|Zdenek, Z., Reddy, C.M., Teuten, E.L., Xu, L. McNichol, A.P., and Gustafsson, Ö. , Evaluation of gas chromatographic isotope fractionation and process contamination by carbon in compound- specific radiocarbon analysis. , Analytical Chemistry, 2007; v79, 2042-2049.|
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 14C values of -996 ± 20 and -985 ± 20” agreed with a 14C 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 14C value of 165 ± 20” agreed with 14C 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 13C and 14C values was observed when only parts of the eluting peak were collected (13C values ranged from -15.75 to -49.91” and 14C values from -82.4 to +4.71”). 14C values, which are normalized to a 13C 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 13C 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.