Automated measurement of 13 C enrichment in carbon dioxide derived from submicromole quantities of L‐(1‐ 13 C)‐leucine

The 13 C enrichment of the carboxyl carbon of leucine was measured by isotope ratio mass spectrometry after conversion to CO 2 by reaction with ninhydrin in a Vacutainer and cryogenic purification using the Finnigan MAT Breath Gas Analysis System designed for processing 13 C breath test samples. The sources of error which arise with submicromole samples are examined and corrections provided for suboptimal mass spectrometer signals and contamination of the evolved CO 2 with CO 2 from the reaction medium. The main limitations to the accuracy and precision of the method are not instrumental but arise from the contamination with residual CO 2 in the reaction medium, and this sets a lower limit of around 0.25 μmol leucine on the practical sample size. This is an improvement of about five‐fold on the previous manual method of CO 2 isolation.