Precise Water Isotope Analysis in the Natural Concentration Range Using Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)

A compact sensor utilizing off-axis integrated cavity output spectroscopy at a wavelength of 1.389 μm was developed to measure the δ¹⁸o and δ²h isotopic ratios in liquid water samples within the natural concentration range. allan variance analysis indicated an optimal averaging time of approximately 21 s. the stability and long-term reproducibility of the laser system were assessed using 1-second averaging over 75 min. the minimum detectable absorption (mda) was determined to be approximately 8.4 × 10⁻⁴ hz⁻¹/² with a 17 s averaging time. to assess accuracy and reproducibility, the setup was tested within the natural concentration range (δ²h: -400 to 0‰ and δ¹⁸o: -54 to 0‰) using five standard samples, achieving an average accuracy and reproducibility of better than 2.5‰ for δ²h and 0.5‰ for δ¹⁸o.