Determination of lidocaine in plasma by direct solid-phase microextractioncombined with gas chromatography

Direct-immersion solid-phase microextraction (SPME) has been used to extrac t the local anesthetic lidocaine from human plasma. A simplified model show s the relationship between the total amount of drug in plasma and the amoun t of drug extracted. The model takes into account that the drug participate s between the fiber, sample and proteins. Therefore the model can also be u sed to obtain a good approximation of the drug-protein binding. Extraction yields of lidocaine in plasma are <1%, and the protein binding of lidocaine was found to be about 74% at pH 9.5. A SPME method has been developed for the determination of the total amount of lidocaine in plasma. The protein b inding was reduced by acidification and, subsequently, the sample was depro teinized with trichloroacetic acid. With a 100-mu m polydimethylsiloxane-co ated fiber and addition of sodium chloride to the sample an extraction yiel d of about 12% at equilibrium (45 min) has been obtained. The relative stan dard deviation of this method is <10%. A linear range was found from 25 to 2000 ng ml(-1) lidocaine in plasma (r=0.998) with a detection limit of 5 ng ml(-1) in plasma. An extraction yield of about 80% could be obtained after an overnight extraction by use of a 65-mu m polydimethylsiloxane-divinylbe nzene-coated fiber. if an extraction time of 10 min is used with this fiber , the same yield is obtained as with the single-phase fiber in 45 min. Howe ver, the drawback of this mixed-phase fiber is its much shorter lifetime. ( C) 2000 Elsevier Science B.V. All rights reserved.