Elsevier Science B.V. Acta Medica Okayama 0003-2670 620 1-2 2008 A caffeine-sensitive membrane electrode: Previous misleading report and present approach 50 54 EN Takashi Katsu Yumi Tsunamoto Nobumitsu Hanioka Keiko Komagoe Kazufumi Masuda Shizuo Narimatsu Although a previous study [S.S.M. Hassan, M.A. Ahmed, M.M. Saoudi, Anal. Chem. 57 (1985) 1126] had shown that a caffeine-sensitive electrode made with picrylsulfonate and 1-octanol as a cation-exchanger and a solvent mediator, respectively, had a wide working pH range (5.5-9.5) and exhibited a Nernstian response, we could not find such response in this electrode. The present result was reasonable, because the pK, value of caffeinium ion was reported to be around 0.7 and the neutral form of caffeine was predominant in the pH range examined. Thus, we reinvestigated the response characteristics of a caffeine electrode, taking into consideration the pKa value, and constructed a new electrode with a combination of the lipophilic cation-exchanger, tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate (HFPB), and the solvent mediator with high degree of dielectric constant, 2-fluoro-2'-nitrodiphenyl ether (FNDPE). This electrode showed a pH-dependent response to caffeinium ion and gave a detection limit of 50 mu M with a slope of 55 mV per concentration decade at pH 2. The use of other solvent mediators was less effective than that of FNDPE. The electrode was applied for the determination of caffeine in some central stimulants. No potential conflict of interest relevant to this article was reported.

Springer Berlin / Heidelberg Acta Medica Okayama 1618-2642 387 6 2007 S,S,S-Tris(2-ethylhexyl) phosphorotrithioate as an effective solvent mediator for a mexiletine-sensitive membrane electrode 2057 2064 EN Takashi Katsu Yumi Tsunamoto Nobumitsu Hanioka Keiko Komagoe Kazufumi Masuda Shizuo Narimatsu S,S,S-Tris(2-ethylhexyl) phosphorotrithioate proved to be an effective solvent mediator for constructing a mexiletine-sensitive membrane electrode in combination with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate. Among a series of phosphorus compounds containing phosphoryl (P=O) groups, this solvent mediator showed the highest sensitivity to mexiletine in phosphate-buffered physiological saline containing 0.15 mol L-1 NaCl and 0.01 mol L-1 NaH2PO4/Na2HPO4 (pH 7.4), giving a detection limit of 2 x 10(-6) mol L-1 with a slope of 58.8 mV decade(-1). This is the best reported detection limit of any mexiletine-sensitive electrode developed to date. Owing to its high selectivity toward inorganic cations, the electrode was used to determine the level of mexiletine in saliva, the monitoring of which is quite effective for controlling the dose of this drug noninvasively. The mexiletine concentrations determined with the mexiletine-sensitive electrode compared favorably with those determined by high-performance liquid chromatography. No potential conflict of interest relevant to this article was reported.