O-16-O-16 by measuring the intensities of rotationally resolved absorption features of each species. As in isotope ratio mass spectrometry (IRMS), the isotopic ratio of a sample is compared to that of a standard CO2 sample calibrated to the Pee Dee Belemnite scale and reported as the sample's delta(13)C value. Measurements of eight replicate CO2 samples standardized by IRMS and consisting of 5% CO2 in N-2 at atmospheric pressure demonstrated a precision of 0.22parts per thousand for the technique. delta(13)C values were also obtained for breath samples from individuals testing positive and negative for the presence of Helicobacter pylori, the leading cause of peptic ulcers in humans. This study demonstrates the ability of the instrument to obtain delta(13)C values in breath samples with sufficient precision to serve as a useful medical diagnostic.