Year of Publication: 2012
Abstract:
The purpose of this report is to identify challenges in analyzing naphthenic acids (NAs) associated with oil sands process water (OSPW). Naphthenic acids are present naturally in oil sands bitumen and have the classical formula CnH2n+ZO2. Within this formula n represents the carbon number and Z is an even, negative integer corresponding to hydrogen deficiency mainly due to ring formation in the structure. Thus the absolute value of Z divided by 2 gives the number of the rings in the compounds. A Z-value of 0 means acyclic acids, which are believed to be highly branched rather than linear natural fatty acids. A Z-value of -2 represents monocyclic NAs; -4 represents bicyclic and so on. The Z-value may also include unsaturation in the chemical structure. The generality of the formula allows for a vast array of isomers for each value of n and Z. The challenge in analyzing NAs from OSPW is that microbial activity alters the structure of classical naphthenic acids creating a large number of compounds that are labeled as naphthenic acids but differ from the CnH2n+ZO2 general formula. This increased number of compounds elevates the demands on the analytical methods used to characterize these compounds obtained from OSPW. In this report, issues affecting both qualitative and quantitative data from a variety of analytical methods will be reviewed to generate an awareness of the challenges faced by laboratories conducting NA determinations. The report also highlights the issues of naming these compounds “naphthenic acids” since many of the compounds being extracted from OSPW do not conform to the classical NA formula. The method chosen has a significant effect on the interpretation of the analytical data. Analytical results are dependent on sampling, extraction and clean-up techniques. The report examines various approaches used to prepare samples for analysis based on the following themes: sampling tools and techniques, sample preservation and transport, extraction, and clean-up methods. There are numerous analytical instruments currently being used in the analysis of NAs. Within the field of spectroscopy Fourier Transform Infrared Spectroscopy (FTIR) has been used and is often considered the reference method for quantitative assessment of NAs in OSPW. Both, UV-Vis and fluorescence spectroscopy, and more recently Synchronous Fluorescence Spectroscopy (SFS) have been applied to studies of NAs in OSPW. Each of these methods are limited in the information that can be provided, however, they have value in assessing the types and possible sources of NAs being evaluated in a sample. Major advancements in the analysis of NAs are being accomplished using the power of chromatography to attain a partial separation of thousands of compounds found in a NA extract and mass spectrometry (MS) for their detection. Early methods of analysis using unit mass resolution MS have created problems in properly assessing NAs present in OSPW. This has led to the overestimation of NA concentrations in OSPW. Similar problems have been encountered with FTIR. Misclassification and identification of false positives has been another issue plaguing early adopters of these analytical methods. Fortunately, new analytical tools are being developed which enable high resolution mass spectrometry (HRMS) to be performed enabling these errors in classification to be partially rectified. Although many efforts have been made in the development of analytical methods, no rugged routine method that can separate, identify, and quantify the individual components of NA mixtures has been achieved to date. This review will provide an overview of methods currently used for the analysis of NA class of compounds including sampling, sample preservation, sample transport, extraction and clean-up, analytical techniques, and future needs, with a major focus on NAs from OSPW. No method currently exists that is capable of identifying all isomers of NAs. Without this capability it is impossible to clearly assess the toxicity of individual “naphthenic acids” encountered in OSPW. Additionally it makes it difficult to fully understand the potential for biodegradation and remediation of NAs in fluid tailings or their long term impacts in the reclaimed landscape.
