Gas chromatography–mass spectrometry
In combination with isotopic labeling of metabolic compounds, the GC-MS is used for determining metabolic activity. Most applications are based on the use of 13C as the labeling and the measurement of 13C-12C ratios with an isotope ratio mass spectrometer (IRMS). A IRMS is an mass spectrometer with a detector designed to measure a few select ions and return values as ratios.