An evaluation of alumina reaction tube conditioning for high- precision 2 H/ 1 H isotope measurements via gas chromatography/ thermal conversion/isotope ratio mass spectrometry Yunning Cao 1 , Weiguo Liu 1,2 * , Peter E. Sauer 3 , Zheng Wang 1 and Zheng-hua Li 4 1 State Key Lab of Loess and Quaternary Geology, IEE, CAS, Xi’an 710075, China 2 School of Human Settlement and Civil Engineering, Xi’an Jiaotong University, Xi’an 710049, China 3 Department of Geological Sciences, Indiana University, Bloomington, IN 47405-1405, USA 4 Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA RATIONALE: The condition of the pyrolysis reactor is very important for obtaining stable, precise hydrogen isotopic ratios using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC/TC/IRMS). However, few studies of the conditioning process have been conducted, and little is known about the best methods for high-precision hydrogen isotope analysis. METHODS: We investigated d 2 H variations and observed the changes in carbon coating using six different conditioning methods for the pyrolysis alumina tube: (i) no treatment; (ii) conditioning with 4 mL hexane; (iii) conditioning with 2 mL hexane; (iv) conditioning with 2 mL hexane followed by backflushing overnight; (v) conditioning with 10 s of backflushing with methane; (vi) conditioning with 3 s of backflushing with methane. RESULTS: Conditioning the alumina tube can improve the pyrolysis efficiency of organic compounds because a moderate amount of carbon acts as a catalyst in high-temperature regions of the alumina tube. Carbon actually flows in the tube and is difficult to confine to the high-temperature region. Insufficient amounts of carbon in the high-temperature regions lead to incomplete pyrolysis of organic compounds and lower d 2 H values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher d 2 H values, probably due to enrichment of deuterium in the hydrocarbon residue. CONCLUSIONS: The d 2 H values obtained by Method 6 are closest to the TC/EA d 2 H values and are more precise than those obtained by other methods, perhaps because this method introduces a moderate, consistent amount of carbon with each sample injection. Copyright © 2012 John Wiley & Sons, Ltd. Gas chromatography coupled with isotope ratio mass spectro- metry via a thermal conversion interface (GC/TC/IRMS) has been widely used to determine the 2 H/ 1 H isotope ratios of indi- vidual organic compounds since its introduction in 1997. [1] This on-line analytical method is convenient, highly efficient and highly precise, and has advanced molecular hydrogen isotope research in many fields. These include paleoclimate and paleoenvironmental research, [2–5] biogeochemistry, [6–9] envir- onmental research, including hydrogen compound specific isotope analysis of n-alkanes in petroleum crudes, hydrogen isotopic fractionation due to evaporation, degradation, GC oven programs, [10–16] atmospheric chemistry, [17,18] and food research. [19–21] Due to the very low abundance of 2 H (0.015%) in nature, high accuracy and precision are necessary for high- resolution research and routine isotopic analysis. Many factors influence the measurement of 2 H/ 1 H isotope ratios. Among them, the condition of the pyrolysis tube is very important. [22] In order to obtain reliable d 2 H values, conditioning of the pyrolysis tube is recommended. [22,23] However, little research into the best conditioning process has been performed. In early work Burgoyne and Hayes [23] graphitized reactor tubes by passing a flow of pure propane (approximately 10 mL/min) through the tube while heating it in an oxygen-methane flame. At present, two approaches are used to condition the alumina reactor: passing methane in back- flushing mode [23] or, more commonly, the injection of hex- ane. [22] However, many studies have not used any conditioning method. [20,24,25] The lack of a widely accepted alumina tube conditioning protocol could cause system deviation or error in the measured d 2 H values, thus affecting any comparison of the data obtained. We have investigated the measurement of 2 H/ 1 H isotope ratios by GC/TC/IRMS using six different conditioning methods and compared the data obtained with the d 2 H values generated using TC/EA/IRMS. We compared changes in the carbon coating inside the alumina tube arising from the different conditioning methods. Our aim was to understand how the conditioning of the pyrolysis tubes affected the pyrolysis efficiency of organic compounds (and thus the d 2 H V-SMOW values) and to define optimal experimental condi- tions for hydrogen isotope analysis. * Correspondence to: W. G. Liu, State Key Laboratory of Loess and Quaternary Geology, IEE, CAS, Xi’an 710075, P.R. China. E-mail: liuwg@loess.llqg.ac.cn Copyright © 2012 John Wiley & Sons, Ltd. Rapid Commun. Mass Spectrom. 2012, 26, 2577–2583 Research Article Received: 28 June 2012 Revised: 17 August 2012 Accepted: 23 August 2012 Published online in Wiley Online Library Rapid Commun. Mass Spectrom. 2012, 26, 2577–2583 (wileyonlinelibrary.com) DOI: 10.1002/rcm.6378 2577