DETECTION LIMIT OF ION-SELECTIVE BULK OPTODES AND CORRESPONDING ELECTRODES

The detection limit of a highly selective Pb2+-sensitive optode based on a solvent polymeric membrane phase is compared to the corresponding Pb2+-electrode. For this purpose, a practical definition of the detection limit for optodes is presented. Whereas the optode phase has an incorporated Pb2+-selective ionophore and H+-selective chromoionophore, the ion-selective electrode has no chromoionophore, but otherwise equal composition. It is shown that both optode and electrode are responding to the free ion activity of Pb2+ in the sample, which may be buffered by the use of suitable ligands such as NTA and EDTA. The detection limit of the optode (10(-13.1) M) has been found to be much lower than that of the electrode (10(-9.2) M), but with sample activities of less than 10(-11.5) M, response times of more than 60 h have been observed with EDTA as metal ion buffer. This is possibly due to a slow decomplexation step of the EDTA-Pb2+ complex.