Derivatization/Solid-Phase Microextraction: New Approach to Polar Analytes

Abstract

Trace analysis of fatty acids in water and/or air was enhanced by coupling solid-phase microextraction (SPME) with derivatization of the target analytes to less polar and more volatile species prior to GC analysis. Derivatization was performed in three ways: in the sample matrix, in the SPME fiber coating, and in the GC injector port. Derivatization converts polar analytes into less polar analogues, therefore increasing their coating/water or coating/gas partition coefficients and improving SPME efficiency and method sensitivity. Derivatization changes analytes with low volatilities into more volatile derivatives, thus improving their GC separations, detection, and quantitation. Pentafluorobenzyl bromide and (pentafluorophenyl)diazoethane (PFPDE) were used to derivatize short-chain fatty acids directly in sample matrices for selective and sensitive ECD detection. Diazomethane and pyrenyldiazomethane (PDAM) were used for effective in-fiber derivatization of long-chain and short-chain fatty acids, respectively, to increase their detectability. Tetramethylammonium hydroxide and tetramethylammonium hydrogen sulfate were employed to convert long-chain fatty acids into their volatile methyl esters via in-injector derivatization, for adequate and convenient GC analysis. For aqueous sample analysis, all six reagents were employed. The limits of detection (LODs) for short-chain fatty acids were in low nanogram per milliliter to the high picogram per milliliter levels. For air sample analysis, PDAM and PFPDE were employed. The LODs for short-chain fatty acids using both PFPDE and PDAM were in femtogram per milliliter to low picogram per milliliter levels. The LODs with derivatization/SPME for the analysis of short-chain fatty acids were 1−4 orders of magnitude lower than direct SPME without derivatization, therefore improving the sensitivity of SPME technique significantly.