title: Evaluation of gas chromatographic isotope fractionation and process contamination by carbon in compound-specific radiocarbon analysis.
abstract: 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 Delta14C values of -996 +/- 20 and -985 +/- 20 per thousand agreed with a Delta14C of -995 +/- 20 per thousand 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 Delta14C value of 165 +/- 20 per thousand agreed with Delta14C of 155 +/- 12 per thousand 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 delta13C and delta14C values was observed when only parts of the eluting peak were collected (delta13C values ranged from -15.75 to -49.91 per thousand and delta14C values from -82.4 to +4.71 per thousand). Delta14C values, which are normalized to a delta13C of -25 per thousand, did not deviate significantly (-58.9 to -5.8 per thousand, considering the uncertainty of approximately +/-20 per thousand). This means that normalization of radiocarbon results to a delta13C of -25 per thousand, 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.
authors.affiliation: Department of Applied Environmental Science (ITM), Stockholm University, 10691 Stockholm, Sweden.
authors.forename: Zdenek
authors.initials: Z
authors.lastname: Zencak
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Reddy
authors.forename: Emma L
authors.initials: EL
authors.lastname: Teuten
authors.forename: Li
authors.initials: L
authors.lastname: Xu
authors.forename: Ann P
authors.initials: AP
authors.lastname: McNichol
authors.forename: Orjan
authors.initials: O
authors.lastname: Gustafsson
mesh_terms: D016477:Artifacts; D001547:Benzaldehydes / Q000737:chemistry; D002244:Carbon / Q000032:analysis* / Q000302:isolation & purification; D002250:Carbon Radioisotopes / Q000032:analysis* / Q000302:isolation & purification; D005591:Chemical Fractionation / Q000379:methods*; D002849:Chromatography, Gas / Q000379:methods* / Q000592:standards*; D011078:Polychlorinated Biphenyls / Q000032:analysis; D011786:Quality Control
chemical_list: D001547:Benzaldehydes; D002250:Carbon Radioisotopes; D002244:Carbon; C100058:vanillin; D011078:Polychlorinated Biphenyls; C027309:3,4,5,3',4',5'-hexachlorobiphenyl