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 ¹³C/¹²C isotopic ratios in CO₂ in human breath. The system, which has a demonstrated minimum detectable absorption loss of 3.2 × 10^-11 cm^-1 Hz^-1/2, determines the isotopic ratio of ¹³C¹⁶O¹⁶O/¹²C¹⁶O¹⁶O 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 δ¹³C value. Measurements of eight replicate CO₂ samples standardized by IRMS and consisting of 5% CO₂ in N₂ at atmospheric pressure demonstrated a precision of 0.22‰ for the technique. δ¹³C 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 δ¹³C values in breath samples with sufficient precision to serve as a useful medical diagnostic.