In-situ stable carbon isotope measurements with laser ablation and on-chip laser absorption spectroscopy

Measuring stable carbon isotope ratios is a powerful method to study both the environment and ecosystems. The isotope ratios can be used as evidence of geological development or in climate sciences to study system interactions where it provides crucial insights about the ecosystem gas exchange. Mass spectrometry has remained the golden standard for stable carbon isotope analysis in solids and Isotope Ratio Mass Spectrometry (IRMS) has been applied in numerous use cases due to its precision and selectivity. However, the demand for on-site and in-situ capable carbon isotope monitoring methods is increasing. 

In this work, we present an in-situ-capable, all-optical method for stable carbon isotope ratio measurements in solid samples that combines laser ablation and on-chip tunable diode laser absorption spectroscopy (TDLAS). Laser ablation is used to transform the samples from solid to gas phase. The gas is then transported with a carrier flow through a microfluidic system, passing a particle filter and into the µl detector volume. The small sample volume is enabled by a suspended nanophotonic waveguide-based TLDAS sensor which was recently demonstrated for isotope-specific CO₂ detection with a 20 ppb detection limit and isotope ratio accuracy of 0.2 ‰. Thus, the combination of an on-chip CO₂ detector and laser ablation enables the construction of a compact and portable, all-optical sensor for stable carbon isotope ratio measurements. 

The presented measurement technology generates a paradigm shift in studies integrating the ecological, biological, and geochemical processes. This approach has high sample throughput with ~1 min measurement time due to the minimal requirement of sample treatment, enabling measurement of large sample sets. In addition, the measurement system can be applied to both solid and liquid samples enabling rapid, on-site screening of ecosystem carbon cycle.