Acyclic neutral carrier-based polymer membrane electrode for a stimulant, phentermine

Groups of dioxadicarboxylic diamides, which were developed as potential ion ophores for inorganic cations, were found to act as ionophores for a stimul ant, phentermine. Especially, N,N-dioctadecyl-N',N'-dipropyl-3,6-dioxaoctan ediamide, which was originally developed as a lead ionophore and is commerc ially available from Fluka as lead ionophore I, was suitable for making a p hentermine-selective electrode. The electrode constructed using this ionoph ore and bis(2-ethylhexyl) sebacate as a solvent mediator in a poly(vinyl ch loride) membrane matrix discriminated between phentermine and analogous com pounds more effectively than an electrode based on dibenzo-18-crown-6, a re presentative ionophore for organic ammonium ions. Moreover, the present ele ctrode showed remarkably little interference by inorganic cations, such as Na+ and K+, as well as lipophilic quaternary ammonium ions including (C2H5) (4)N+ and(C3H7)(4)N+. The electrode exhibited a near-Nernstian response to phentermine in the concentration range of 2 x 10(-6) to 1 x 10(-2) M with a slope of 54.8 mV per concentration decade in 0.1 M MgCl2. The lower limit of detection was 7 x 10(-7) M. This electrode was applied to determine phen termine in a cationic-exchange resin complex of this stimulant, which is th e general dosage form in medical use.