Zero-current ion transport through polymeric membranes of ion-selective electrodes (ISEs) alters the composition of the inner solution of these sensors. This causes long-term drifts of the electromotive force that are relevant for miniaturized ISEs as well as for most solid-contacted ISEs in which a thin aqueous layer is formed between the membrane and the metal electrode during conditioning. By a similar mechanism, the composition of the sample can be significantly altered during the measurement if the sample volume is small relative to the volumes of the membrane and the inner solution. This paper provides a formal model that describes the response behavior to be expected for such cases.

Lipophilic polymer membranes incorporating binding sites are widely used in various potentiometric, amperometric, and optical sensors. Here, we report on the biofunctional modification of the surface of a Ca(2+)-selective membrane. A photoactivatable biotin derivative was synthesized and covalently immobilized on a soft polyurethane membrane. The modified polymer was characterized by X-ray photoelectron spectroscopy (XPS) as well as by potentiometric measurements. The selective binding of streptavidin by the photo-cross-linked biotin derivative was demonstrated. The surface coverage obtained with different experimental protocols was analyzed by autoradiography using [(35)S]-streptavidin. The new approach may significantly extend the scope of applicability of potentiometric sensors.

A new solid phase is presented to be used for the solid-phase extraction (SPE) of organic compounds from aqueous solutions and as a stationary phase for the separation of organic compounds in "reversed-phase" HPLC. The material consists of spherical clay conglomerates (SCCs) in the size ranges of 2-5, 5-10, and 10-20 μm. SCCs are especially well suited for the extraction and separation of aromatic compounds with electron-withdrawing substituents, because of the formation of specific electron donor-acceptor (EDA) complexes of such compounds with natural clay minerals. A series of nitroaromatic compounds (NACs), e.g., nitrophenols, and nitrotoluenes, served as probe substances for the characterization of the SPE with SCCs online coupled to a C18-HPLC-DAD system. Breakthrough volumes were > 1 L and method detection limits (MDLs) < 100 ng/L for compounds with moderate to high affinity towards clay minerals. The performance of the material is hardly affected by matrix effects and because of its excellent physical properties, i.e., regenerability and pressure-resistance, it meets the requirements for fully automated routine trace analysis of several primary pollutants, such as 6-methyl-2,4-dinitrophenol (DNOC) or 2,4,6-trinitrotoluene (TNT), in various natural waters. Offline SPE with SCCs was superior or equivalent to commercial SPE products for analysis of such compounds. Finally, SCCs are shown to be well suited as a stationary phase in reversed-phase HPLC. This opens a wide range of applications, e.g., as an easy and fast separation technique that is orthogonal to C18 reversed-phase HPLC.

No abstract