Evaluating the accuracy and reliability of compound-specific carbon isotopic analysis by GC/C/IRMS with the addition of a reduction furnace.

Abstract

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. We attempted to add a separate reduction furnace to the GC IsoLink interface and compared the δ13 C 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. For n-alkanes and fatty acid methyl esters, the δ13 C 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 δ13 C differences between the measured values and their standard values were much larger without the independent reduction furnace (1.0 to 3.71 ‰ for USGS61, 1.78 to 2.19 ‰ for USGS62, and 0.39 to 1.13 ‰ for 9-Ethylcarbazole) than adding a reduction furnace (-0.31 to 0.68 ‰ for USGS61, -0.44 to 0.06 ‰ for USGS62 and -0.04 to 0.25 ‰ for 9-Ethylcarbazole). The adding of an independent reduction furnace had no significant effect on the δ13 C of n-alkanes and fatty acid methyl esters, but had a significant effect on the δ13 C of nitrogen-containing compounds. It is suggested that GC IsoLink need an independent reduction furnace to effectively eliminate the interference of NOx on CO2 isotopic determination to improve the accuracy of δ13 C for nitrogen-containing compounds.