Analytical Sciences

Acyclic Neutral Carrier-Based Polymer Membrane Electrode for a Stimulant, Phentermine

Takashi KATSU,*  Kazuyuki IDO,* and Kenji KATAOKA**

*Faculty of Pharmaceutical Sciences, Okayama University, Tsushima, Okayama 700-8530, Japan
**Central Customs Laboratory, Ministry of Finance, Iwase, Matsudo, Chiba 271-0076, Japan

Groups of dioxadicarboxylic diamides, which were developed as potential ionophores for inorganic cations, were found to act as ionophores for a stimulant, phentermine. Especially, N,N-dioctadecyl-N',N'-dipropyl-3,6-dioxaoctanediamide, which was originally developed as a lead ionophore and is commercially available from Fluka as lead ionophore I, was suitable for making a phentermine-selective electrode. The electrode constructed using this ionophore and bis(2-ethylhexyl) sebacate as a solvent mediator in a poly(vinyl chloride) membrane matrix discriminated between phentermine and analogous compounds more effectively than an electrode based on dibenzo-18-crown-6, a representative ionophore for organic ammonium ions. Moreover, the present electrode showed remarkably little interference by inorganic cations, such as Na⁺ and K⁺, as well as lipophilic quaternary ammonium ions including (C₂H₅)₄N⁺ and (C₃H₇)₄N⁺. 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 phentermine in a cationic-exchange resin complex of this stimulant, which is the general dosage form in medical use.

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