Potentiometric sensors for trace-level analysis
Section snippets
Introduction – What are potentiometric sensors good for?
In recent years, the well-established field of potentiometric sensors has undergone a quiet revolution that did not go unnoticed in the general analytical chemistry community. While it has been traditional wisdom that such sensors may reach only mediocre limits of detection (LODs) around the micromolar range, they have now been improved to make possible true trace-level analysis at sub-nanomolar (low parts per trillion) concentrations. Clearly, this improvement asks for new applications for
What does LOD mean?
In general, the lower LOD is defined as the concentration of the analyte at which the signal is increased relative to the background level by three times the standard deviation of the noise [7]. According to IUPAC recommendations [8], the definition of the lower LOD in potentiometry is unique. This is somewhat unfortunate because a direct comparison with corresponding figures of other methods is not appropriate. This is especially confusing because of the recent improvement of potentiometric
Potentiometric sensors at trace levels: the state of the art
Traditionally, ISEs are distinguished by the underlying membrane material. Polymeric membrane-based sensors are a group of very high chemical versatility and tunability because the selectivity is given by the extraction of ions into a polymer and complexation with a selective receptor that may be chemically designed [3]. Glass electrodes, including chalcogenide glasses, are an attractive material for a variety of ions, including H+, but the fine tuning of their electrochemical response is
Predictability of interference effects with potentiometric sensors
An important characteristic of potentiometry is that the response function may be predicted on the basis of fundamental relationships and measurable parameters. In the case of polymer membrane electrodes, the response function is related to thermodynamic constants and the composition of the membrane. The contribution of the individual ions to the EMF can be calculated with potentiometric selectivity coefficient , which is determined from measurements on simple solutions (pure solutions or
Applications of ISEs with low LODs
The focused development of polymeric membrane electrodes for trace analysis started less than 10 years ago [39], [40] although some early examples of trace-level measurements with a Cu2+-selective electrode are known [56], [57]. More recently, different solid-state electrodes have been applied for trace-level measurements in seawater. The Cu2+-electrode based on an optimized jalpaite membrane in the rotating disk configuration was used to analyze San Diego Bay seawater samples [18]. The LODs
Future directions
The last few years have witnessed significant activity in understanding the principles that may dictate the low LODs of potentiometric sensors and in finding protocols and examples of successful improvements. Because of this, perhaps, a novice in the field may seem somewhat overwhelmed by the various choices. It will therefore be crucial to see a unified, simplified approach to producing potentiometric sensors with low LODs, rapid response time, sufficient chemical ruggedness and long lifetime,
Acknowledgments
The authors thank the US National Institutes of Health (Grant R01-EB02189 and R01-GM071623) for supporting their research on potentiometric sensors for trace-level analysis (joint grant) and instrumentally controlled ion sensors (E.B.). E.P. also acknowledges financial support from the Swiss National Science Foundation and an internal research grant from ETH Zurich.
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