Automated measurement of13C enrichment in carbon dioxide derived from submicromole quantities of L-(1-13C)-leucine

Abstract

The ¹³C enrichment of the carboxyl carbon of leucine was measured by isotope ratio mass spectrometry after conversion to CO₂ by reaction with ninhydrin in a Vacutainer and cryogenic purification using the Finnigan MAT Breath Gas Analysis System designed for processing ¹³C 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 CO₂ with CO₂ 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 CO₂ 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 CO₂ isolation.