Evaluating MTBSTFA derivatization reagents for measuring naphthenic acids by gas chromatography-mass spectrometry

Naphthenic acids (general formula CnH2n+zO2) arc found in petroleum and oil sands deposits Release of these acids to aquatic environments is a concern because of their potential toxicity Naphthenic acids consist of a complex mixture of carboxylic acids, and estimating their concentrations in environmental samples is a challenge Two recent reports have described gas chromatography-mass spectrometry (GC-MS) methods to selectively detect these acids in fish flesh and water samples. The methods use N-methyl-N-(t-butyldimethylsilyl)-trifluoroacetamide (MTBSTFA) with 1% t-butyldimethylchlorosilane (t-BDMCS) to derivatize naphthenic acids to their t-butyldimethylsilyl derivatives. Single ion monitoring (S M) was used to detect the fragment m/z 267, which corresponds to the derivatives of one isomer class (n = 13 and Z = -4) of naphthenic acids The SIM chromatograms give a characteristic naphthenic acids hump between retention times 15 and 20 min and a sharp peak for the t-butyldimethylsilyl derivatized surrogate standard. 9-fluorenecarboxylic acid Integration of this hump and the peak from the surrogate standard allows quantification of the naphthenic acids. Using newly purchased MTBSTFA containing 1% t-BDMCS (from three different suppliers) yielded SIM chromatograms with one or two large contaminating peaks (eluting at 16.7 and 18 8 min) that interfered with integration of the hump, rendering the method unreliable The contaminants were traced to the presence of t-BDMCS. Each of the three suppliers sells MTBSTFA devoid of t-BDMCS, and using MTBSTFA without 1% t-BDMCS was found to be suitable for the GC-MS method.