Salting-out effects on the characterization of naphthenic acids from Athabasca oil sands using electrospray ionization

Abstract

There is growing interest in the mass spectrometric characterization of oil sands acids present in natural waters and contaminated soils. This interest stems from efforts to isolate the principal toxic components of oil sands acid extractable organics in aquatic environment. Salting-out effects are demonstrated for nanospray ionization mass spectra of Athabasca oil sands acid extractable organics (naphthenic acids), using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry. The differences in spectra obtained for the sodium naphthenates in dichloromethane/acetonitrile cosolvents compared to spectra obtained in the absence of saturated sodium chloride salts, are used here as a surrogate to indicate the more bioavailable or toxic components in natural waters. Whereas, monocarboxylic compounds (C_nH_2n+ZO₂) were prevalent in the Z =−4, −6, and −12 (2, 3 and 6-ring naphthenic acids respectively) family in the carbon number range of 13 to 19 in the dichloromethane/acetonitrile cosolvent systems, salting-out effects resulted in a general enhancement of Z =−4 species, relative to others. Likewise, the shift in relative intensities of species containing O₁, O₃, O₄, O₂S and O₃S was dramatic for systems with and without saturated salts present. The O₄ and O₃S species for example, were prevalent in the dichloromethane/acetonitrile cosolvent but were non-detected in the presence of saturated salts. Interactions of oil sands acids with salts are expected to occur in oil sands processed waters and natural saline waters. As evident by the distribution of species observed, salting-out effects will play a major role in limiting the bioavailability of oil sands acids in aquatic systems.

Keywords:

• Salt effects
• naphthenic acids
• oil sands acids
• electrospray
• mass spectrometry

Acknowledgments

This work was funded in part by the Program of Energy Research and Development.