Optimizing the Conditions for pH Measurement with an Automated pH Measurement System Using a Flow-Through-Type Differential Sensor Probe Consisting of pH-FETs

In previous papers we have reported on the construction of a flow-through-type differential pH sensor probe for a fully automated pH measurement system. Our flow-through-type sensor probe is quite different from the dipping-type sensor probes now in common use. To realize highly accurate and high-speed measurements with our probe, it is essential to optimize measurement conditions such as the volume of the sample solution suctioned into the probe, the washing time and related parameters, and the measurement time required for stabilization of the electrode potential. In this paper we describe a method to derive suitable measurement conditions for a pH measurement system equipped with a flow-through-type differential sensor probe consisting of pH-sensitive Field-Effect Transistors (pH-FETs). After establishing the optimum measurement conditions, we confirmed the performance of our system for the pH measurement of 26 μl solutions over a 106 sec cycle. The measurement error with diluted low-buffering-capacity solutions was on the order of 0.01 pH.