In-line tubular ion-exchanger to enhance selectivity in enzyme-based flow-injection potentiometry; application to determination of -glutamine in bioreactor media

Abstract

A new approach for reducing positive errors caused by endogenous cationic interference when using ammonium ion-selective electrodes as detectors in immobilized enzyme-based flow-injection analysis systems is described. The method involves the use of an in-line tubular cation-exchange unit (e.g., Nafion) placed between the injection valve and the downstream immobilized enzyme reactor/electrode detector portion of the system. Interferent cation species within the sample slug are exchanged for other cations (replacement ions; e.g., Li+) contained within a reservoir solution surrounding the ion-exchange tubing. The membrane electrode exhibits much less response toward the replacement cations; consequently, the detected concentration of ammonium ions generated downstream within the enzyme reactor is directly proportional to the level of analyte substrate present in the sample. The influence of various experimental parameters on the efficiency of the in-line exchanger as well as the general advantages and limitations of this approach are examined. The analytical utility of the concept is demonstrated by the rapid and accurate determination of -glutamine in bioreactor media via the use of immobilized glutaminase enzyme.