Cationic or anionic sites? Selectivity optimization of ion-selective electrodes based on charged ionophores

The influence of ionic sites on the selectivities of ionophore-based ion-selective electrodes (ISEs) is described on the basis of a phase boundary potential model. The discussion presented here is significantly more general than previous ones. It is formulated for primary and interfering ions of any charges and it is valid for ISEs based on electrically charged or neutral ionophores. Furthermore, it also applies to membranes that contain more than one type of complex of the primary or interfering ion. It has been believed thus far that only ionic sites of the same charge sign as the primary ion improve the selectivities of ISEs based on charged ionophores. However, it is shown here that the charge sign of the ionic sites that give the highest potentiometric selectivities depends on the charge number of the primary and interfering ions and on the stoichiometry of their complexes with the ionophore. The validity of our model was confirmed experimentally with three ISEs based on different charged ionophores. ISEs based on lasalocid or 1-(N,N-dicyclohexylcarbamoyl)-2- (N,N-dioctadecylcarbamoyl)ethylphosphonic acid monomethyl ester (ETH 5639) as the ionophore responded selectively to Sr2+ or Mg2+, respectively, and discriminated well against other alkaline earth cations when their membranes contained anionic sites. These two electrodes are the first examples of ISEs based on charged ionophores for which maximum selectivities are obtained with membranes containing ionic sites with a charge sign opposite to that of the primary ion. On the other hand, the experimental F- selectivities of membranes based on oxo(5,10,15,20-tetraphenylporphyrinato)molybdenum-(V) improved gradually when the concentration of anionic sites was increased from 0 to 75 mol%. The selectivity-modifying influence of ionic sites for these three types of ISEs can be explained by considering the different stabilities of the 1:2 ion-ionophore complexes of the primary and of the interfering ions.

The effect of lung compliance and experience on manual hyperinflation.

This study was designed to investigate the factors which affect the safe and effective performance of manual hyperinflation by physiotherapists. To determine this, experienced and student physiotherapists were requested to deliver manual hyperinflations at tidal volumes of 1000mLorpeak inspiratory pressures of 15cm H2O during a series of trials with variations in lung compliance. These hyperinflations were delivered into an artificial lung system. Subjects were blind to random changes in lung compliance and were only allowed the feel of the bag for feedback during testing. It was found that compliance had a significant effect on the tidal volumes and pressures generated, and bothgroups of subjects allowed peak inspiratory pressure to rise significantly when lung compliance was low. Experience, hand size and grip strength of individuals did not influence these results.

Characterization procedure for ion-selective electrode assays of magnesium activity in aqueous solutions of physiological composition.

A magnesium ion-selective liquid membrane electrode based on a synthetic neutral carrier is presented. The selectivity for Mg2+ over Na+, K+, and H+ is sufficient for assays in the physiological range, but a chemometric correction of approximately 10% is still necessary for the Ca2+ interference. We optimized the membrane composition especially with respect to selectivity and accuracy by performing a new characterization procedure in which we simulated the variety of blood serum samples with 10 aqueous solutions. After a three-point calibration, these solutions were measured alternately with the middlerange calibrator for 3 min. The magnesium activity was evaluated after correcting for the calcium interference according to the Nikolsky equation. The selectivity coefficient, KpotMgCa, was itself fitted to give the minimum deviations from the assigned magnesium activities. Electrodes with an optimized membrane composition show an average deviation from theoretical activity values of about 1.6% with logKpotMgCa = -0.8.

Characterization of sodium-selective optode membranes based on neutral ionophores and assay of sodium in plasma.

Plasticized poly(vinyl chloride) membranes that incorporate a highly lipophilic sodium-selective neutral ionophore (ETH 4120) and novel H(+)-selective chromo-ionophores, lipophilic isologs of Nile Blue, are used in the competitive binding of Na+ and H+ for a reversible, optical determination of sodium activities in buffered solutions at different selected pH values. These optode membranes are used to assay total sodium concentrations in pH-buffered human blood plasma.

Lifetime of neutral-carrier-based liquid membranes in aqueous samples and blood and the lipophilicity of membrane components.

On the basis of previously reported correlations between the lipophilicity of membrane components, their partition coefficient between the membrane and the sample, and the lifetime of corresponding neutral-carrier-based sensors, the lipophilicities of ionophores and plasticizers in analytically relevant ion-selective electrodes, ISFETs, and optodes are analyzed and reported. Equations for the estimation of the lifetimes of liquid membranes in continuous-flow systems are presented, and the experimental determination of the lipophilicity values by thin-layer chromatography (TLC) is described. The required lipophilicities for the lifetimes of liquid membranes over 30 24-h days for different applications in aqueous solutions as well as in blood are presented. A comparison of the experimental results of lifetime measurements with calculated theoretical values is given. The experimental results of the determination of the lipophilicity by TLC are compared with the lipophilicities estimated on the basis of Hansch parameters.

Neutral ionophore-based selective electrode for assaying the activity of magnesium in undiluted blood serum.

The concentration of free, ionized magnesium in undiluted blood serum can be determined potentiometrically with a new magnesium-selective carrier in polymeric membrane electrodes (by weight 66% plasticizer, approximately 33% polyvinyl chloride and approximately 1% ionophore). However, discrimination of calcium by the new ionophore-added membrane is not sufficient to keep calcium interference to less than 1%. To correct for the interference by calcium, we determine the calcium activity of the serum sample with a calcium-selective electrode and calibrate the magnesium-selective electrode with standard solutions containing calcium in the concentrations found. In this way we determined the free ionized magnesium in five plasma samples from five different persons and found it to be within the range expected for magnesium in undiluted serum.

Intracellular magnesium ion selective microelectrode based on a neutral carrier.

A magnesium ion selective microelectrode based on a synthetic neutral carrier is presented. The selectivity of Mg2+ over Na+, K+, H+, and Ca2+ is sufficient for assays of intracellular magnesium ion activities. The microelectrodes with an optimized membrane composition have a resistance of about 5 x 10(10) omega and a 90% response time of less than or equal to 3 s for a tip diameter around 1 microns. The lifetime of the microelectrode cell assembly is longer than 1 week and the emf drift after equilibration is less than or equal to 0.3 mV/h.