Separation of free and glucuronidated opioids by capillary electrophoresis in aqueous, binary and micellar media

Abstract

The separation behavior of free and glucuronidated opioids, including codeine, dihydrocodeine, morphine, norcodeine, normorphine, codeine-6-glucuronide, morphine-3-glucuronide, dihydromorphine, nordihydrocodeine, nordihydromorphine and dihydrocodeine-6-glucuronide, by capillary zone electrophoresis (CZE) and electrokinetic chromatography (EKC) in aqueous and binary media with ethylene glycol is reported. The opioids' charges and thus their separations are shown to be strongly dependent on buffer pH, particularly in the pH range 6–12. Unconjugated codeinoids are shown to be able to form cations whereas unconjugated morphinoids and the glucuronides are of amphoteric nature with pI values in the 9–10 range and around 5, respectively. Due to the similarity of most pKa values and chemical structures, all 12 opioids cannot be fully separated by aqueous and binary CZE. However, they can be resolved completley by aqueous EKC using 80 mM sodium dodecyl sulfate at pH 10.6. In EKC at pH 2.2 the cationic opioids strongly interact with the negatively charged dodecyl sulfate, migrate poorly resolved toward the positive electrode, are detected in the order of decreasing capacity factors and their elution times decrease with increasing surfactant concentration. With addition of ethylene glycol (up to 60%, v/v), the interaction between solutes and dodecyl sulfate is weakened and resolution is attained. An approach to measuring the viscosity of running buffers using a capillary electrophoresis apparatus is discussed.