Electrochemical Sample Matrix Elimination for Trace-Level Potentiometric Detection with Polymeric Membrane Ion-Selective Electrodes

Abstract

Potentiometric sensors are today sufficiently well understood and optimized to reach ultra-trace level (sub-nanomolar) detection limits for numerous ions. In many cases of practical relevance, however, a high electrolyte background hampers the attainable detection limits. A particularly difficult sample matrix for potentiometric detection is seawater, where the high saline concentration forms a major interfering background and reduces the activity of most trace metals by complexation. This paper describes for the first time a hyphenated system for the online electrochemically modulated preconcentration and matrix elimination (EMPM) of trace metals, combined with a downstream potentiometric detection with solid contact polymeric membrane ion-selective microelectrodes. Following the preconcentration at the bismuth coated electrodes, the deposited metals are oxidized and released to a medium favorable to potentiometric detection, in this case calcium nitrate. Matrix interferences arising from the saline sample medium are thus circumvented. This concept is successfully evaluated with cadmium as a model trace element and offers potentiometric detection down to low parts per billion levels in samples containing 0.5 M NaCl background electrolyte.Recent improvements in the detection limits of ISEs based on polymeric membranes containing selective receptors (ionophores) yielded potentiometric sensors for the direct measurement of ions down to the subnanomolar concentration range. 1 This was made possible by minimizing diffusional ion fluxes from the membrane into the sample solution. 2, 3 Today potentiometry routinely achieves detection limits in the nanomolar or lower concentration range without any accumulation step and with essentially no sample perturbation. 1 Unfortunately, ISEs are normally incapable of reaching the same detection limits in undiluted seawater samples, primarily owing to the very high levels of electrolyte (salt) background and the complexation of the target metal ion. Additionally, seawater contains natural organic matter and surfactants that may interfere with the ISE response via adsorption and extraction processes.To overcome related seawater matrix effects, detection techniques such as ICP-MS 4 and electrospray MS, 5 among others, make use of preconcentration methods that separate the target analyte from the complex matrix. The methods used for matrix separation include liquid-liquid extraction, 6, 7 sorption on solid sorbents or exchangers, 8, 9 coprecipitation 10 and electrodeposition. 5 It is the last one which is considered most suitable for preconcentration, because the procedures are relatively fast compared with chemical preconcentration and the technique can be automated to provide continuous accumulation. In addition, when used in an on-line format, these preconcentration methods minimize manual sample handling and offer NIH Public Access Experimental ReagentsThe ionophores N, N,N′,, N,NDicyclohexyl-N′,N′-dioctadecyl-diglycolic diamide (ETH 5234), lipophilic cation exchanger s...

Nanomaterial/Ionophore-Based Electrode for Anodic Stripping Voltammetric Determination of Lead: An Electrochemical Sensing Platform toward Heavy Metals

“…However, ion-selective electrodes are normally incapable of reaching the same detection limits for samples with high levels of electrolyte (salt) background. 54 For the proposed electrode, the detection limit of 1 nM can be achieved with 0.2 M acetate buffer solution.…”

Electrochemical Sensing System for Determination of Heavy Metals in Seawater

“…The UV digestion unit was comprised of a UV lamp (200 W), a quartz spiral coil (2.5 m × 0.8 mm i.d., 5.2 cm in diameter) used as digestion reactor, a temperature control device with a small power fan and a reflection foil. The electrochemical accumulation cell had a three-electrode system with a bismuth-coated glass carbon (for Cd 2+ and Pb 2+ ) or a gold (for Cu 2+ ) working electrode, an Ag/AgCl reference electrode (3 M KCl), and a platinum disk as counter electrode [10] . The ISE indicator and Ag/AgCl disk electrodes were imbedded into the detection cell body face to face.…”

“…ISEs can be used for monitoring trace levels of analytes in the presence of high concentrations of background electrolytes [10,11] . In this paper, a flow sensing system was described for determination of heavy metal elements in seawater using solid contact polymeric membrane ISEs as detectors, which combined with online sample pretreatment …”

Paper-based microfluidic sampling and separation of analytes for potentiometric ion sensing

“…This approach, however, requires sophisticated and relatively expensive instrumentation. Furthermore, pre-concentration and matrix elimination procedures based on electrochemical accumulation, magnetic sep-aration or paper-based filtration are alternative choices to enhance the performance of ISEs for potentiometric measurements [8][9][10][11]. In all these methods, medium exchange is an essential step for subsequent potentiometric detection, which has the drawbacks of tedious measurement protocols.…”