Compound-specific carbon isotope analysis of low-molecular organic acids and its application to geochemical samples

点击翻译按钮获取中文摘要

Carbon isotopic compositions of individual low-molecular carboxylic acids such as acetic acid and oxalic acid were measured on sub-nano mol level using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). While free monocarboxylic acids are hard to oxidaze to COィイD22ィエD2 quantatively in a micro volume furnace, decyl ester derivatives of the acids are easier to convert to COィイD22ィエD2. Also the dicarboxylic acids were derivatized to dibutyl esters for the GC/C/IRMS measurement. Using internal isotopic standards of n-alkanes, analytical precision was within ±0.3‰. Isotopic mass balance calculation shows that the corrected analytical precisions are 2-3‰ for the free acids.Carbon isotopic compositions of individual organic acids in oil-field waters from Sarukawa Oil Field (Funakawa Formation, Miocene) in the northern Japan. The δィイD113ィエD1C values of acetic acid and propanoic acid range widely from -61 to -23‰ (relative to PDB) and -from -51 to -28‰, respectively. The low-molecular acids are more depleted in ィイD113ィエD1C than the bulk crude oil (ca. -22‰) produced from the same formation. The δィイD113ィエD1C values of acetic acid correlate well (rィイD12ィエD1=0.94) with those of propanoic acid (n=7 with one exception). Isotopically heavy acids tend to be observed in higher temperature water. In addition we conducted hydrous pyrolysis of kerogen purified from the same formation. The isotopic composition of acetic acid and propanoic acid produced by the pyrolysis experiment is plotted on the correlation that is observed in natural samples. These results suggest that the low-molecular monocarboxylic acids are produced by cracking of kerogen.