| Analysis of the soluble fraction of a soil sample is commonly performed using ion chromatography, atomic absorption, or inductively coupled plasma. However, an electrochemical approach to this application is examined here using ion selective electrode arrays. These devices offer minimal cost, low power requirements, fast response times, wide dynamic ranges, small instrument volume, and very selective detection of their target ion. It is established that the use of ion selective electrodes (ISEs) provides data no less accurate than any of the above techniques. The design and fabrication of three arrays of ion selective electrodes is presented. Each successive design represents a substantial improvement. While the primary function of the arrays is to measure the ionic content of a given soil sample, the eventual goal of this research is to complete an in-situ analysis of Martian soil. To accomplish this task, all electrodes must have sufficient durability to survive shock and vibration, as well as the temperature and pressure extremes faced during transit and on the Martian surface. Additionally, the electrodes must possess long lifetimes. This seemingly daunting task can be achieved through relatively facile modifications of classical ISE components.; All arrays contained a minimum of potentiometric sensors specific for sodium, ammonium, potassium, calcium, magnesium, and chloride. Other arrays incorporated additional electrodes such as nitrate, lithium, perchlorate, and barium (for sulfate analysis). The ISEs were all extensively characterized through various multiple calibrations, in which the selectively, linear range, and overall responses were observed. To simulate analyses of Martian soils, several simulant soils were devised, being composed of either entirely aqueous solutions or of geochemical minerals. Furthermore, soils either from desert locations or Hawaii, serving as possible Martian soil analogs were tested. The results obtained from the various arrays were compared to data received from ion chromatography and inductively coupled plasma. It was found that as the arrays were decreased in overall volume, the accuracy was not affected. Overall, the electrochemical approach to analysis of leached soil samples proved highly accurate. The activities determined for all ions were often less than 10% in error relative to the chromatography data. The final, and best, array was composed of nine different ISEs, each in triplicate. The overall volume of this array (including wiring) was such that analysis in a 10cm 3 test tube having a diameter of 1.25cm (0.5&inches;) was possible. |
