Non-equilibrium solid-phase microextraction coupled directly to ion-trap mass spectrometry for rapid analysis of biological samples

To determine sub-ppb levels of drugs in biological samples, selective, sensitive and rapid analytical techniques are required. This work shows the possibilities for high-throughput analysis of solid-phase microextraction (SPME) directly coupled to an ion-trap mass spectrometer equipped with an atmospheric pressure chemical ionisation source. As no chromatographic separation is performed, the SPME procedure is the time-limiting step. Direct immersion SPME under non-equilibrium conditions permits the determination of lidocaine in urine within 10 min. After a 5 min sorption time with a 100 microm polydimethylsiloxane-coated fibre, the extraction yield of lidocaine from urine is about 7%. When applying 4 min desorption, using a mixture of ammonium acetate buffer (pH 4.5) and acetonitrile (85 + 15 v/v), about 10% of the analyte is retained on the fibre. An extra cleaning step of the fibre is therefore used to prevent carry-over. By use of tandem MS, no matrix interference is observed. The detection limit for lidocaine is about 0.4 ng ml(-1) and the intraday and interday reproducibility are within 14% over a concentration range of 2-45 ng ml(1).