Rotating electrode potentiometry: Lowering the detection limits of nonequilibrium polyion-sensitive membrane electrodes

A rotating electrode configuration is evaluated as a means to lower the det ection limits of newly devised polyion-sensitive membrane electrodes (PSEs) . Planar potentiometric polycation and polyanion PSEs are prepared by incor porating tridodecylmethylammonium chloride and calcium dinonylnaphthalenesu lfonate, respectively, into plasticized PVC or polyurethane membranes and m ounting disks of such films on an electrode body housed in a conventional r otating disk electrode apparatus. Rotation of the PSEs at 5000 rpm results in an enhancement in the detection limits toward heparin (polyanion) and pr otamine (polycation) of at least 1 order of magnitude (to 0.01 unit/mL for heparin; 0.02 mug/mL for protamine) over that observed when the EMF respons es of the same electrodes are assessed using a stir-bar to achieve convecti ve mass transport. A linear relationship between omega (-1/2), where omega is the rotating angular frequency, and C-1/2, the polyion concentration cor responding to half the total maximum Delta EMF response toward the polyion species, is observed. It is further shown that the rotating polycation sens or can be used as an end-point detector to greatly enhance (relative to non rotated indicator electrode) the analytical resolution and precision for me asurement of low concentrations of heparin when such samples are titrated w ith protamine. The theoretical basis for lowering the detection limits by r otating PSEs is discussed based on the unique nonequilibrium response mecha nism of such sensors.