Elimination of the acetate interference in potassium-selective valinomycin-PVC-membranes for process-controlling during haemodialysis

Summary Valinomycin-PVC-membranes of potassium-selective electrodes, which were used up to now, show for process controlling during haemodialysis in the presence of dialytic solutions including acetate, an anion interference which has to be corrected. On the basis of experimental examinations on the influence of plasticizers, lipophilic anions and different concentrations of ionophoric transport-antibiotics on the nature of membranes, an optimized application could be realized. But surprisingly, the incorporation of lipophilic anions like tetrakis(4-chlorophenyl)borate induces a significant acetate anion interference. Otherwise, when avoiding this lipophilic anion in PVC-membranes with a saturation of valinomycin on oxygen-independent solid-state contact sensors, an elimination of acetate anion interference is achieved if bis(2-ethylhexyl)sebacate is used as plasticizer.     

The influence of an anionic additive on the properties of cation-exchanger based calcium-selective electrodes

Potentiometric and electrochemical impedance spectroscopic investigations of calcium-selective membranes containing poly(vinylchloride), dioctylphenylphosphonate, calcium (bis[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate) and different amounts of the lipophilic anionic additive tridodecylmethylammonium chloride were carried out. The addition of the lipophilic additive changes the properties of calcium-selective electrodes, e.g. slope and calcium selectivity. The selectivity for calcium in presence of H⁺, Na⁺, K⁺, NH₄ ⁺, Mg²⁺, Ba²⁺, Sr²⁺ and (C₂H₅)₄N⁺ was measured by three different methods, namely separate solution method, fixed interference method and matched potential method. Membranes with different concentration ratios between the calcium-exchanger and tridodecylmethylammonium chloride were investigated within half a year. The tendency of changing from cationic into anionic response for membranes containing nearly equivalent concentrations of cation- and anion-exchanger was shown. This inversion of the electrode response depends not only upon the concentration ratio of both ion-exchangers but also upon the total concentration of calcium-exchanger. Electrochemical impedance spectroscopy was used for monitoring the development of membrane resistances during a soaking period of one month. Based on these results dielectric constants for the calcium-selective membranes depending on the membrane composition were determined. Furthermore, the dependence of the membrane resistance on the membrane thickness and the concentration of tridodecylmethylammonium chloride was evaluated.     

The influence of an anionic additive on the properties of cation-exchanger based calcium-selective electrodes

Potentiometric and electrochemical impedance spectroscopic investigations of calcium-selective membranes containing poly(vinylchloride), dioctylphenylphosphonate, calcium (bis[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate) and different amounts of the lipophilic anionic additive tridodecylmethylammonium chloride were carried out. The addition of the lipophilic additive changes the properties of calcium-selective electrodes, e.g. slope and calcium selectivity. The selectivity for calcium in presence of H⁺, Na⁺, K⁺, NH₄ ⁺, Mg²⁺, Ba²⁺, Sr²⁺ and (C₂H₅)₄N⁺ was measured by three different methods, namely separate solution method, fixed interference method and matched potential method. Membranes with different concentration ratios between the calcium-exchanger and tridodecylmethylammonium chloride were investigated within half a year. The tendency of changing from cationic into anionic response for membranes containing nearly equivalent concentrations of cation- and anion-exchanger was shown. This inversion of the electrode response depends not only upon the concentration ratio of both ion-exchangers but also upon the total concentration of calcium-exchanger. Electrochemical impedance spectroscopy was used for monitoring the development of membrane resistances during a soaking period of one month. Based on these results dielectric constants for the calcium-selective membranes depending on the membrane composition were determined. Furthermore, the dependence of the membrane resistance on the membrane thickness and the concentration of tridodecylmethylammonium chloride was evaluated. Received: 21 July 1998 / Revised: 16 October 1998 / Accepted: 23 October 1998     

New calcium-selective electrodes based on crown compounds bearing ester groups in the macrocyclic ring

