Subnanomolar Sensitive Stable Isotopic Determination in CO₂ by Tunable Infrared Laser Absorption Spectroscopy

Oxygen and carbon stable isotope ratios (¹⁸O/¹⁶O, ¹³C/¹²C, and ¹⁷O/¹⁶O) of CO₂ have been crucial in helping us understand Earth and planetary systems. These ratios have also been used in medicine for the noninvasive diagnosis of diseases from exhaled breath and for quantifying biochemical or metabolic reactions and in determining the production area of agricultural products. The current method for measuring the stable isotope ratios of CO₂ is primarily gas-source isotope ratio mass spectroscopy (IRMS). Due to the recent demand for isotopic microanalysis of carbonates and organic compounds, the sample size required for isotopic measurements has been reduced to approximately 2 nmol CO₂ (equivalent to 0.2 μg CaCO₃ and 24 ng carbon) by using high-precision IRMS. We report a novel method using tunable mid-infrared laser direct absorption spectroscopy (TILDAS) for sensitive measurements of ¹⁸O/¹⁶O and ¹³C/¹²C in subnanomolar CO₂. This method can accurately measure ¹⁸O/¹⁶O and ¹³C/¹²C in CO₂ with a repeatability of less than 0.03‰ (n = 28) in a range of 0.3 nmol (equivalent to 0.03 μg CaCO₃ and 3.8 ng carbon) to 30 nmol. This is a sample size 1 order of magnitude smaller than currently available sensitive analytical techniques. In addition, the TILDAS system measures ¹⁷O/¹⁶O simultaneously with a repeatability of less than 0.06‰ (n = 28). Our method is a major advance in supersensitive CO₂ stable isotopic analyses for various fields.