GAS-CHROMATOGRAPHIC ISOLATION OF INDIVIDUAL COMPOUNDS FROM COMPLEX MATRICES FOR RADIOCARBON DATING

This paper describes the application of a novel, practical approach fo r isolation of individual compounds from complex organic matrices for natural abundance radiocarbon measurement. This is achieved through th e use of automated preparative capillary gas chromatography (PCGC) to separate and recover sufficient quantities of individual target compou nds for C-14 analysis by accelerator mass spectrometry (AMS). We devel oped and tested this approach using a suite of samples (plant lipids, petroleums) whose ages spanned the C-14 time scale and which contained a variety of compound types (fatty acids, sterols, hydrocarbons), Com parison of individual compound and bulk radiocarbon signatures for the isotopically homogeneous samples studied revealed that Delta(14)C val ues generally agreed well (+/- 10%). Background contamination was asse ssed at each stage of the isolation procedure, and incomplete solvent removal prior to combustion was the only significant source of additio nal carbon, Isotope fractionation was addressed through compound-speci fic stable carbon isotopic analyses, Fractionation of isotopes during isolation of individual compounds was minimal (< 5 parts per thousand for delta(13)C), provided the entire peak was collected during PCGC, T rapping of partially coeluting peaks did cause errors, and these resul ts highlight the importance of conducting stable carbon isotopic measu rements of each trapped compound in concert with AMS for reliable radi ocarbon measurements, The addition of carbon accompanying derivatizati on of functionalized compounds (e.g., fatty acids and sterols) prior t o chromatographic separation represents a further source of potential error, This contribution can be removed using a simple isotopic mass b alance approach, Based on these preliminary results, the PCGC-based ap proach holds promise for accurately determining C-14 ages on compounds specific to a given source within complex, heterogeneous samples.