Strategies for the Analysis of Polar Solvents in Liquid Matrixes

Abstract

Various approaches to the analysis of polar compounds in different matrixes by solid-phase microextraction (SPME) were studied. The analysis of polar analytes in nonpolar matrixes was performed with custom-made SPME fibers coated with Nafion perfluorinated resin. The sensitivity of this fiber in this type of analysis was better by 1 order of magnitude on average as compared to those of any of the commercially available fibers. The fiber was the most sensitive for the most polar of the compounds studied, i.e., methanol. Determination of methanol, ethanol, and 2-propanol in unleaded gasoline was illustrated. Except for methanol, the fiber did not perform very well in the analysis of alcohols in water. The fiber was capable of extracting water from benzene. SPME analysis of polar compounds in water was studied using aqueous solutions of acetone, methyl ethyl ketone, methyl isobutyl ketone (MIBK), 2-propanol, 2-methyl-2-propanol, and tetrahydrofuran. Fibers coated with poly(dimethylsiloxane)/divinylbenzene yielded the highest sensitivity in this type of analysis. Low- or sub-ppb detection limits were obtained for all the analytes with FID detection when the samples were saturated with NaCl. Since fibers of this type extract analytes by adsorption rather than absorption, nonlinear responses were observed when all the analytes were allowed to equilibrate because of the limited number of adsorption sites on the surface of the coating and displacement of compounds with low distribution ratios by compounds with high distribution ratios (mainly MIBK). Two approaches allowed a significant improvement in linearity: extraction of a vigorously stirred sample for a short time, or extraction under static conditions for a time much shorter than that required for equilibration of all the analytes. In both cases the amount of MIBK extracted was significantly reduced, while the remaining analytes were affected to a much lesser degree. The sensitivity of acetone determination was greatly improved by in-solution derivatization with o-(2,3,4,5,6-pentafluorobenzyl)hydroxylamine hydrochloride and extraction of the oxime formed.