Preparation of the calibration standards and quality control (QC) samples The stock solutions of Clindipine and Nifidipine were prepared using water and acetonitrile mixture 1:1 at a concentration of 1.0 mg/mL each. Clindipine working solution was used to prepare the spiking stock solutions for construction of six-point calibration curve (10.0-125.0 ng/mL) and QC samples at three different levels (25.0, 50.0, 100.0 ng/mL). All the stock solutions were refrigerated (2-8°C) when not in use. Calibration standards and QC samples were prepared in bulk by spiking 25.0µL of respective spiking stock solutions. These were stored at -70°C until analysis