Determination of drug substances using ion-selective electrodes

The use of ion-selective electrodes for the determination of drug substances is reviewed. Although the primary emphasis is placed on drugs in the field of forensic chemistry, for example, paraquat, methamphetamine, amphetamine, and cocaine, other clinically important drugs such as procainamide, bretylium, disopyramide, and salicylate, as well as serotonin, a biogenic amine, are also discussed. Sensor membranes of these ion-selective electrodes can be prepared by incorporating a lipophilic ion-exchanger (or neutral carrier) and an appropriate membrane solvent in a poly(vinyl chloride) membrane matrix. In many cases, the combination of the ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate and the membrane solvent, 2-fluoro-2'-nitrodiphenyl ether afforded the most sensitive sensor membrane for drugs. In some cases, the choice of membrane solvent was important and tris(2-ethylhexyl) phosphate, tricresyl phosphate, and tetrakis (2-ethylhexyl) pyromellitate became excellent membrane solvents for making sensor membranes responding to serotonin, methamphetamine and cocaine, respectively. Neutral carriers may be useful to make highly selective electrodes to specific drugs, but have not been studied in detail. We discussed here amphetamine-selective and salicylate-selective electrodes using the neutral carriers, dicyclohexano-18-crown-6 and heptyl-4-trifluoroacetyl-benzoate, respectively, as typical examples.