Organic compounds in aquatic sediments analyzed by pyrolysis–GC–MS with tetramethylammonium hydroxide (TMAH) and alkaline CuO oxidation methods

Abstract

Thermally assisted hydrolysis and methylation with tetramethylammonium hydroxide (TMAH) can analyze various organic compounds, e.g., lignin phenols, fatty acids, sterols, etc., thus, suitable for profiling organic matter in sediments. Although the method uses pyrofoils with different reaction temperatures, changes in the yield of organic compounds in sediment samples at different temperatures are unclear and requires clarification. This study characterizes the compounds released during the TMAH reaction by testing the effect of reaction temperatures and comparing the results with a conventional method (alkaline CuO oxidation). The reaction temperatures (315 °C, 358 °C, 445 °C, 500 °C, 590 °C, and 670 °C) effects via thermochemolysis with TMAH using a Curie point pyrolyzer–GC–MS instrument (TMAH method) on lignin phenols, alkanoic acids, cutin acids, alcohols, sterols, and phytol in marine (offshore California) and lake (Lake Baikal) sediments were determined. Comparison between two different sediment locations provided a variety of compounds derived from marine and terrestrial environments. Lower temperatures (≤ 358 °C) were suitable for n-C14–C18 alkanoic acids, unsaturated n-C16:1 and C18:1 acids, cutin acids, n-C16–C28 alcohols, sterols, and phytol, whereas higher temperatures (from 445 °C to 500 °C or 590 °C) were appropriate for lignin phenols, n-C20–C30 alkanoic acids, ω-hydroxyalkanoic acids, and n-alkanedioic acids. However, relatively high concentrations determined under high temperature conditions may be due to the apparent high concentrations obtained by the decomposition of the internal standard (n-C19 alkanoic acid). Vertical distribution of lignin phenols, alkanoic acids, and cutin acids collected from lake sediments using the TMAH method at 358 °C and 590 °C were compared with those obtained by alkaline CuO oxidation (CuO method). Significant linear relationships were observed for lignin phenols and alkanoic acids, except for n-alkanedioic acids. Differences were found in some yield amounts between the methods; the yields of cinnamyl phenols (at 358 °C and 590 °C) and n-C14–C18 alkanoic acids (at 358 °C) obtained by the TMAH method significantly exceeding those obtained by the CuO method. Additionally, cutin acid yields obtained by the TMAH method were substantially lower than those obtained from the CuO method. We recommend using lower temperatures for the TMAH reaction to optimize this analytical method for different compound classes.