The new wave of ion-selective

Abstract

During the last decade, the capabilities of potentiometric analysis have changed fundamentally in that the lower limit of detection (LOD) of ion-selective electrodes (ISEs) has improved by a factor of up to one million and the discrimination factor of interferences from ions by up to one billion. These spectacular improvements are related to the control of ion fluxes through the ion-selective membrane. Nowadays, ISEs can be used for trace measurements in environmental samples. However, by reducing the volume of the samples, the LOD in terms of the amount of analytes has been reduced to the attomole range. This is promising for bioanalysis using metal nanoparticle labels. Other recent progress includes the excellent fundamental understanding of the working mechanism, the introduction of a novel kind of calibration procedure that reduces the demands on signal stability and reproducibility, and the advent of pulsed amperometric methods.

Figure 1

Calibration curves for two types of a Ca²⁺-selective membrane electrode, each with two different inner solutions, as indicated. Left: ISE based on a monolithic capillary (200 μm i.d.) and a membrane solution without PVC [50]. Right: ISE with conventional plasticized PVC membrane (5 mm o.d., ca. 150 μm thickness) [50]. The EMF responses were recorded in CaCl₂ solutions at pH 6.2 (successive dilutions) with 10⁻⁴ M NaCl as background electrolyte. With the conventional PVC membrane, because of ion fluxes from the membrane into the sample (inner solution 10⁻¹ M CaCl₂) or in the case of 5 × 10⁻² M Na₂EDTA (pH 9.0) from the sample into the membrane, LODs are less than optimal. With the monolith ISE, the composition of the inner solution has virtually no effect on the response.

Figure 2

Left: Speciation analysis of drinking water spiked with 10 ppb of Pb²⁺. Data points for % of free Pb²⁺ as a function of the sample pH. Dashed line: Calculated free Pb²⁺ activity for a total carbonate concentration of 4.14 mM [51]. Right: Comparison of the values obtained by potentiometry at pH 4.0 with ICP-MS measurements [51].

Figure 3

Top: Detail of the 3-μL measuring cell. The Ag⁺-ISE (left, indicator electrode) and a Na⁺-ISE reference electrode (right) are inserted into a 1-mm i.d. silicone tube and put in contact with the aqueous sample plug (3 μL). Bottom: Potentiometric detection of 300 attomol of Ag⁺ (10⁻¹⁰ M in 3 μL) at a constant background of 10⁻⁶ M NaNO₃ [56].

Figure 4

Calibration curves for protamine in 0.1 M NaCl. A: with pulsed galvanostatic technique: B: with a conventional protamine ISE. Logarithmic protamine concentrations (mg/L) are indicated on the traces (first and last samples are 0.1 M NaCl) [81].