Measuring amperometric oxygen analyzers

Either the CO2 formation is followed potemiometrically (243) or the O2 consumption is measured amperometrically at an oxygen electrode (245). In the first method, the enzyme is physically immobilized with a dialysis membrane. The response is linear in the range 5-300 pg/mL of salicylate. The second technique uses chemically immobilized enzyme (GA -F BSA) attached to a pig intestine mounted on the tip of the O 2 electrode. Samples containing from 10 pM to 2 mM salicylate were analyzed. An elegant microelectrode (244) has the enzyme and the cofactor immobilized in the electrode matrix (carbon paste) and the catechol formation is monitored at -F 300 mV versus Ag/AgCl. The electrode consists of a disposable strip, allowing measurements to be made on a drop of blood within 1 min. 

Electrochemical analyzers based on the amperometric measurement of oxygen are used to measure the rate of oxidase enzyme reactions. For example, the substrate glucose is determined by measuring the rate of oxygen consumption in the presence of glucose oxidase. Results are obtained in less than a minute. Similarly,... 

A reducing gas can also be analyzed indirectly by first oxidizing it, for instance, with an excess of oxygen produced by an oxygen pump (I = constant). The Oj excess is then measured by an amperometric oxygen sensor.A schematic representation is given in Figure 10.13. 

Enzymes themselves can be analyzed by measuring the amount of substrate transformed in a given time or the product that is produced in a given time. The substrate, houldJ2eJn ccess so that the reaction rate depends oiily on the enzyme concentration. The results are expressed as international units of enzyme. For example, the activity of a glucose oxidase preparation can be determined by measuring manometrically or amperometrically the number of micromoles of oxygen consumed per minute. On the other hand, the use of enzymes to develop specific procedures for the determination of substrates, particularly in clinical chemistry, has proved to be extremely useful. In this case, the enzyme concentration is in excess so the reaction rate is dependent on the substrate concentration. 

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