Characterization of dicarboxylic naphthenic acid fraction compounds utilizing amide derivatization: proof of concept

Rationale The characterization of naphthenic acid fraction compounds (NAFCs) in oil sands process affected water (OSPW) is of interest for both toxicology studies and regulatory reasons. Previous studies utilizing authentic standards have identified dicarboxylic naphthenic acids using two-dimensional gas chromatography hyphenated to time-of-flight mass spectrometry (GCxGC/TOFMS). The selective derivatization of hydroxyl groups has also recently aided in the characterization of oxy-NAFCs, and indirectly the characterization of dicarboxylic NAFCs. However, there has been no previous report of derivatization being used to directly aid in the standard-free characterization of NAFCs with multiple carboxylic acid functional groups. Herein we present proof-of-concept for the characterization of dicarboxylic NAFCs utilizing amide derivatization. Methods Carboxylic acid groups in OSPW extract and in a dicarboxylic acid standard were derivatized to amides using a previously described method. The derivatized extract and derivatized standard were analyzed by direct-injection positive-mode electrospray ionization ((+)ESI) high resolution mass spectrometry (MS), and the underivatized extract was analyzed by (-)ESI MS. Tandem-MS was carried out on selected ions of the derivatized standard and derivatized OSPW. Data analysis was carried out using the Python programming language. Results The distribution of monocarboxylic NAFCs observed in the amide-derivatized OSPW sample by (+)ESI-MS was generally similar to that seen in underivatized OSPW by (-)ESI-MS. The dicarboxylic acid standard shows evidence of being doubly-derivatized, although the second derivatization appears to be inefficient. Furthermore, a spectrum of potential diacid NAFCs is presented, identified by both charge state and derivatization mass. Interference due to the presence of multiple derivatization products is noted, but can be eliminated using on-line separation or an isotopically labelled derivatization reagent. Conclusions Proof of concept for the characterization of dicarboxylic NAFCs utilizing amide derivatization is demonstrated. Furthermore, high-resolution (+)ESI-MS of the derivatized monocarboxylic NAFCS yields similar information to (-)ESI-MS analysis of underivatized NAFCs, with the benefit of added selectivity for carboxylic acid species and the characterization of diacids.