Porous polymer monoliths have been used to develop an online solid-phase extraction with liquid chromatography method for determination of dopamine in urine as well as for a continuous monitoring of dopamine in flowing system. A polymerization mixture containing 4-vinylphenylboronic acid monomer has been used to prepare a trapping column based on specific ring formation reaction with dopamine cis-diol functionality. Additionally, a monolithic stationary phase with zwitterion functionality has been used to prepare capillary column for the separation of dopamine. Experimental conditions including molarity, pH, and flow rate of the loading buffer together with a valve switching time have been optimized to provide the highest recovery for dopamine. Experimental setup has been used to determine dopamine in a urine. By using both calibration curve and standard addition method, the dopamine level was determined to be 1.19 and 1.28 mg/L, respectively. Further, we have used experimental design to optimize coupling of two extraction monolithic loops to separation capillary column with monolithic phase for a comprehensive monitoring of dopamine. After multivariate analysis, sample loading flow-rate and a flow-rate of flushing buffer were selected as the most significant variables. Optimized experimental setup was applied to continuously monitor dopamine degradation.
In this work, we aimed to prepare a monolithic capillary column that allowed an isocratic separation of ten dopamine precursors and metabolites in a single run. Segments of five zwitterion sulfobetaine polymer monoliths have been modified by zwitterion phoshorylcholine by using an ultraviolet‐initiated two‐step photografting. Columns with 0, 33, 50, 66, and 100% of modified length were prepared. Effect of length of the modified segment and mobile phase composition has been tested. All columns provided dual‐retention mechanism with reversed‐phase retention in highly aqueous mobile phase and hydrophilic interaction mechanism in highly organic mobile phase. The retention mechanism was controlled by the composition of the mobile phase and has been described by a three‐parameter model. We have used regression parameters to characterize the retention of analyzed compounds and to study individual pathways of dopamine metabolism. Comprehensive optimization of mobile phase composition allowed to find an optimal composition of the mobile phase and stationary phase surface chemistry arrangement to achieve desired separation. Optimized columns provided an isocratic separation of all tested compounds in less than nine min.
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