Flow injection analysis using pH/pF ISFET combinations for determination of very low fluoride concentrations

Abstract A flow injection analysis system has been developed using a fluoride ISFET as detector and a pH ISFET as reference. We found that the application of acid solutions decreases the response time of the fluoride sensor. Optimum conditions are in a range around pH 2, which can be met using 1 mol dm −3 acetic acid. The sample rate can be increased by a factor of 8 in this solution compared to standard TISAB and very low concentrations down to 1 ppb can be determined. The reason for the influence of pH on the response rate has been investigated using impedance spectroscopy and XPS showing that chemical modification of the LaF 3 surface reduces the energy barrier for fluoride ion transfer.