Sensitivity study of naphthenic acids from flow assurance deposits characterized by low-resolution mass spectrometry

Soaps constitute one of the more recent challenges in exploration and production flow assurance. In this work, the naphthenic acids from a field soap deposit were extracted and analyzed using low-resolution mass spectrometry, with the aim of examining the effects of ionization sources, operational settings, and the impact of solvents. Results suggest that fast atom bombardment (FAB), atmospheric pressure chemical ionization (APCI), and electrospray (ES) ionization sources have different effects on the samples and lead to different spectra. FAB led to the highest signal for the so-called ARN acids, as well as a range of signals for monocarboxylic acids. The signals for ARN acids obtained using both the ES and APCI sources were similar to each other in terms of intensity. Nevertheless, because of the differences in monocarboxylic acid signals between these sources, the spectrum obtained with APCI, most likely, suggests favorable ionization of ARN acids. This was supported by the analysis of the spectra of a commercial naphthenic acid mixture using the three ionization sources. Mass spectrometry data also showed that the APCI source led to the formation of multimers of higher molecular-weight naphthenic acid for the concentrations studied. More detailed investigations of the operation of the ES source demonstrated that a decrease in source voltage resulted in favored ionization and detection of the ARN acids over the monocarboxylic acids. Moreover, the use of more polar solvents in combination with the ES source also resulted in favored ionization of ARN acids. Use of selected conditions with liquid chromatography followed by tandem mass spectrometry allowed for the identification of four carboxylic acid functionalities for a selected ARN acid ion. Overall, this information points to the importance of various mass spectrometry variables in naphthenic acid detection, particularly with different acid families, and will aid in the development of a combination of quantitative methods for use with field samples. On the basis of our work and the sources examined, the ES source would be the preferred setting for an overview of the general fingerprinting of the naphthenic acids in deposits. The APCI source may be the preferred setting for work focusing on ARN identification and characterization. The ES source was also used to investigate soaps formed in the laboratory conditions using aqueous phases of varying ionic composition. The presence of ARN and non-ARN species in the soap generated in the laboratory suggests competing effects during the soap formation process, as well as the availability of mono-or divalent ions in the parent aqueous phase for the precipitation mechanism.