Fiber-Enhanced Raman Gas Spectroscopy for ¹⁸O-¹³C-Labeling Experiments

Stable isotopes are used in ecology to track and disentangle different processes and pathways. Especially for studies focused on the gas exchange of plants, sensing techniques that offer oxygen (O₂) and carbon dioxide (CO₂) sensitivity with isotopic discrimination are highly sought after. Addressing this challenge, fiber-enhanced Raman gas spectroscopy is introduced as a fast optical technique directly combining ¹³CO₂ and ¹²CO₂ as well as ¹⁸O₂ and ¹⁶O₂ measurements in one instrument. We demonstrate how a new type of optical hollow-core fiber, the so-called revolver fiber, is utilized for enhanced Raman gas sensing. Carbon dioxide and oxygen isotopologues were measured at concentrations expected when using ¹³C- and ¹⁸O-labeled gases in plant experiments. Limits of detection have been determined to be 25 ppm for CO₂ and 150 ppm for O₂. The combination of measurements with different integration times allows the creation of highly resolved broadband spectra. With the help of calculations based on density functional theory, the line at 1512 cm^-1 occurring in the oxygen spectrum is assigned to ¹⁸O¹⁶O. The relative abundances of the isotopologues ¹⁸O¹⁶O and nitrogen ¹⁵N¹⁴N were in good agreement with typical values. For CO₂, fiber-enhanced Raman spectra show the Fermi diad and hotbands of ¹²C¹⁶O₂, ¹³C¹⁶O₂, and ¹²C¹⁸O¹⁶O. Several weak lines were observed, and the line at 1426 cm^-1 was identified as originating from the (0 4 0 2) → (0 2 0 2) transition of ¹²C¹⁶O₂. With the demonstrated sensitivity and discriminatory power, fiber-enhanced Raman spectroscopy is a possible alternative means to investigate plant metabolism, directly combining ¹³CO₂ and ¹²CO₂ measurements with ¹⁸O₂ and ¹⁶O₂ measurements in one instrument. The presented method thus has large potential for basic analytical investigations as well as for applications in the environmental sciences.