A comparison between14CO2 and13CO2 measurements in the respired breath of rats fed sodium acetate

13CO2/12CO2 ratios in the respired breath of rats fed carbon-13 labeled compounds have been continuously assayed using a mass spectrometer to monitor m/e 44 and 45 in real time. Rats dosed with sodium [1-13C]acetate were restrained in a specially designed glass metabolism cage. A constant air flow through the cage was continuously monitored for respired carbon dioxide by a commercial analytical organic high resolution mass spectrometer. The mass spectrometer was operated in the peak matching mode while alternately recording the ion intensities of m/e 44 and 45 at a cycle rate of 30 s. The appearance and decay in the breath of excess 13CO2 was interpreted for the metabolism of sodium acetate. The minimum dose of label was found to be 1.4 × 10−4 mol kg−1. The precision of the mass ratio measurement was ± 1%. Details of the experimental design and limitations of the technique are discussed. In order to compare 13CO2 results with an established radiochemical method the rats were also dosed with sodium [1-14C]acetate. The appearance of 14CO2 in the respired breath was followed by bubbling the breath through sodium hydroxide traps with quantitation of radioctivity by means of liquid scintillation counting. A comparison of the rates of respiration of 13CO2 and 14CO2 showed that the two methods produced comparable results in rats. This finding further substantiates the feasibility of using carbon-13 labeled compounds in metabolism studies.