Obtaining stable isotopic measurements of important elements like carbon and oxygen using conventional technology requires either a relatively large amount of sample or a highly sensitive, but very expensive instrument, such as an isotope ratio mass spectrometer. PNNL scientists have developed a patented new method and system for isotopic analyses of small samples using a novel combination of laser ablation, catalytic combustion, and infrared absorption measurements in a hollow capillary. The system incorporates a mid-wave-length, highly tunable quantum cascade laser (QCL) to provide light for spectroscopic interrogation of a gas sample. A tubular hollow wave guide is used to confine the sample for obtaining absorbance measurements correlating to abundance of specific isotopologues using an indium antimonide (InSb) or other type of detector.

The system can perform measurements with ~1‰ (¹³C, in delta notation) isotopic precision on samples as small as 20 to 100 picomoles of CO₂. Other instrument platforms (primarily those based on mass spectrometry) have reported comparable measurement sensitivities to that of the new technology, but capillary absorption spectrometry offers several key advantages over traditional instrumentation including relatively low cost, high potential for field deployment, and immunity from isobaric interference of a mass spectrometry instrument. Further, the capillary absorption spectrometry system can be coupled to a laser ablation sample introduction device to enable highly spatially resolved isotope measurements over a sample surface. The potential applications for this technology are numerous, including forensic analysis of hair samples, investigations of tightly woven microbial systems, and surface analyses where there is a wide delineation between different components of the sample. It should also be readily adaptable for direct studies of CO₂ concentrations for niche bio-compartments like waste gases from soil bacteria.