Solid-phase microextraction and on-line methylation gas chromatography for aliphatic carboxylic acids.
Methylation of carboxylic acids upon syringe injection of a mixture of the acid sample and phenyltrimethylammonium hydroxide (PTMAH) into the GC injection port is a convenient but under-utilized derivatization procedure. To minimize potential instrumental problems due to the sample matrix, it was shown that solid-phase microextraction (SPME) is effective for the absorption of both the carboxylic acid (RCOOH) and PTMAH permitting on-line methylation from the fiber. A comparison of three fibers, polydimethylsiloxane (PDMS), polyacrylate (PA), and carboxene/PDMS for decanoic and stearic acids showed the carboxene/PDMS fiber was about five times more effective for the extraction of the RCOOH-PTMAH mixture dissolved in methanol. The optimum fiber absorption time was about 20 min and the optimum desorption time in the injection port held at 280 degrees C was about 5-10 min. The optimum PTMAH/RCOOH ratio was about 125:1. Linearity for C18:0 at 3.3 x 10(-6)-3.3 x 10(-4) M was demonstrated by GC-MS with a detection limit of 1 microM. This SPME method is also effective for the methylation of C18:1, C18:2, and C18:3 fatty acids. Transesterification of olive oil using PTMAH and then on-line methylation either by the syringe method or by SPME gave comparable fatty acid methyl ester profiles.
Liu
Yaqian
Y
Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056, USA.
Cho
Seong R
SR
Danielson
Neil D
ND
D002138:Calibration; D002264:Carboxylic Acids / Q000302:isolation & purification*; D004952:Esters; D008401:Gas Chromatography-Mass Spectrometry / Q000295:instrumentation / Q000379:methods; D008745:Methylation; D008832:Microchemistry / Q000379:methods; D009862:Online Systems
D002264:Carboxylic Acids; D004952:Esters