Calcium-selective electrodes (Ca-SEs) based on the geometric isomers of di-(1,2,3,6-tetrahydrobenzo)-crown esters bearing four ester fragments in the macrocyclic ring were prepared, and their electrochemical properties were studied. It was demonstrated that these electrodes offer promise for medical and biological studies and clinical medicine. The electrodes exhibit high selectivity at crown compound concentrations, ensuring the formation of a 1 : 1 Ca-ionophore complex in the membrane.__________Translated from Zhurnal Analiticheskoi Khimii, Vol. 60, No. 4, 2005, pp. 417–419.Original Russian Text Copyright © 2005 by Shabanov, Khandar, M. Gasanova, U. Gasanova, Asadov.     

Some alkylphosphoric acid esters for use in coated-wire calcium-selective electrodes: Part I. Response characteristics

The use of several alkylphosphoric acid esters in the production of PVC membranes for use as calcium-sensitive, coated-wire electrodes is reported. The response characteristics, reproducibility, region of linear response for, and effect of plasticizer on, the electrodes are discussed.     

Glassy carbon electrodes coated with cellulose acetate for adsorptive stripping voltammetry

The utility of glassy carbon electrodes coated with cellulose acetate for adsorptive stripping voltammetry of oxidizable organic compounds is evaluated. This surface modification alleviates the co-adsorption problem commonly encountered at conventional electrodes. Interferences from electro-inactive surfactants and, in certain situations, from adsorbable electroactive substances, are minimized. Quantitation of the drugs, chlorpromazine and trimipramine, is not affected by the presence of up to 120 mg 1^?1 albumin or gelatin. The chlorpromazine response is not affected by the bilirubin or perphenazine peaks which overlap at uncoated electrodes. The adsorptive stripping response at the coated electrode is evaluated with respect to hydrolysis time, preconcentration time, concentration dependence, reproducibility, and other variables. The detection limit for chlorpromazine is 1.3 × 10^?8 M (5-min preconcentration). Applicability to assays of urine and serum samples is illustrated.     

Studies of quaternary ammonium ion interference on potassium ion-selective electrodes

Summary The influence of quaternary ammonium chlorides on potassium ion-selective electrodes is described. The observed decrease of sensitivity towards potassium ions, up to quasi-Nernstian towards chloride ions in the extreme case, is explained by ion-partition and ion-exchange processes at the membrane surface. A scheme of interference pathways is presented and a possibility of eliminating undesirable influences is given.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Bipolar pulse conductometric monitoring of ion-selective electrodes: Part 1. Method development with a calcium-selective electrode and elucidation of the basic principles involved

The potential generated by a plastic-membrane calcium ion-selective electrode (i.s.e.) is shown to be indirectly measurable by a non-zero current method based on bipolar pulse conductance. Linear current—voltage curves are obtained using 0–5-V pulses; the current axis intercept is related to the i.s.e. potential. A simple electrical contact (e.g., platinum or stainless steel) can be used instead of a poised reference electrode as the counter electrode in this two-electrode system. Long-term exposure of the i.s.e. to calcium solutions causes an upward drift in the measured current. This drift is minimized by avoiding long exposure times to solution, rinsing the electrode between measurements, and constructing current—voltage curves for determination of the current axis intercepts. Voltage pulses lasting 100 μs are optimum for this method. Shorter pulses are subject to error from capacitive charging currents, and longer pulses yield poorer precision, and degrade the electrode through faradaic reactions. The measured signal is dependent upon Ca²⁺ concentration (rather than activity), making ionic strength adjustment unnecessary. The concentration dependence is induced by application of voltage pulses greater than ~ 15 mV in amplitude. Selectivities of the potentiometric and conductometric methods are shown to be comparable for a variety of interfering monovalent and divalent cations. The conductometric method yields a fast i.s.e. response because of induced migration of Ca²⁺ into the membrane. Response time decreases as the pulse height increases. Pulses greater than 2 V in magnitude yield response times limited by the solution mixing time rather than by the electrode.     

