Use of ionomer membranes to enhance the selectivity of electrode-based biosensors in flow-injection analysis.

The use of ionomer membranes to enhance the selectivity of potentiometric enzyme electrodes in flow-injection measurement arrangements is examined. The ionomer membranes employed are permeable to analyte substrates but relatively impermeable to detectable ions that would normally interfere with the measurement of the substrates if the enzyme electrodes were in direct contact with the sample. As a model system, the selectivity of enzyme electrodes prepared with nonactin-based ammonium-sensitive polymeric membranes is evaluated. In the preferred configuration, a thin hydrophilic anion-exchange membrane is incorporated within a flow-through dialysis unit upstream from the enzyme-electrode detector. As the sample passes through the dialysis unit, neutral or anionic analyte molecules (urea or glutamine) move through the membrane while the permeation of endogenous ammonium ions and other cations in the sample is retarded. A flowing recipient buffer on the other side of the membrane carries the analyte substrate to the enzyme-electrode detector. Enhancements in selectivity for analyte substrates over endogenous ammonium and potassium ions are greater than or equal to 9-fold when compared to enzyme-electrode flow-injection analysis (FIA) systems assembled without the ionomer membrane unit. The analytical utility of the proposed system is demonstrated by the accurate measurements of urea in blood serum and L-glutamine in hybridoma bioreactor media.