The Use of Internal Surface Reversed-Phase Packing for the Solid Phase Extraction of Drugs from Serum

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Abstract
The utilization of Internal Surface Reversed Phase (ISRP) packing as a solid phase extraction (SPE) matrix was investigated. Evaluation of the relative retention of nineteen medicinal agents on ISRP material was monitored using HPLC. The effects of altering pH and/or buffer concentration on retention were further studied using verapamil, phenelzine and tamoxifen as model compounds. Spiked serum samples containing the model compounds plus amitriptyline were also subjected to ISRP-SPE and HPLC. Verapamil, phenelzine, and tamoxifen were all strongly retained on ISPR as pH increased. The buffer concentration of the sample was not as critical on retention of these compounds as pH. Verapamil was quantitatively recovered from spiked serum samples (103 /pm 8.5%, n = 6), with a limit of detection of 10 ng/mL using fluorescent detection (γex = 280 nm, γem = 310 nm). Amitriptyline recovery was also quantitative (99.0/pm 5.3%, n = 6), and its limit of detection was 10 ngmL employing short wavelength UV detection (γ = 214 nm). The ISRP packing as an SPE matrix failed to adequately disrupt the binding of either phenelzine or tamoxifen from serum proteins; therefore, recoveries of these compounds were poor (<50%). A comparison of ISRP to conventional C18 and C2 SPE columns indicated that the ISRP packing eliminated interferences as well or better than the conventional columns.