Evaluation of Gas Chromatographic Isotope Fractionation and Process Contamination by Carbon in Compound-Specific Radiocarbon Analysis

The relevance of both modern and fossil carbon contamination as well as isotope fractionation during preparative gas chromatography for compound-specific radiocarbon analysis (CSRA) was evaluated. Two independent laboratories investigated the influence of modern carbon contamination in the sample cleanup procedure and preparative capillary gas chromatography (pcGC) of a radiocarbon-dead 3,3‘,4,4‘,5,5‘-hexachlorobiphenyl (PCB 169) reference. The isolated samples were analyzed for their 14C/12C ratio by accelerator mass spectrometry. Sample Δ14C values of −996 ± 20 and −985 ± 20‰ agreed with a Δ14C of −995 ± 20‰ for the unprocessed PCB 169, suggesting that no significant contamination by nonfossil carbon was introduced during the sample preparation process at either laboratory. A reference compound containing a modern 14C/12C ratio (vanillin) was employed to evaluate process contamination from fossil C. No negative bias due to fossil C was observed (sample Δ14C value of 165 ± 20‰ agreed with Δ14C of 155 ± 12‰ for the unprocessed vanillin). The extent of isotopic fractionation that can be induced during pcGC was evaluated by partially collecting the vanillin model compound of modern 14C/12C abundance. A significant change in the δ13C and δ14C values was observed when only parts of the eluting peak were collected (δ13C values ranged from −15.75 to −49.91‰ and δ14C values from −82.4 to +4.71‰). Δ14C values, which are normalized to a δ13C of −25‰, did not deviate significantly (−58.9 to −5.8‰, considering the uncertainty of ∼ ±20‰). This means that normalization of radiocarbon results to a δ13C of −25‰, normally performed to remove effects of environmental isotope fractionation on 14C-based age determinations, also cor-rects sufficiently for putative isotopic fractionation that may occur during pcGC isolation of individual compounds for CSRA.