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Recently direct plasma injection LC/MS/MS technique has been increasingly used in pharmaceutical research and development due to the demand for higher throughput of sample analyses. In this work, two on-line extraction methods including high flow LC/MS/MS and high flow column switching LC/MS/MS were investigated. The evaluations were conducted and focused on their performances with respect to peak responses, separation efficiency, and signal to-noise ratio in a multiple-component LC/MS/MS assay. Two HPLC pumps were used-with one for high flow delivery and one for gradient elution. A CTC autosampler was used to inject plasma samples. High flow LC was achieved by the use of 4 ml/min flow rate on a 1 x 50 mm Waters Oasis column. A 2 x 100 mm YMC column was coupled via a column-switching valve. The extracted analytes were analyzed in multiple-reaction-monitoring (MRM) mode using a triple quadrupole MS/MS. As a rapid and simple procedure, vortex-mixing plasma and internal standard directly in sample vials completed sample preparation. The high flow column switching method (two-column system) provided sharper peak shape than the conventional high flow method. This effect increased analyte signal-to-noise ratio and sensitivity. Narrower peak width resulted in much better separation efficiency, which was required for multiple compound (N-in-1) analysis. A 2 mm I.D. column resulted in better peak shape and resolution than using a smaller I.D. column. The selected method achieved acceptable recoveries for most of the compounds tested, and it was successfully applied to a 10-in-1 pharmacokinetic (PK) study. The results showed that the dynamic range, lower limit of quantitation, assay accuracy and precision were acceptable for all compounds. Rapid sample preparation eliminated labor intensive and time consuming processes and improved productivity. This high through-put on-line extraction high flow column switching method has been proven particularly useful for multiple component analysis in PK studies. (C) 2002 Dupont Pharmaceuticals. Published by Elsevier Science B.V. All rights reserved.