Trace analysis of basic drugs in micro-volume human plasma samples using a simple device combined with a gas chromatography-flame ionization detector

In this work, a drop-to-drop microextraction system in a capillary tube (i.d. ∼1.1 mm) assisted by a micro-scale homemade magnetic bar was introduced. The assembled setup demonstrated remarkable simplicity, rendering it ideal for monitoring the presence of methadone, tramadol and pethidine in plasma samples. This system facilitated easy extraction from sample volumes as minimal as 7 μL. The analytes were efficiently extracted from a 7 μL aqueous sample at pH 8 using a 0.5 μL toluene +20% hexane mixture as the extraction solvent. The sample was stirred at 700 rpm for 8 minutes, employing a micro-magnetic bar with a size of 430 μm, and the detection was achieved using a gas chromatography-flame ionization detector. The new setup offered good linearity in the range of 0.04-5 μg mL^-1 for pethidine, 0.1-10 μg mL^-1 for tramadol and 0.2-10 μg mL^-1 for methadone with a coefficient of determination (<i>r</i>²) higher than 0.998. The extraction efficiency achieved during sample stirring was nearly double that observed in the static solution, and the relative standard deviation (RSD%) remained below 6.1%, as determined through three replicate measurements. The limits of detection were 0.012, 0.04, and 0.06 μg mL^-1 for pethidine, tramadol, and methadone, respectively.