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NA and Hexadecane Belt Skim Test

  1. Mix 80 mL water, 0.11 mL commercial naphthenic acid, and 5 mL hexadecane into beaker.
  2. Using predetermined ideal belt sample (refer to belt selection tests), dip belt into solution for 3 seconds and scrape skimmed sample into a separate beaker.
  3. Using GC-MS, determine concentration level of NAs in skimmed solution as outlines below.

    Extraction and analysis of skimmed samples for the presence of naphthenic acids (NAs)

    After skimmers were scraped, the water and hydrocarbon layers were separated in a separatory funnel. The water layer (on the bottom) was collected, acidified to pH <2, and extracted with dicholoromethane. The resulting layer was derivatized with MTBSFTA (N-methyl-N-(t-butyldimethylsilyl)trifluoroacetamide, Thermo Fisher Scientific) prior to GC/MS analysis. This reagent adds t-butyldimethylsilyl groups to the carboxylic acid groups of NA to make them amenable to the analysis, and allows for quantification of a specific NA within a complex mixture to be based on a characteristic [M+57]+ ion. This technique is commonly used to analyze mixtures of NA (Clemente and Fedorak, 2005). The hydrocarbon layer was derivatized directly with MTBSTFA. Samples were then analyzed by GC/MS on an Agilent GC/MS system (model 7890A GC, model 5975C inert XL MSD) equipped with an HP-1 capillary column (50 m x 0.32 mm i.d. x 0.2 μm film, Agilent Technologies, Inc.). The samples were analyzed in splitless mode, with the inlet held at 270°C. The oven was initially held at 100°C for 5 min, then increased at a rate of 8°C/min to a final temperature of 300°C that was held for 5 min. The various NA were then quantified using a customized program in Excel (Clemente et al., 2003).


    Clemente, J.S., Prasad, N.G.N., Michael D. MacKinnon, M.D., and Fedorak, P.M. 2003. A statistical comparison of naphthenic acids characterized by gas chromatography–mass spectrometry. Chemosphere, 50: 1265-1274.

    Clemente, J. S. and Fedorak, P.M. 2005. A review of the occurrence, analyses, toxicity, and biodegradation of naphthenic acids. Chemosphere, 60: 585-600.