Stable Isotope Ratios Using Cavity Ring-Down Spectroscopy:  Determination of 13C/12C for Carbon Dioxide in Human Breath

• Published in: Analytical chemistry (Washington) 2002-05, Vol.74 (9), p.2003-2007
• Main Authors: Crosson, Eric R, Ricci, Kenneth N, Richman, Bruce A, Chilese, Frank C, Owano, Thomas G, Provencal, Robert A, Todd, Michael W, Glasser, Jessica, Kachanov, Alex A, Paldus, Barbara A, Spence, Thomas G, Zare, Richard N
• Format: Article
• Language: English
• Subjects: Air breathing; Biological and medical sciences; Breath Tests - instrumentation; Breath Tests - methods; Carbon dioxide; Carbon Dioxide - analysis; Carbon Isotopes; Diagnostic Equipment; Fundamental and applied biological sciences. Psychology; Helicobacter Infections - diagnosis; Humans; Isotopes; Lasers; Respiration; Respiratory system: anatomy, metabolism, gas exchange, ventilatory mechanics, respiratory hemodynamics; Spectroscopy, Fourier Transform Infrared - instrumentation; Spectroscopy, Fourier Transform Infrared - methods; Spectrum analysis; Vertebrates: respiratory system
• ISSN: 0003-2700; 1520-6882
• DOI: 10.1021/ac025511d

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.