Electrospray-Mass Spectrometric Analysis of Reference Carboxylic Acids and Athabasca Oil Sands Naphthenic Acids

Naphthenic acids (NAs) are complex mixtures of naturally occurring acyclic and cyclic aliphatic carboxylic acids that are responsible for the toxicity of the water in the tailings ponds associated with the recovery of bitumen from the Athabasca oil sands. NAs are difficult to analyze due to their complexity and the lack of commercially available NA standards. This paper describes the use of negative ion electrospray ionization mass spectrometry for the analysis of NAs. Model carboxylic acids, alone and in mixture, afforded mass spectral signal intensities that were highly dependent on extractor and cone voltages and on molecular structure. These effects were also observed for authentic NAs. Under conditions that were close to optimal for all the model compounds, their calibration sensitivities varied by a factor of <2, and there were minimal interactions when the model compounds were examined in mixture. Under the same conditions, the authentic NAs showed apparent congener distributions similar to those observed previously by GC/MS for derivatized NAs. The similar calibration sensitivities among congeners allowed the use of the standard addition method to determine the approximate absolute concentrations of NA congeners in an authentic sample.