Simultaneous high-temperature gas chromatography with flame ionization and mass spectrometric analysis of monocarboxylic acids and acylglycerols in biofuels and biofuel intermediate products.

Triacyl-, diacyl- and monoacylglycerols (TAGs, DAGs, MAGs) along with monocarboxylic acids (MCAs) are intermediate products in many triacylglycerol oil-to-biofuel conversion pathways. Accumulation of these compounds leads to poor biofuel characteristics and may result in fuel system damage. We developed a method for simultaneous identification and quantification of a wide range of MCAs (C4-C18), MAGs, DAGs, and TAGs. The method is based on trimethylsilylation followed by high temperature GC with programmed temperature vaporizer (PTV) injection coupled to parallel FID and MS detectors (HTGC-FID/MS). To minimize the discrimination of both low and high molecular weight species typically occurring on the injector, we optimized injection conditions using a central composite design. The critical variables were the time at initial temperature (40 °C), splitless time, and the interaction between these two parameters. Among three tested electron ionization source/quadrupole analyzer temperatures, a 350/200 °C setting provided the highest response and signal-to-noise ratio for TAGs and did not have an effect on MAGs and DAGs. Similar results were obtained when quantifying target analytes in intermediate products of soybean oil cracking with FID and MS (using specific acylglycerol fragmentation ions). The instrumental FID limits of detection (LODs) were 0.07-0.27 ng for most of the target analytes. Selected ion monitoring (SIM) LODs were 0.01-0.05 ng for MCAs and 0.03-0.14 ng for acylglycerols. For the total ion current (TIC), LODs observed increased with acyl chain length and degree of unsaturation, resulting in an increase from 0.05 to 0.18 ng for MCAs (C5 to C18) and from 0.03 to 1.8 ng for acylglycerols (TAGs C8 to C22). Deviations in the repeatability of sample preparation, intra- and inter-day analyses, including sample stability over an eight-day time period, did not exceed 10% variance. These results demonstrate that the developed method is accurate and robust for the determination of acylglycerols and MCAs produced during the processing of TAGs into biofuels.

Simultaneous determination of trace concentrations of aldehydes and carboxylic acids in particulate matter.

Carboxylic acids and aldehydes are present in ambient air particulate matter (PM) originating from both primary emission and secondary production in air and may, due to their polarity have, an impact on formation of cloud condensation nuclei. Their simultaneous determination may provide improved understanding of atmospheric processes. We developed a new analytical method allowing for a single step determination of majority of carboxylic acids and aldehydes (+95 compounds). This sample preparation employed O-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine (PFBHA·HCl) in methanol to yield oximes (for aldehydes) and methyl esters (for majority of acids); with the limits of detection of 0.02-1 ng per injection, corresponding to approximately 0.4-20 µg/gPM. Subsequent trimethylsilylation with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) was employed only for aromatic acids, which were not completely esterified, and for hydroxyl groups. Our method, in contrast to previous primarily qualitative studies, based on derivatization with an aqueous PFBHA followed by BSTFA derivatization, is less labor-intesive and reduces sample losses caused by an evaporation. The method was tested with a broad range of functionalized compounds (95), including monocarboxylic, dicarboxylic and aromatic acids, ketoacids, hydroxyacids and aldehydes. The developed protocol was applied to wood smoke (WS) and urban air standard reference material 1648b (UA) PM. The observed concentrations of aldehydes were 10-3000 µg/gPM in WS PM and 10-900 µg/gPM in UA PM, while those of acids were 20-1800 µg/gPM in WS PM and 15-1200 µg/gPM in UA PM. The most prominent aldehydes were syringaldehyde and vanillin in WS PM and glyoxal in UA PM. The most abundant acids in both PM samples were short-chain dicarboxylic acids (≤C10). WS PM had a high abundance of hydroxyacids (vanillic and malic acids) as well as ketoacids (glutaric and oxalacetic) while UA PM also featured a high abundance of long-chain monocarboxylic acids (≥C16).