An Ion-Selective Electrode/Flow-Injection Analysis Experiment Determination of Potassium in Serum Mark E. Meyerhoff and Paul M. Kovach University of Michigan. Ann Arbor, MI 48109
The use of ion-selective membrane electrodes for the determination of discrete ions in physiolozical samples has grown rapidly in recent y e a n (1):1ndeed,most new clinical chemistry analyzers used in large hospitals now employ ionselective membrane electrode technologies to determine H+, Na+, CI-, Ca+2,and K+ in blood and urine samples. There are several distinct advantages for using ion-selective electrode (ISE) detection, including, simple low cost instrumentation reouirements..ranid . analvsis time. measurement of onlv free ionic activity, and, above-all, l i t t i or no pretreatment of the sample is reauired (i.e.. . .turhiditv and color of the samole are not problem). T o increase the number of samples which can be determined in a given time period, large clinical laboratories turn to automated analytical methods. One approach which has received considerable attention in the p a i t few years is Flow Injection Analysis (FIA). Pioneered by Ruzicka and Hansen (2,3),this technique involves the injection of a sample "plug" directly into a flowing reagent stream which carries the sample to a suitable analytical detector. Through the use of small diameter connecting tuhing, small volume flow-through detectors, and fast flow rates, the resulting analytical system offers rapid assay times, high precision, and tequires small volumes of sample (usually less than 200 pL). To introduce students to the modern analytical methods described above, we have devised the following low cost experiment intended for senior level instrumental analysis courses. The exueriment consists of oreoarine a home-made potassium seleciive tubular flow-thrbug'h electrode and then incorporatine that electrode into an FIA svstem. Students can obtai" the relevant theory of lSEs and FIA hy reading several excellent books and reviews on these suhejcts (4-8). Once constructed, students are asked to study and optimize the resulting ISE-FIA arrangement in terms of selectivity properties, sample through-put capabilities, precision, dispersion properties, etc. Students can then use their home-made system to determine the potassium content of commercially available pre-assayed control serum, and can compare analytical results to other methods. In our course, students also determine potassium in serum by flame photometry (traditional method) and thus, they can first hand compare and contrast the two techniques.
a
Experlrnental Reagents All reagents were reagent grade and all aqueous solutions were oreoared with distilled-deionizedwater. ' \ialinmycin (Styma ('hem~ml('
