We have developed a flow-through fast liver investigation packet (FLIP) to quantitatively measure the amount of exhaled (13)CO(2), as a result of liver metabolization processes. The FLIP system allows investigation of every single breath in real time. Line width variations due to interactions with other gas components disturb traditional measurements and limit their sensitivity to the ppm range. Detection of the complete breath volume and the spectrally fully resolved line shape allows sensitivity in the ppb range with a standard deviation of approximately 80 ppb, a prerequisite to quantitatively analyze liver metabolization processes.