title: Improved online δ18O measurements of nitrogen- and sulfur-bearing organic materials and a proposed analytical protocol.
abstract: It is well known that N(2) in the ion source of a mass spectrometer interferes with the CO background during the δ(18)O measurement of carbon monoxide. A similar problem arises with the high-temperature conversion (HTC) analysis of nitrogenous O-bearing samples (e.g. nitrates and keratins) to CO for δ(18)O measurement, where the sample introduces a significant N(2) peak before the CO peak, making determination of accurate oxygen isotope ratios difficult. Although using a gas chromatography (GC) column longer than that commonly provided by manufacturers (0.6 m) can improve the efficiency of separation of CO and N(2) and using a valve to divert nitrogen and prevent it from entering the ion source of a mass spectrometer improved measurement results, biased δ(18)O values could still be obtained. A careful evaluation of the performance of the GC separation column was carried out. With optimal GC columns, the δ(18)O reproducibility of human hair keratins and other keratin materials was better than ± 0.15 ‰ (n=5; for the internal analytical reproducibility), and better than ± 0.10 ‰ (n=4; for the external analytical reproducibility).
authors.affiliation: US Geological Survey, Reston, VA 20192, USA. haipingq@usgs.gov
authors.forename: Haiping
authors.initials: H
authors.lastname: Qi
authors.forename: Tyler B
authors.initials: TB
authors.lastname: Coplen
authors.forename: Leonard I
authors.initials: LI
authors.lastname: Wassenaar
mesh_terms: D000818:Animals; D008401:Gas Chromatography-Mass Spectrometry / Q000295:instrumentation / Q000379:methods; D006197:Hair / Q000737:chemistry; D006371:Helium / Q000737:chemistry; D006724:Hoof and Claw / Q000737:chemistry; D006733:Horns / Q000737:chemistry; D006358:Hot Temperature; D006801:Humans; D053564:Keratins, Hair-Specific / Q000737:chemistry*; D009584:Nitrogen / Q000032:analysis; D010103:Oxygen Isotopes / Q000032:analysis*; D012015:Reference Standards; D015203:Reproducibility of Results; D013457:Sulfur Compounds / Q000032:analysis
chemical_list: D053564:Keratins, Hair-Specific; D010103:Oxygen Isotopes; D013457:Sulfur Compounds; D006371:Helium; D009584:Nitrogen