DEVELOPMENT OF A PORTABLE COCAINE-SELECTI VE ELECTRODE

We have designed a portable cocaine-selective electrode which can be u sed at the location of a crime. The cocaine electrode is based on a po ly (vinyl chloride) membrane containing sodium tetrakis[3,5-bis(triflu oromethyl)phenyl]borate as an ion-exchanger and tetrakis (2-ethylhexyl ) pyromellitate as a membrane solvent. The following two points were c onsidered in miniaturizing an instrument. First, we used a digital mul timeter having a small size (16 X 7.5 X 3 cm) for a potential measurem ent. Second, the structure of a reference electrode was changed from a double to a single junction in order to reduce the size (10 cm in len gth and 4 mm in diameter). Calibration plots between the electric pote ntials and cocaine concentrations were measured in the presence of 0.5 M MgCl2 in order to adjust the ionic strength of a solution. The dete ction limit was 4 X 10(-7) M and the slope of the electrode in the lin ear region was 56 mV per concentration decade. Interference by inorgan ic ions (Mg2+, Ca2+, H+, Na+, K+ and NH4+), drugs (morphine and codein e) and stimulants (methamphetamine and amphetamine) was almost negligi ble. However, lipophilic medicines, such as diphenhydramine, chlorprom azine and imipramine, interfered significantly.