Validation of the membrane composition effect on the flow-injection signal profile of chalcogenide-based ion-selective sensors: a model study using electrochemical approach: Hg(II) flow-injection detector case

The membrane composition effect on signal profile of the combined ISE-flow-injection system is examined on the example of Hg(II) flow-injection potentiometric (FIP) detector based on the secondary response to Hg(II) of different thin layer silver chalcogenide membranes, obtained by cathodic electrodeposition at controlled potential. The potential of the electrochemical approach to produce a great diversity of membrane compositions and the possibility for fine tuning of their stoichiometry made it possible to select the stoichiometry with respect to silver (intrinsic defects factor) and the inclusion of dopant (extrinsic defects factor) as the two variable composition parameters. The following membranes have been tested: Ag₂Se_1−xTe_x, Ag₂Se, Ag_2+δSe and Ag_2+δSe_1−xTe_x (δ=0.24). The experimental conditions have been varied in a wide range to include four flow-rates (within the 2.5-6 ml min⁻¹ interval), and three typical carrier compositions to which either Ag(I) or Hg(II) have been added as pilot ions. The results obtained in this study show unambiguously that the membrane composition factors are an important figure of merit, the weight of which in some particular cases can be co-measurable with that of the flow manifold factors. A new important information concerning the active role of the “pilot ion” added to the carrier in controlling the rising part of the signal, through changing the membrane response rate order, is also provided.