“…As for the sample preparation procedures employed prior to the HPLC determination of verapamil, there is a diversity in the speed and complexity of the clean-up methods starting from direct determination of verapamil from rat plasma [7], through using only deproteinization of the plasma samples [11,13] to the more complex ones such as liquid-liquid extraction (LLE) using various organic solvents [3,4,6,9,10,15] and solid-phase extraction (SPE) with various adsorbents [4,8,9,17]. The most sophisticated sample preparation procedures employ automated column switching techniques for on-line sample preparation [5,7,8,12].…”