Selective focusing of catecholamines and weakly acidic compounds by capillary electrophoresis using a dynamic pH junction

A systematic study of selective analyte focusing in a multisection electrol yte system by capillary electrophoresis (CE) is presented. It was found tha t a dynamic pH junction between sample and background electrolyte zones can be used to focus zwitterionic catecholamines and weakly acidic compounds w ithout the use of special ampholytes. Differences in pH and concentration o f complexing agents, such as berate, in the sample and background electroly te zones were determined to cause focusing through changes in the local vel ocity of the analyte in two different segments of the capillary. Velocity-d iffererence induced focusing (V-DIF) of analytes using a dynamic pH junctio n allowed the injection of large sample volumes and significantly improved the concentration sensitivity of CE. Under optimized conditions, the limit of detection for epinephrine was determined to be about 4 x 10(-8) M (the o riginal sample) with conventional UV absorbance detection. Moreover, separa tion efficiencies greater than a million theoretical plates can be achieved by focusing such large sample volumes into narrow zones. Multisection elec trolyte systems, which lead to the formation of a dynamic pH junction, can be tuned toward improving the concentration sensitivity of specific analyte s their chemical properties are known.