Mitigating matrix effects in oil and gas wastewater analysis: LC-MS/MS method for ethanolamines†

Abstract

The high salinity and organic content in oil and gas wastewaters can cause ion suppression during liquid chromatography mass spectrometry (LC/MS) analysis, diminishing the sensitivity and accuracy of measurements in available methods. This suppression is severe for low molecular weight organic compounds such as ethanolamines (e.g., monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), N-methyldiethanolamine (MDEA), and N,N-ethyldiethanolamine (EDEA)). Here, we deployed solid phase extraction (SPE), mixed-mode LC, triple quadrupole MS with positive electrospray ionization (ESI), and a suite of stable isotope standards (i.e., one per target compound) to correct for ion suppression by salts and organic matter, SPE losses, and instrument variability. The method was evaluated in produced water samples from Italy (NaCl salinity from 8110–18 100 mg L⁻¹; diesel range organic compounds ranging from 5.1–7.9 mg L⁻¹). After correcting for matrix effects, ethanolamines in produced water samples were quantified. The first batch of samples (March 2019) had 37–646 μg L⁻¹ total ethanolamines. The second batch of samples (September 2019) had greater ethanolamine content of 77–3976 μg L⁻¹ which was attributed to a reduced water cut during oil production, enhancing the proportionate abundance of these compounds in the aqueous phase. In all samples, DEA and MEA were the dominant ethanolamine species. Possible sources (e.g., corrosion inhibitor and biotransformation) and natural attenuation potential during storage (e.g., at different temperatures, acidification, and addition of sodium azide) were investigated. The developed analytical method enables further investigation of the fate of low molecular weight organic additives in oil and gas development and provides an enhanced ability to evaluate risks associated with chemical release to the environment.