20-4 Art: 714382-00D Rev. Date: 02/20/06Activity VersusConcentrationIon-selective electrodes measure activity rather than concentration. Activity (a)is related to concentration (c) through the activity coefficient (γ): a = γc.While ion activities, which reflect free rather than total ion concentrations,are the physiologically relevant quantity, activity values are converted toconventional concentration units so that values obtained by direct ISEmeasurements can be compared to values obtained from methods that measuretotal ion concentrations. The latter includes the indirect methods, whichhave activity coefficients close to unity or one, and flame photometric, atomicabsorption and titration methods.AmperometricSensorsIn amperometric measurements, a potential is applied to the measuringelectrode while current generated by the resulting oxidation or reductionreactions in the test system is measured. The current generated is directlyproportional to the concentration of the analyte. An enzyme can be added to alayer on or near an amperometric sensor to produce electroactive species fromanalytes of interest that cannot themselves be oxidized or reduced.ConductometricSensorsIn a conductometric measurement, an alternating current is applied betweentwo electrodes in contact with the test solution and the resulting voltagedifference is measured. The conductivity of the solution is proportional tothe magnitude of the voltage difference. In aqueous solutions, conductivity isdependent upon the concentration of electrolytes; an increase in the electrolyteconcentration causes an increase in conductivity.Where S replaces the constant term which defines the slope of the sensor.The slope is the change in millivolts per tenfold change in the activity of theanalyte. For a positively-charged monovalent ion, the theoretical slope wouldbe 59.1 mV at 25°C.DETERMINATION OF TEST RESULTSDeterminationof AnalyteConcentrationPotentiometric and amperometric sensors are used for the determination ofanalyte concentration. For both sensors, the concentration of the analytecan be calculated using:1) the known value of the analyte concentration in the calibrantsolution,2) the measured voltage (potentiometric) or current (amperometric)signal generated by the analyte in the calibrant, and3) the measured signal generated by the analyte in the test solution.For potentiometric sensors, the analyte activity in the sample is calculated fromthe Nernst equation according to:Esample - Ecalibrant = S log (asample/acalibrant).Complex solutions such as blood deviate slightly from Nernstian behavior dueto interfering ions and matrix effects that result in junction potentials. Byincluding selectivity coefficients in the Nernst equation (Nikolsky equation),these effects can be minimized. By characterizing the reference electrode indifferent solutions, effects of matrix on the reference junction potential canalso be minimized.
