title: Influence of electrolyte nature on the separation selectivity of amphetamines in nonaqueous capillary electrophoresis: protonation degree versus ion pairing effects.
abstract: The simultaneous analysis of Ecstasy and its derivatives in an acetonitrile-methanol (80:20 v/v) mixture was previously shown to be strongly dependent on the nature of the electrolyte (acetate versus formate). To elucidate the phenomena involved, systematic experiments were conducted in this solvent medium. Conductivity measurements allowed to evaluate the ion-pairing rate in the background electrolyte (BGE) and thereby distinguish between electrolyte concentration and ionic strength. The influence of electrolyte concentration on analyte effective mobilities micro(eff)) was studied by capillary electrophoresis (CE). As micro(eff) extrapolated to infinite dilution proved to be independent of the nature of the electrolyte, selectivity changes could not be attributed to a modification in the protonation degree of amphetamines. Experimental mobility data were then confronted to existing theoretical mobility models to discriminate between ion pairing or simple ionic strength effect. Ion-pair formation in a BGE containing acetate was highlighted with an ion-pairing model and ion-pair formation constants between each amphetamine and acetate ion were calculated.
authors.affiliation: Laboratoire d'Electrochimie et Chimie Analytique, UMR CNRS 7575, Ecole Nationale Supérieure de Chimie de Paris, Paris, France.
authors.forename: Stéphanie
authors.initials: S
authors.lastname: Descroix
authors.forename: Anne
authors.initials: A
authors.lastname: Varenne
authors.forename: Laurent
authors.initials: L
authors.lastname: Geiser
authors.forename: Samir
authors.initials: S
authors.lastname: Cherkaoui
authors.forename: Jean-Luc
authors.initials: JL
authors.lastname: Veuthey
authors.forename: Pierre
authors.initials: P
authors.lastname: Gareil
mesh_terms: D000662:Amphetamines / Q000302:isolation & purification*; D004573:Electrolytes / Q000737:chemistry*; D019075:Electrophoresis, Capillary / Q000379:methods*; D009994:Osmolar Concentration; D011522:Protons
chemical_list: D000662:Amphetamines; D004573:Electrolytes; D011522:Protons