A silver electrode based surface acoustic wave (SAW) mercury vapor sensor: a physio-chemical and analytical investigation

We developed for the first time a silver electrode-based surface acoustic wave (SAW) elemental mercury (Hg0) vapor sensor and carried out detailed physio-chemical and analytical studies to better understand Hg-Ag interaction as well as to assess the sensor's feasibility for real world applications. The mercury sorption and desorption rates on the Ag surface were calculated by differentiating the sensor's dynamic response which indicated that fast adsorption process occurred at the initial stage of the 30 minute Hg0 exposure before the relatively slower diffusion process started to take place. Furthermore, the sensor's dynamic response magnitudes and maximum sorption/desorption rates were found to follow the Langmuir extension isotherm at all tested operating temperatures indicating that relatively small concentrations of Hg0 vapor interacting with Ag surface can be detected. Moreover, the operating temperature was found to have little effect on the maximum sorption rate of the sensor while the maximum desorption rate was found to increase at elevated operating temperature. Following extensive testing and analysis, the developed Ag based SAW Hg0 vapor sensor was found to have a limit of detection (LoD) as low as 0.5 ppbv (at 35 °C), which is comparable with the LoD reported for an Au-electrode based SAW sensor (0.7 ppbv). Further investigation indicated that the low concentrations (< 400 ppbv) of Hg0 vapor can be detected selectively at various operating temperatures between 35 and 105 °C even when common industrial gases (i.e. ammonia, acetaldehyde, ethylmercaptan, dimethyl disulphide, methyl ethyl ketone and humidity) are present without showing spurious artifacts, significant response degradation or unmanageable memory effects.