Analysis of naphthenic acid mixtures as pentafluorobenzyl derivatives by gas chromatography-electron impact mass spectrometry

作者：Gutierrez Villagomez Juan Manuel; Vazquez Martinez Juan; Ramirez Chavez Enrique; Molina Torres Jorge; Trudeau Vance L^*

来源：Talanta, 2017, 162: 440-452.

DOI：10.1016/j.talanta.2016.10.057

• Naphthenic acids
• Derivatization
• PFBBr
• Mixtures
• GC/EIMS
• Oil

In this study, we report for the first time the efficiency of pentafluorobenzyl bromide (PFBBr) for naphthenic acid (NA) mixtures derivatization, and the comparison in the optimal conditions to the most common NAs derivatization reagents, BF3/MeOH and N-(t-butyldimethylsilyl)-N-methyltrifluoroacetamide (MTBSTFA). Naphthenic acids are carboxylic acid mixtures of petrochemical origin. These compounds are important for the oil industry because of their corrosive properties, which can damage oil distillation infrastructure. Moreover, NAs are commercially used in a wide range of products such as paint and ink driers, wood and fabric preservatives, fuel additives, emulsifiers, and surfactants. Naphthenic acids have also been found in sediments after major oils spills in the United States and South Korea. Furthermore, the toxicity of the oil sands process affected water (OSPW), product of the oil sands extraction activities in Canada's oil sands, has largely been attributed to NM. One of the main challenges for the chromatographic analysis of these mixtures is the resolution of the components. The derivatization optimization was achieved using surface response analysis with molar ratio and time as factors for derivatization signal yield. After gas chromatography-electron impact mass spectrometry (GC/EIMS) analysis of a mixture of NA standards, it was found that the signal produced by PFB-derivatives was 2.3 and 1.4 times higher than the signal produced by methylated and MTBS-derivatives, respectively. The pentafluorobenzyl derivatives have a characteristic fragment ion at 181m/z that is diagnostic for the differentiation of carboxylic and non-carboxylic acid components within mixtures. In the analysis of a Sigma and a Merichem derivatized oil extract NA mixtures, it was found that some peaks lack the characteristic fragment ion; therefore they are not carboxylic acids. Open column chromatography was used to obtain a hexane and a methanol fraction of the Sigma and Merichem mixtures. The components in the hexane fraction, presumably hydrocarbons that did not react with PFBBr were similar to 7% by weight. The effectiveness of PFBBr was confirmed when the two NA oil extracts were spiked with 8 distinct NA standards and identified by GC/EIMS in the methanol fraction. Here, we also report retention indices of the methyl, MTBS and PFB derivatives of these 8 NM. The use of PFBBr increases sensitivity, chromatographic resolution, and identification accuracy for the analysis of standards and mixtures of NM compared to MTBSTFA and BF3/MeOH. This methodology will have wide applications in the elucidation of NA mixtures.

• 出版日期2017-1-1