Sample Preparation with Bio-Sensors

Here again, it is difficult to give detailed procedures, because each of the large number of possible reactions has its own set of optimal parameters. One problem which often arises is the proper pH value. The optimal pH value for the biologically active material does not always coincide with that of the ion-selective electrode. This may be because H ions are directly sensed by a cation-sensitive glass electrode, or because the species to be indicated is associated (for example, NHj ion formation when NH3 is to be detected, or HCN formation when measuring CN ). In the case of urease, a compromise can be reached between pH 7 [183] and 8.5 [184]. When this is not possible, the reaction and indication must be broken down into two separate steps. This was done in the first attempts to measure urea in whole blood [190]. In such cases it is best to use a flow cell apparatus in which the biological reaction is separated from the indicator electrode, and the optimal indicator electrode pH is established immediately before 

Concerning the analyzable concentration range, care must be taken not to work with too high a substrate concentration. The nearly Nemstian response is obtained only in concentration regions in which the Michaelis-Menten kinetics are followed. With substrate saturation (reaching the maximum rate), the EMF no longer follows the substrate concentration. 

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