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Rationale: Gas chromatography‐combustion‐isotope ratio mass spectrometry (GC/C/IRMS) is widely used for compound‐specific carbon isotopic analysis. However, current isotopic analysis systems utilize the GC IsoLink combustion reactor, and independent reduction furnaces are not implemented. Therefore, whether this limitation in furnace use affects the precision of compound‐specific carbon isotopic analysis needs to be evaluated. Methods: We attempted to add a separate reduction furnace to the GC IsoLink interface and compared the δ13C values of n‐alkanes (including C and H elements), fatty acid methyl ester (including C, H, and O elements), caffeine (USGS61 and USGS62, including C, H, O, and N elements), and 9‐ethylcarbazole (including C, H, and N elements) before and after the addition of the reduction furnace using the GC IsoLink combustion reactor. Results: For n‐alkanes and fatty acid methyl esters, the δ13C differences between the measured values and their standard values were basically falling within 0.5‰ whether or not an independent reduction furnace was added. However, for the nitrogen‐containing compounds (caffeine and 9‐ethylcarbazole), the δ13C differences between the measured values and their standard values were much larger without an independent reduction furnace (1.0–3.71‰ for USGS61, 1.78–2.19‰ for USGS62, and 0.39–1.13‰ for 9‐ethylcarbazole) than with a reduction furnace (−0.31–0.68‰ for USGS61, −0.44–0.06‰ for USGS62, and −0.04–0.25‰ for 9‐ethylcarbazole). Conclusions: The addition of an independent reduction furnace had no significant effect on the δ13C of n‐alkanes and fatty acid methyl esters, but it had a significant effect on the δ13C of nitrogen‐containing compounds. It is suggested that GC IsoLink needs an independent reduction furnace to effectively eliminate the interference of NOx on CO2 isotopic determination to improve the accuracy of δ13C for nitrogen‐containing compounds. [ABSTRACT FROM AUTHOR]

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 δ2H 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 μL hexane; (iii) conditioning with 2 μL hexane; (iv) conditioning with 2 μL 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 δ2H values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher δ2H values, probably due to enrichment of deuterium in the hydrocarbon residue. CONCLUSIONS The δ2H values obtained by Method 6 are closest to the TC/EA δ2H 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. [ABSTRACT FROM AUTHOR]

RATIONALE In recent experiments, we found that compound-specific δ2H values can vary as a result of changing the gas chromatography temperature program under common pyrolysis conditions. To achieve better precision, it is necessary to examine the details and find a solution to this problem when using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC-TC-IRMS) for hydrogen isotope analysis. METHODS A test was designed to find the possible temperature effect under four different GC temperature ramp rates using n-alkanes ( n-C21, n-C27, and n-C31) and fatty acids ( n-C12, n-C18, and n-C24). The common 'hexane' method was used initially to condition the pyrolysis reactor. Experiments were then carried out using the 'methane condition' method because it was considered to improve pyrolysis efficiency. RESULTS Under the 'hexane condition' the measured hydrogen isotope ratios of the n-alkanes and n-fatty acids became more positive with increasing GC temperature ramp rate. The ion current intensity of hydrogen also generally increased. However, when the 'methane condition' method was used, the measured δ2H values of the n-alkanes and n-fatty acids showed little change under different GC temperature ramp rates. CONCLUSIONS Higher pyrolysis efficiency could reduce the tailing of the H2 peak and the related isotopic variations at increased GC temperature ramp rates. In addition, too slow a temperature ramp rate could broaden the peak width and thus increase the background effect and possible isotopic fractionations in the split interface; this could also influence the hydrogen isotope values. We therefore suggest that the appropriate temperature ramp rate is an important factor in improving the precision in analyzing compound-specific hydrogen isotopes. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]