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

Oxygen and carbon stable isotope ratios (O-18/O-1, C-13/C-12, and O-17/O-16) 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 mu g CaCO3 and 24 ng carbon) by using high-precision IRMS. We report a novel method (TILDAS) for sensitive measurements of O-18/O-16 and C-13/C-12 using tunable mid-infrared laser direct absorption spectroscopy in subnanomolar CO2. This method can accurately measure O-18/O-16 and C-13/C-12 in CO2 with a repeatability of less than 0.03 parts per thousand (n = 28) in a range of 0.3 nmol (equivalent to 0.03 mu g 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 O-17/O-16 simultaneously with a repeatability of less than 0.06 parts per thousand (n = 28). Our method is a major advance in supersensitive CO2 stable isotopic analyses for various fields.