No abstract

The aim of the investigations was to show the analytical use of an atomic force microscopy (AFM) tip coated with an ion-selective membrane and to show that the ion-selective boundary potential is detectable as a force induced by ion-selective electrostatic interactions, which are more pronounced than double-layer forces. This new technique allows the area-specific ion exchange over boundaries to be displayed with a destruction-free technique by AFM in an aqueous buffer. From experiments with ISEs (ion-selective electrodes), a boundary potential for valinomycin was assumed to be established in close vicinity to a K+-releasing surface. To trace this boundary potential, an AFM tip was coated with a potassium-selective polymer film containing valinomycin as used in preparing ISEs. The K+-releasing substrate was prepared by incorporating a lipophilic potassium salt into a plasticized PVC layer. In contact with an electrolyte such as sodium chloride solution, an ion exchange takes place. Analogue experiments were performed using a sodium-selective ionophore, DD16C5, incorporated into the coating of the AFM tip, with a Na+-releasing substrate. The boundary potential was traced and investigated with the help of force vs distance curves. The resulting adhesion forces for a valinomycin-coated tip in a 150 mM NaCl solution were 9.8+/-3.275 nN using a blank PVC substrate and 330.15+/-113.0 nN using a substrate containing 10 wt % potassium tetrakis(4-chlorophenyl) borate. The selectivity of the ion-selective AFM tips was measured on sodium relative to potassium-releasing substrates and studied in different salt solutions with concentrations between 10 mmol L(-1) and 1 mol L(-1). For valinomycin, a force selectivity coefficient log Kf(K,Na) of -2.5+/-0.5 for K+ against Na+ and a selectivity coefficient log Kf(Na,K) of -4 +/- -0.5 for DD16C5 were measured. In addition, the surface of the polymer substrate was imaged by AFM in contact mode with and without lipophilic potassium salt. The modulation of the force-distance curves induced by the ion exchange was compared to the experimental change in elasticity of the blank and ion-exchanging substrate. The Young's moduli measured with strain force analysis on a potassium-containing substrate were 5 times smaller than the ones registered with nanoindentation and did not explain the modulation of the force vs distance curves.

Summary The relationships of the serum concentration profiles of the ionized fractions of calcium and magnesium (Ca2+, Mg2+) to those of the respective total concentrations (Catot, Mgtot) were examined during a 5‐h infusion of 5 % Na2EDTA in six Brown Swiss dairy cows. The mean concentration of Catot decreased in two phases whereas that of Ca2+ decreased in one phase. At the end of the infusion, calcium was presumably either bound to EDTA or free. Mean Mgtot and Mg2+ serum concentrations did not change significantly during the course of the infusions. Mean Pi serum concentrations decreased until 1 h after the end of the infusion and increased regularly to reach pre‐infusion concentrations the following day. The longitudinal changes of the serum concentrations of Catot and Pi are discussed. It is concluded that the concentrations of Ca2+ are more adequate than those of Catot for the evaluation of the calcium mobilization rate. Zusammenfassung Serumkonzentrationen des ionisierten und gesamten Calciums und Magnesiums und des anorganischen Phosphors während einer Na2EDTA‐Infusion bei der Milchkuh Bei 6 Braunvieh Kühen wurde der Einfluss einer fünfstündigen Infusion einer 5 %igen Na2EDTA‐Lösung auf die Serumkonzentrationen des ionisierten Calciums und Magnesiums (Ca2+, Mg2+) und des gesamten Calciums und Magnesiums (Catot, Mgtot) untersucht. Die Mittelwerte der Catot‐Konzentrationen nahmen während der ersten 4 Stunden der Infusion nur langsam ab. Danach folgte ein rascher Abfall bis zum Ende der Infusion. Die Mittelwerte der Ca2+‐Konzentrationen sanken während der EDTA‐Infusion kontinuierlich ab. Am Ende der Infusion war das Calcium vermutlich entweder an EDTA gebunden oder frei. Auf die Mgtot‐ und Mg2+‐Serumkonzentrationen hatte die Na2EDTA‐Infusion keinen wesentlichen Einfluss. Die Pi‐Serumwerte nahmen während der Infusion ebenfalls ab. Der zeitliche Verlauf der Änderungen der Catot‐ und Pi‐Serumkonzentrationen wird diskutiert, und es wird gezeigt, daß die Calciumsmobilisierungsrate an Hand der Werte der Ca2+‐Konzentrationen besser beurteilt werden kann als an Hand des Gesamtcalciums. Résumé Concentrations sériques ionisées et totales du calium et magnésium et du phosphate inorganique pendant une perfusion de Na2EDTA chez la vache laitière La relation entre les concentrations sériques ionisées du calcium et du magnesium (Ca2+, M2+) avec celles de leur concentrations totales respectives (Ca2+, Mgtot) ont été examinées au cours d'une perfusion d'une durée de 5 heures d'une solution de 5 % de Na2EDTA chez 6 vaches laitières de la race brune suisse. La concentration moyenne de Catot a diminué en 2 phases alors que celle de Ca2+ a diminué en 1 phase. A la fin de la perfusion, le calcium était soit lié à l'EDTA ou libre. Les concentrations moyennes de Mgtot et de M2+ sont restées constantes au cours de la perfusion alors que les concentrations moyennes de Pi ont également diminués. Les changements longitudinaux des concentrations de Catot et de Pi sont discutés. En conclusion, les concentrations de Ca2+ sont plus adéquate...

Two types of magnesium-selective microelectrodes based on a new synthetic neutral carrier ETH T 5504 are presented. One type of microelectrode is applicable to the determination of intracellular free Mg 2þ molality without interference from alkaline or alkaline-earth metal ions versus a typical intracellular ion background of 10 mmol/L NaCl, 100 mmol/L KCl, and 1 mM CaCl 2 . The response function showed a slope of 28.6 Ϯ 0.4 mV (1 SD per decade), and a detection limit at 10 ¹5.1 Ϯ 0.1 mol/L. A magnesium-selective electrode with different composition of the membrane cocktail has been employed for assays of both intracellular and extracellular Mg 2þ -activities, with improved discrimination of Ca 2þ and other physiological cations compared to electrodes based on ETH 1117, ETH 5214 and ETH 7025, respectively. This is the first report where Mg 2þ -selective microelectrodes can be applied to extracellular Mg 2þ -measurements. The microelectrodes based on ETH T 5504 perfectly discriminate the physiological extracellular ion background. Electrodes with tip diameters of typically 1 mm have an average electrical resistance of 3.0 × 10 10 Q, and a response time, t 90 % , of 3 s. The lifetime of the microelectrodes is longer than 1 week and the emf-drift after 1day conditioning period is Յ0.3 mV/ h. An example of measurements of the Mg 2þ -molality in low volumes of specimen is described in this article for blood serum of rats.