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

Abstract

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 x 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 delta13C value. Measurements of eight replicate CO2 samples standardized by IRMS and consisting of 5% CO2 in N2 at atmospheric pressure demonstrated a precision of 0.22/1000 for the technique. Delta13C 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 delta13C values in breath samples with sufficient precision to serve as a useful medical diagnostic.

High-precision optical measurements of 13 C/ 12 C isotope ratios in organic compounds at natural abundance

“…Thus, 1 unit of ␦ 13 C represents a change of Ϸ1 in the fifth decimal place of the 13 C/ 12 C isotope ratio. R sample is calculated by dividing the integrated area of the 13 Fig. 3 shows the change in ␦ 13 C values for different injection amounts of ethane and propane, respectively, in the GC-C-CRDS instrument.…”

“…A continuous-flow cavity ring-down spectroscopy (CRDS) system integrating a chromatographic separation technique, a catalytic combustor, and an isotopic 13 C/ 12 C optical analyzer is described for the isotopic analysis of a mixture of organic compounds. A demonstration of its potential is made for the geochemically important class of short-chain hydrocarbons.…”

“…The resulting combustion products are combined with dry N 2 before entering a cavity ring-down spectrometer. Ring-down rates are recorded for the R (12) line of 13 13 C/ 12 C isotope ratio. We refer to this setup as GC-C-CRDS.…”

“…The sensitivity gain of CRDS compared with single-pass absorption measurements comes from the much larger path length of CRDS, which is often kilometers in extent, and from the relative insensitivity of the ring-down rate to fluctuations in the intensity of the light source. Previously, carbon dioxide concentrations as low as a few parts per million have been measured (13), using CRDS and the same technique has been used to determine 13 C/ 12 C isotope ratios (13)(14)(15)(16), for example, as a diagnostic for ulcer-forming bacteria (Helicobacter pylori) by CO 2 breath analysis (13). CRDS has also been used successfully to analyze hydrogen and oxygen isotopes in water samples, introduced under either a liquid or vapor form, with isotopic measurements that parallel the high-precision yielded with dual-inlet IRMS systems (17,18).…”

Monitoring CO2 concentration and δ13C in an underground cavity using a commercial isotope ratio infrared spectrometer

“…6). Human breath has δ 13 C values between −20 and −23 ‰, with CO 2 concentration of several percent [23][24][25]. As the isotope signature of human breathing is lower than that of the background in the tunnel (δ 13 C ≈ −8 to −15 ‰), the resulting δ 13 C decreases during CO 2 peaks.…”