Automated pH-ISFET measurements under hydrostatic pressure for marine monitoring application

Association of microsensors and flow measurement techniques could provide many advantages for the in-situ monitoring of marine environment. First, adaptability of such sensors to autonomous measurement and under pressure conditions has to be considered. This paper presents a laboratory study devoted to seawater pH flow measurement using ISFETs. The measurement principle had to circumvent drift problems associated with these sensors. This was performed by measuring potential steps produced by a fast alternation of the sample and a standard buffer over the sensor. In addition, interference of seawater ionic matrix on ISFET response was circumvented by the use of saline buffers. The precision of the method is better than 0.01 pH and could be further increased. Its optimal accuracy will be limited to 0.01 pH, due to uncertainties on the pH of saline buffers. The ability of ISFETs to operate under pressure, up to 30 bars, over 43 h was demonstrated, without significant decrease in sensitivity. The method could also be suitable for long-term unmanned measurements. These first results incline to further examination of the potentialities of this technique for pH monitoring at depth.