13CO
2 in exhaled CO
2 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 CO
2 samples.
13CO
2/
12CO
2 in the sample was measured and
13C abundance was expressed as δ
13C‰(per mil). The CO
2 gas of which
13CO
2/
12CO
2 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, Δ
13C, it was calculated to be 3.8‰. When the accuracy for the determination of Δ
13C was defined as the relative deviation of the Δ
13C values, it was calculated to be 1.3 ‰/Δ
13C. 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
14CO
2 and
13CO
2 analysis was also performed. Rats were given
13C and
14C labeled compounds with same labeled position simultaneously, the labeled CO
2 in the breath was collected and analyzed.
14CO
2 was measured by the liquid scintillation method. The recoveries of
14CO
2 and
13CO
2 were, as expected from the sensitivity of the
13CO
2 analysis, identical when the
13C increase was more than 5‰. Comparing the sensitivity of
14CO
2 and
13CO
2 detection, lμCi gave the
14CO
2 signals equivalent to those of
13CO
2 given by 10 mM
13C. On the basis of these studies, the application of the analytical method to the medical sciences is in progress.
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