Anal. Chem. 1997, 69, 196-205
Derivatization/Solid-Phase Microextraction: New Approach to Polar Analytes Lin Pan and Janusz Pawliszyn*
The Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G1
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 infiber 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 shortchain 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. The biological and environmental importance of carboxylic acids has led to extensive method development for their measurements, the importance of which is reflected by the estimated 25% of GC publications devoted to these compounds.1 Fatty acids are intermediates in many biological processes.2 Evidence suggests that these acids might have beneficial physiological effects.3,4 More often, fatty acids are produced from humic substances during (1) Krupcik, J.; Hrivnak, J.; Janak, J. J. Chromatogr. Sci. 1976, 14, 4. (2) Brill, J. H.; Narayanan, B. A.; McCormick, J. P. Appl. Spectrosc. 1991, 45, 1617. (3) Ghoos, Y.; Geypens, B.; Hiele, M.; Rutgeerts, P.; Vantrappen, G. Anal. Chim. Acta 1991, 247, 223.
196 Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
water treatment processes. These acids play an important role as corrosive agents in water distribution nets, steam generation processes, etc.5 Recently, a few review articles for the analysis of long-chain fatty acids,6,7 fatty acids in lipid samples,8,9 and physiologically important carboxylic acids10 have been published. Fatty acids are most commonly analyzed by gas chromatography (GC) either as free acids or as derivatives. In the underivatized form, the sensitivity and reproducibility of analyses are poor.11 For short-chain acids, this is because that they are very polar and soluble in water3 and are possibly adsorbed in the GC column, especially at low concentrations (
