Isotope-ratio-monitoring gas chromatography--mass spectrometry

A 7500C cupric-oxide-packed combustion furnace is inserted between the gas chromatographic column outlet and a GC/MS interface attached to a computer-controlled beam-switching isotope ratio mass spectrometer. Monitoring of N2 and CO2 ion currents due to the combustion products allows continuous measurement of 15N/14N or 13C/12C ratios, providing directly comparable isotopic analyses for all eluting compounds regardless of composition and mass spectrometric behavior of the parent compounds. Carbon and nitrogen isotope ratios can be measured with a precision of 0.5% or better with 20 nmol of CO2 or 100 nmol of N2. Enrichments of 13C and 15N as low as 0.004 at.% excess can be detected. 5 figures, 3 tables