Application of ultrahigh-performance liquid chromatography–quadrupole time-of-flight mass spectrometry for the characterization of organic aerosol: Searching for naphthenic acids

Abstract

Naphthenic acids (NAs) are naturally occurring constituents of hydrocarbon deposits (petroleum, oil sands bitumen, and crude oils), and present in any facilities that extract, process or use crude oil or bitumen for manufacturing. In the Athabasca oil sands region (AOSR) of Alberta, Canada, this diverse group of saturated acyclic, monocyclic, and polycyclic carboxylic acids is present in bitumen and in tailing ponds. Little is known about the occurrence of residual oil sands derived organic material, including NAs, in atmospheric particulate matter (PM). This work describes the optimization of an ultrahigh-performance liquid chromatography–quadrupole time-of-flight mass spectrometry (UPLC/QTOF-MS) method to characterize and identify classical NAs in atmospheric PM. Under the optimum conditions, analysis of the Sigma-Aldrich technical NA STD Mix showed the absence of saturated fatty acids, while branched and cyclic NAs isomers with carbon number ranged between 10 and 30, and Z families between 0 and −12 were detected. Higher molecular weight NAs representing compounds with carbon number ranging between 30 and 40 were identified in the analyzed PM samples collected in AOSR. In contrast, isomeric branched acyclic NAs were not detected in the background (PM-R) samples collected far from AOSR. Except for fatty acids (Z=0), other Z homologues were also not detectable or present at very low concentration in the analysed PM-R samples. Preliminary examination of NA profiles showed that the composition of NAs in PM collected in close proximity to surface mining operations is predominantly “refractory” high molecular weight branched NAs, differing from that collected closer to upgraders and tailings ponds. It is suspected that dust released from the mine faces and dry tailings in the AOSR are sources of NAs to atmosphere. Further samples should be examined to confirm findings. This preliminary study presents, to our knowledge, the first direct identification of branched acyclic and cyclic NAs in atmospheric PM.