In this study we present the results of total hydrolysable amino acids (THAA) and amino acid D/L‐enantiomers in northeastern Atlantic continental margin sediments. There is increasing evidence that intrinsically labile amino acids are present in old marine sediments as part of a refractory network of peptide‐like material. We used amino acid enantiomers to identify the contribution of amino acids from bacterial cell walls to THAA in organic matter ranging from relatively young to 18,000 yr old. The ratio of D/L‐amino acids increased with depth in the sediment mixed layer. Application of a transport‐racemization‐degradation model excludes a significant production of D‐amino acids by racemization and implies in situ bacterial production as the main source. Amino acids associated with a refractory pool of bacterial cell walls could account for approximately one third of the THAA deeper in the sediments. We propose that in situ bacterial production and the primary flux of labile organic matter from the water column result in a small but highly reactive pool of amino acids in the surface mixed sediment only, whereas amino acids associated with refractory cell walls persist in marine sediments.

In this work, a sensitivity study was conducted on naphthenic acid derivatization agents. Four silylation chemistries and one methylation chemical were initially evaluated on 10 model naphthenic acids using gas chromatography. An experimental design procedure was setup to look at a number of effects, including contact time, catalyst presence, and reagent concentration. Overall, the silylation agents resulted in higher derivatization yields compared to the methylation agent. Moreover, the silylation agents did not show major evidence of selective derivatization as a function of the naphthenic acid structure. A silylation agent [(N,O-bis-(trimethylsilyl)trifluoroacetamide (BSTFA)] was then used to test commercial naphthenic acid mixtures with two-dimensional gas chromatography coupled to time-of-flight mass spectrometry using the electron-impact source (GC×GC−TOFMS). The results enabled identification of many different naphthenic acid species. To test the derivatization agents on realistic samples, naphthenic acid extracts obtained from two crude oils of flow assurance significance were separated with a liquid-phase extraction procedure. The naphthenic acids were then treated with a silylation agent (BSTFA) and a methylation agent (BF3/methanol). The derivatized naphthenic acids together with the non-derivatized naphthenic acids from both crude oils were further examined using medium-resolution time-of-flight mass spectrometry with an electrospray source (TOFMS). Differences were observed in the TOFMS spectra for the naphthenic acid extracts. Extracts that did not contain ARN naphthenic acid species did not show major differences between non-derivatized and derivatized spectra in the negative mode. Extracts that contained ARN did show differences between derivatized and non-derivatized samples in the negative mode. Use of BSTFA resulted in enhanced signals for ARN, particularly the second ionization. Use of BF3/methanol resulted in a poor ARN response compared to the non-derivatized spectra. ARN species were also observed in the positive mode after treatment with both BSTFA and BF3/methanol, but signals were very poor. Moreover, use of BF3/methanol resulted in poor solubility of the naphthenic acid extracts from the crude oil containing ARN species. No solubility issues were observed with the use of BSTFA. Overall, the results point to the shortcomings of the application of methylation chemicals as derivatization agents, particularly for naphthenic acids extracted from crude oil samples containing high-molecular-weight acids of flow assurance significance (e.g., ARN species).

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Nonaqueous capillary electrophoresis was used for the separation and characterization of asphaltene samples from different sources. For the separation medium (background electrolyte), mixtures of tetrahydrofuran and a high-permittivity organic solvent could be used. The best results were obtained with an 80:20 mixture of tetrahydrofuran and acetonitrile, containing 1−10 mM of lithium perchlorate. In this separation medium, asphaltene samples were found to be composed of two fractions that could be clearly separated: one fraction of neutral species and a fraction that carries a positive charge in the solvent mixture employed. Between samples of different origin, differences were found in the relative amounts of the neutral and the charged fractions and in the average electrophoretic mobility of the charged components. Taylor dispersion analysis was applied to estimate the average diffusion coefficient of the asphaltene species in the solvent mixture used. From the results, it is concluded that the asphaltenes are present as nanoaggregate clusters of 3000−4000 Da and that the charged aggregates carry a net charge of approximately +1. The possible correlation between the electrophoretic properties of asphaltenes in crudes of different origin and their field behavior is discussed.

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