IN-LINE TUBULAR ION-EXCHANGER TO ENHANCE SELECTIVITY IN ENZYME-BASED FLOW-INJECTION POTENTIOMETRY - APPLICATION TO DETERMINATION OF L-GLUTAMINE IN BIOREACTOR MEDIA

A new approach for reducing positive errors caused by endogenous catio nic interferences when using ammonium ion-selective electrodes as dete ctors in immobilized enzyme-based flow-injection analysis systems is d escribed. The method involves the use of an in-line tubular cation-exc hange unit (e.g., Nafion) placed between the injection valve and the d ownstream immobilized enzyme reactor/electrode detector portion of the system. Interferent cation species within the sample slug are exchang ed for other cations (replacement ions; e.g., Li+) contained within a reservoir solution surrounding the ion-exchange tubing. The membrane e lectrode exhibits much less response toward the replacement cations; c onsequently, the detected concentration of ammonium ions generated dow nstream within the enzyme reactor is directly proportional to the leve l 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 e xamined. The analytical utility of the concept is demonstrated by the rapid and accurate determination of L-glutamine in bioreactor media vi a the use of immobilized glutaminase enzyme.