Investigation of dissociation constants for individual and total naphthenic acids species using ultra performance liquid chromatography ion mobility time-of-flight mass spectrometry analysis

Abstract

Ultra-performance liquid chromatography ion mobility time-of-flight mass spectrometry (UPLC-IM-TOFMS) was utilized for the analysis of naphthenic acids (NAs) in fractions of pH-dependent sequential liquid-liquid extractions from oil sands process-affected water. Ion-mobility separation technique allowed the differentiation of OyS−NAs (2 ≤ y ≤ 4) from Ox−NAs (2 ≤ x ≤ 5) via drift time versus retention time separations. The results indicated that the addition of S atom to the O2−NA molecule led to a lower increase in the dissociation constant (pKa) compared to that caused by the addition of O atom. Because additional O is present as OH while the S atom is present as the CSC structure, the latter does not involve into the deprotonation process directly. The pKa value decreased along with increasing carbon number and |Z| number for O2−, O3−, O4−, and O2S−NA species, except for O5−, O3S−, and O4S−NA species, each of which are comprised of chemical structures with distinct functional groups. A calculation model was developed to estimate pKa values for individual and total NA species via nonlinear regression curve fitting, utilizing the relative abundances of detected NA species. pKa values were calculated as 3.9 for total NAs, 3.3 for O2−NAs, 4.4 for O3−NAs, 7.3 for O4−NAs, and 4.1 for O2S−NAs. Knowledge of NAs pKa is crucially important for the comprehensive understanding of their potential transformation route and toxicity as well as for the development of water remediation applications. Both the ion-mobility separation technique and the new calculation model could be widely applied for the investigation of other complicated pollutants present in water and wastewater.