Abstract
Oxygen and carbon stable isotope ratios ((18)O/(16)O, (13)C/(12)C, and (17)O/(16)O) of CO(2) 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 CO(2) 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 CO(2) (equivalent to 0.2 μg CaCO(3) 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 (18)O/(16)O and (13)C/(12)C in subnanomolar CO(2). This method can accurately measure (18)O/(16)O and (13)C/(12)C in CO(2) with a repeatability of less than 0.03‰ (n = 28) in a range of 0.3 nmol (equivalent to 0.03 μg CaCO(3) 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 (17)O/(16)O simultaneously with a repeatability of less than 0.06‰ (n = 28). Our method is a major advance in supersensitive CO(2) stable isotopic analyses for various fields.