Applications of cavity ring-down spectroscopy to high precision isotope ratio measurement of 13C/12C in carbon dioxide.

Recent measurements of carbon isotopes in carbon dioxide using near-infrared, diode-laser-based cavity ring-down spectroscopy (CRDS) are presented. The CRDS system achieved good precision, often better than 0.2 per thousand, for 4% CO2 concentrations, and also achieved 0.15-0.25 per thousand precision in a 78 min measurement time with cryotrap-based pre-concentration of ambient CO2 concentrations (360 ppmv). These results were obtained with a CRDS system possessing a data rate of 40 ring-downs per second and a loss measurement of 4.0 x 10(-11) cm(-1) Hz(-1/2). Subsequently, the measurement time has been reduced to under 10 min. This standard of performance would enable a variety of high concentration (3-10%) isotopic measurements, such as medical human breath analysis or animal breath experiments. The extension of this ring-down to the 2 microm region would enable isotopic analysis at ambient concentrations, which, combined with the small size, robust design, and potential for frequent measurements at a remote site, make CRDS technology attractive for remote atmospheric measurement applications.

Moving beyond traditional UV-visible absorption detection: cavity ring-down spectroscopy for HPLC.

We describe the use of liquid-phase continuous-wave cavity ring-down spectroscopy for the detection of an HPLC separation. This technique builds on earlier work by Snyder and Zare using pulsed laser sources and improves upon commercially available UV-visible detectors by a factor of up to 50. The system employs a compact doubled-diode single-mode continuous-wave laser operating at 488 nm and a previously described Brewster's-angle flow cell. Ring-down time constants as long as 5.8 micros were observed with liquid samples in a 0.3-mm path length cell. The baseline noise during an HPLC separation was only 2 x 10(-7) absorbance units (AU) peak to peak, as compared to 1 x 10(-5) AU for a state-of-the-art commercial UV-visible detector.

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 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.