PNNL

Abstract

A capillary absorption spectrometer (CAS) suitable for IR laser isotope analysis of small CO2 samples is presented. The system employs a continuous-wave (cw) quantum cascade laser to study nearly adjacent rovibrational transitions of different isotopologues of CO2 near 2307 cm-1 (4.34 µm). This initial CAS system can achieve relative isotopic precision of about 10 ppm 13C, or ~ 1‰ (per mil in delta notation relative to Vienna Pee Dee Belemnite) with 20 to 100 picomoles of entrained sample within the hollow waveguide for CO2 concentrations ~ 400 to 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 13C/12C ratios can be calibrated to ~ 2‰ accuracy with diluted CO2 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 1,000 scans are co-added in ~ 10 sec. The CAS is meant to work directly with converted CO2 samples from a Laser Ablation-Catalytic-Combustion (LA CC) micro-sampler to provide 13C/12C ratios of small biological isolates with spatial resolutions ~ 50 µm.