Separation of opiate alkaloids by electrokinetic chromatography with sulfated-cyclodextrin as a pseudo-stationary phase.
The separation of six related opiate alkaloids (morphine, thebaine, 10-hydroxythebaine, codeine, oripavine and laudanine) was studied using sulfated-cyclodextrin (s-CD) as a cation-exchange pseudo-stationary phase. Cation-exchange interactions between the cationic analytes and the anionic s-CD (7-11 mol of sulfate groups per mole CD) were found to bethe predominant mechanism, allowing the separations to be performed at low pH where the opiates are protonated and exhibit very similar mobilities. The concentrations of the s-CD and the competing ion (Na+ or Mg2+) in the electrolyte were used to govern the extent of the ion-exchange interactions. Interactions with the sulfated-cyclodextrin differed for each analyte, with oripavine exhibiting the strongest interaction and 10-thebaine and laudanine showing the weakest interactions. Despite the very similar structures of the analytes, these differences resulted in significant changes in separation selectivity. The separation was modelled using a migration equation derived from first principles and based on ion-exchange interactions between the s-CD and the opiates. Constants within the model were obtained by non-linear regression using a small subset of experimentally determined migration times. These constants related to the ion-exchange affinities of the s-CD for the various opiates. When the model was used to predict migration times under other experimental conditions, a very good correlation was obtained between observed and predicted mobilities (r2=0.996). Optimisation of the system was performed using the normalised resolution product and minimum resolution criteria and this process provided two optimised separations, each exhibiting a different separation selectivity.
Zakaria
Philip
P
Australian Centre for Research on Separation Science. School of Chemistry, University of Tasmania, GPO Box 252-75, Hobart, Tasmania 7001, Australia.
Macka
Miroslav
M
Haddad
Paul R
PR
D000470:Alkaloids / Q000302:isolation & purification*; D020374:Chromatography, Micellar Electrokinetic Capillary / Q000295:instrumentation / Q000379:methods*; D003505:Cyclodextrins / Q000737:chemistry*; D009294:Narcotics / Q000302:isolation & purification*; D012680:Sensitivity and Specificity
D000470:Alkaloids; D003505:Cyclodextrins; D009294:Narcotics