Naphthenic acids and other acid-extractables in water samples from Alberta: What is being measured?

Abstract

There is increasing international interest in naphthenic acids (NAs, classical formula C nH 2n + Z O 2) found in the oil sands from Alberta, Canada and in petroleum from around the world. The complexity of NAs poses major analytical challenges for their quantification and characterization. We used ultrahigh resolution electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR MS) to probe the make up of NAs from various sources by searching for peaks corresponding to the formula C nH 2n + Z O x, for combinations of n = 8 to 30, Z = 0 to −12, and x = 2 to 5. The sources included three commercial NAs preparations, and the acid-extractable organics from eight oil sand process-affected waters (OSPW) and from six surface fresh waters. Extracts from OSPW contained between 1 and 7% sulfur. The mass spectra showed between 300 and 1880 peaks, with > 99% of the peaks having m/z between 145 and 600. In most cases, < 20% of the peaks were assigned as classical NAs (x = 2) and oxy-NAs (x = 3 to 5). The classical NAs from the OSPW were predominantly Z = −4 and −6, whereas those from the fresh waters were mainly Z = 0, with palmitic and stearic acids being the major components in the fresh waters. Remarkably, when the peak abundances were considered, < 50% of the total abundance could be assigned to the classical and oxy-NAs. Thus, > 50% of the compounds in the extracts of OSPW were not “naphthenic acids”. Based on these findings, it appears that the term “naphthenic acids”, which has been used to describe the toxic extractable compounds in OSPW, should be replaced by a term such as "oil sands tailings water acid-extractable organics (OSTWAEO)". Classical and oxy-NAs are components of OSTWAEO, but this term would not be as misleading as “naphthenic acids”.