Isotope-ratio detection for gas chromatography

Abstract

Instrumentation and methods exist for highly precise analyses of the stable-isotopic composition of organic compounds separated by GC. The general approach combines a conventional GC, a chemical reaction interface, and a specialized isotope-ratio mass spectrometer (IRMS). Most existing GC hardware and methods are amenable to isotope-ratio detection. The interface continuously and quantitatively converts all organic matter, including column bleed, to a common molecular form for isotopic measurement. C and N are analyzed as CO_2 and N_2, respectively, derived from combustion of analytes. H and O are analyzed as H_2 and CO produced by pyrolysis/reduction. IRMS instruments are optimized to provide intense, highly stable ion beams, with extremely high precision realized via a system of differential measurements in which ion currents for all major isotopologs are simultaneously monitored. Calibration to an internationally recognized scale is achieved through comparison of closely spaced sample and standard peaks. Such systems are capable of measuring ^(13)C/^(12)C ratios with a precision approaching 0.1‰ (for values reported in the standard delta notation), four orders of magnitude better than that typically achieved by conventional “organic” mass spectrometers. Detection limits to achieve this level of precision are typically <1 nmol C (roughly 10 ng of a typical hydrocarbon) injected on-column. Achievable precision and detection limits are correspondingly higher for N, O, and H, in that order.