title: A capillary absorption spectrometer for stable carbon isotope ratio (13C∕12C) analysis in very small samples.
abstract: A capillary absorption spectrometer (CAS) suitable for IR laser isotope analysis of small CO(2) samples is presented. The system employs a continuous-wave (cw) quantum cascade laser to study nearly adjacent rovibrational transitions of different isotopologues of CO(2) near 2307 cm(-1) (4.34 μm). This initial CAS system can achieve relative isotopic precision of about 10 ppm (13)C, or ∼1 per thousand (per mil in delta notation relative to Vienna Pee Dee Belemnite) with 20-100 picomoles of entrained sample within the hollow waveguide for CO(2) concentrations ∼400-750 ppm. Isotopic analyses of such gas fills in a 1-mm ID hollow waveguide of 0.8 m overall physical path length can be carried out down to ∼2 Torr. Overall (13)C∕(12)C ratios can be calibrated to ∼2 per thousand accuracy with diluted CO(2) standards. A novel, low-cost method to reduce cw-fringing noise resulting from multipath distortions in the hollow waveguide is presented, which allows weak absorbance features to be studied at the few ppm level (peak-to-rms) after 1000 scans are co-added in ∼10 s. The CAS is meant to work directly with converted CO(2) samples from a laser ablation-catalytic combustion micro-sampler to provide (13)C∕(12)C ratios of small biological isolates currently operating with spatial resolutions ∼50 μm.
authors.affiliation: Pacific Northwest National Laboratory, Richland, Washington 99352, USA. jfk@pnnl.gov
authors.forename: J F
authors.initials: JF
authors.lastname: Kelly
authors.forename: R L
authors.initials: RL
authors.lastname: Sams
authors.forename: T A
authors.initials: TA
authors.lastname: Blake
authors.forename: M
authors.initials: M
authors.lastname: Newburn
authors.forename: J
authors.initials: J
authors.lastname: Moran
authors.forename: M L
authors.initials: ML
authors.lastname: Alexander
authors.forename: H
authors.initials: H
authors.lastname: Kreuzer