Optimization of modified dispersive liquid-liquid microextraction coupled with high-performance liquid chromatography for the simultaneous preconcentration and determination of nitrazepam and midazolam drugs: An experimental design.

A simple, sensitive, and rapid microextraction method, namely, ultrasound-assisted surfactant-enhanced emulsification microextraction based on the solidification of floating organic droplet method coupled with high-performance liquid chromatography was developed for the simultaneous preconcentration and determination of nitrazepam and midazolam. The significant parameters affecting the extraction efficiency were considered using Plackett-Burman design as a screening method. To obtain the optimum conditions with consideration of the selected significant variables, a Box-Behnken design was used. The microextraction procedure was performed using 29.1 μL of 1-undecanol, 1.36% (w/v) of NaCl, 10.0 μL of sodium dodecyl sulfate (25.0 μg mL(-1)), and 1.0 μL of Tween80 (25.0 μg mL(-1)) as an emulsifier in an extraction time of 20.0 min at pH 7.88. In order to investigate the validation of the developed method, some validation parameters including the linear dynamic range, repeatability, limit of detection, and recoveries were studied under the optimum conditions. The detection limits of the method were 0.017 and 0.086 ng mL(-1) for nitrazepam and midazolam, respectively. The extraction recovery percentages for the drugs studied were above 91.0 with acceptable relative standard deviation. The proposed methodology was successfully applied for the determination of these drugs in a number of human serum samples.