Evaluation of the mass spectrometric analysis of ¹³CO₂ in the breath

Abstract

¹³CO₂ in exhaled CO₂ was analyzed by an isotope ratio mass spectrometer equipped with a dual inlet, a magnetic analyzer, and a single collector. Animal experiments using rats were performed for the CO₂ samples. ¹³CO₂/¹²CO₂ in the sample was measured and ¹³C abundance was expressed as δ¹³C‰(per mil). The CO₂ gas of which ¹³CO₂/¹²CO₂ is 0.010920 was used as the reference gas. When the sensitivity of this analysis was defined as the confidence limit in the measurement of small ¹³C increase, Δ¹³C, it was calculated to be 3.8‰. When the accuracy for the determination of Δ¹³C was defined as the relative deviation of the Δ¹³C values, it was calculated to be 1.3 ‰/Δ¹³C. Then if we need to obtain the accuracy with the relative deviation smaller than 5%, more than 26‰¹³C increase is needed. The comparison study between ¹⁴CO₂ and ¹³CO₂ analysis was also performed. Rats were given ¹³C and ¹⁴C labeled compounds with same labeled position simultaneously, the labeled CO₂ in the breath was collected and analyzed. ¹⁴CO₂ was measured by the liquid scintillation method. The recoveries of ¹⁴CO₂ and ¹³CO₂ were, as expected from the sensitivity of the ¹³CO₂ analysis, identical when the ¹³C increase was more than 5‰. Comparing the sensitivity of ¹⁴CO₂ and ¹³CO₂ detection, lμCi gave the ¹⁴CO₂ signals equivalent to those of ¹³CO₂ given by 10 mM ¹³C. On the basis of these studies, the application of the analytical method to the medical sciences is in progress.