where E is the observed potential; E° is a constant potential which varies with the choice of reference electrodes; R is the gas constant; T is the temperature; n is the integral value of the charge on the measured species, A; F is the Faraday constant; and A is the thermodynamic activity of the measured species.

The properties and performance of the electrodes depend on the composition and choice of membrane material. The most well-known ISE is the glass electrode used for pH measurements (Fig. 1). A typical glass electrode consists of a glass tube sealed in one end with a very thin glass membrane and filled with a reference electrolyte. Electrical contact with the inner solution is normally made with a metal wire coated with Ag/AgCl. The glass electrode is immersed in the analyte together with a reference electrode. Hydrogen ions build up a pH dependent voltage across the glass membrane and this voltage is measured by connecting the pH electrode and the reference electrode to a very high input resistance voltmeter.

An example of a food application is the measurement of the sodium content of foods (12). This study described a glass electrode with special composition and having selectivity and sensitivity for sodium ions. The operation was performed in a range where hydrogen ion activity was negligible compared to sodium ion activity. Sodium glassmembrane electrodes contained 11% Na20,18% A1203 and 71% Si02 and could detect sodium ion concentrations as low as 0.23 ppm. Varying percentages of these three oxides caused a desired selectivity for other monovalent ions such as potassium. The electrode was successfully used to find sodium in meat samples. Use of this electrode required less sample preparation than other analytical methods but was still an off-line method for solids since they needed sample pH Reference electrode electrode

Ag/AgCl wire -Inert glass-Sensing glass-KCI/buffer ■ solution m

Sample solution

KCI solution Ag/AgCl wire

Controlled leak preparation. Other reported ion-selective electrodes include measuring sodium and potassium in practically all types of foods (13,14) and measuring nitrite content of animal feed (15).

Practical use of ion-selective electrodes in foods have not been very successful due to an incomplete understanding of the underlying principles and due to the poor performance of some of the older instruments. They are normally not suitable for aggressive environments and sample preparation is often required with these electrodes. However, they are still faster and less expensive than more traditional wet chemistry techniques and have potential for future on-line use in food and agricultural processing, particularly in composition sensing of fluids.

Chemically Sensitive Field Effect Transistor (CHEMFET)

Chemical parameters can be measured with semiconductor devices. The ion-selective electrodes can be combined with a transistor to develop an ion-selective field effect transistor (ISFET). The relationship between ISEs and ISFETs is shown in Figure 2 (16), in which the conventional ISE and a reference electrode 'R' are first shown connected to the insulated gate field effect transistor 2(IGFET) input of a voltmeter (Fig. 2a). An IGFET is commonly called MOSFET (Metal-oxide-semiconductor field effect transistor). It has very high input impedance and is used in ultra-high-input impedance amplifiers as needed in the potentiometric electrode measurement to minimize the flow of current. The integration process is achieved by attaching the ion-selective membrane 'M' directly to the gate of the input transistor (Fig. 2b,c). The resulting device is called an ion-selective field effect transistor (ISFET). The ISFET (or CHEMFET) chip is completely insulated except for the gate whose surface complexation with the specific electrolyte in solution determines the ionic selectivity of the chip.

Food application of CHEMFETs include on-line measurement of pH. Due to the chemical selectivity of the ion-selective membrane, these sensors can potentially be made to respond to other food components (liquid and gaseous) and therefore measure their concentration on-line. Compared to ion-selective electrodes, CHEMFETs offer several advantages including flexibility due to small size, a fast response, and the requirement of less complex laborintensive calibration and preconditioning procedures (18). At present, they are used as research tools in physiology iOJ

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