Minimization of volatile nitrogen oxides interference in the determination of arsenic by hydride generation atomic absorption spectrometry

In this study emphasis was given to minimize the interference of volatile nitrogen oxides from digestion procedures with nitric acid on the determination of arsenic by hydride generation atomic absorption spectrometry (HG AAS). Sulfamic acid (SA) is proposed to minimize this interference by employing three procedures for the digestion of hair in closed systems: conventional and microwave (MW) heating in polytetrafluorethylene (PTFE) vessels and by MW heating in glass vials. Hair samples were digested with H₂SO₄+HNO₃ or HNO₃+H₂O₂ mixtures. Concentrated hydrochloric acid was added for the digestion for the procedure in glass vials. The accuracy of the procedures with PTFE vessels was verified by the spike recoveries of organic (p-aminobenzenearsonic acid and dimethyl arsinic acid, from 92 to 101%) and inorganic (sodium arsenate, from 98 to 102%) arsenic compounds. For the procedure in glass vials the recovery was from 86 to 97% for organic As and from 97 to 102% for inorganic As. The results obtained for a certified hair reference material using the three digestion procedures were well within the 95% confidence interval of the certificate when SA was added to the solutions. However, when SA was not added, recoveries were low and non-reproducible signals and high background levels were observed. Urea, benzoic acid and hydroxylamine hydrochloride were also studied (maximum As recovery of 90% using hydroxylamine hydrochloride) but the best results were obtained with use of SA.     

Minimization of water vapor interference in the analysis of non-methane volatile organic compounds by solid adsorbent sampling

Water vapor can be a significant interference in the analysis of air for non-methane volatile organic compounds (NMVOCs) using solid-adsorbent sampling techniques. The adsorbent materials used in sampling cartridges have different hydrophobic characteristics, and it is therefore necessary to characterize solid-adsorbent cartridges over a wide range of humidity. Controlled humidity experiments were performed to assess the extent of water vapor interference when samples are collected onto AirToxics solid-adsorbent cartridges. It was found that elevating the temperature of the cartridge to 10 degrees C above the temperature of the air sample greatly reduced water vapor adsorption and interferences and resulted in > or = 90% recovery of NMVOCs, biogenic VOCs and chlorofluorocarbons. Similar collection efficiencies were obtained at ambient temperature by reducing the relative humidity to > or = 60% in the sample by dilution with dry, scrubbed ambient air. A procedure also was developed and optimized for dry-purging cartridges prior to analysis. However, under optimized conditions, significant losses of C3-C5 compounds still occurred under highly humid conditions. It was determined that these losses were due to reduced retention during sampling rather than loss during the dry purge procedure. The dry purge method was shown to be adequate at high humidities for sampling NMVOCs with retention indices greater than 500.     

Potentiometric ion- and bio-selective electrodes based on asymmetric cellulose acetate membranes

The potentiometric response properties of ammonium-, carbonate-, and proton-selective electrodes prepared by incorporating appropriate neutral carriers within novel asymmetric cellulose acetate membranes are reported. The membranes are formed by first casting a thin layer of cellulose triacetate without carrier, hydrolyzing one side of this film with base, and then on the other side casting a second layer of cellulose triacetate containing the membrane active components. The resulting asymmetric ion-selective membranes function equivalently, in terms of selectivity and response slopes, to non-asymmetric cellulose triacetate membranes and conventional poly(vinyl chloride)-based membranes. The hydrolyzed surface of the asymmetric membranes can be activated in aqueous solution with carbonyldiimidazole for the direct immobilization of proteins on the surface of the membranes, without loss in potentiometric ion-response. As an example, the immobilization of urease on the surfaces of ammonium- and carbonate-selective membranes yields potentiometric bio-selective urea-probes with desirable dynamic response properties.     

Application of heated electrodes operating in a non-isothermal mode for interference elimination with amperometric biosensors

Heated electrodes were applied for the non-isothermal operation of amperometric glucose biosensors based on glucose oxidase immobilised on the electrode surface by entrapment within a polymer layer. The localised deposition of the polymer film under simultaneous entrapment of the enzyme was achieved by an electrochemically induced pH-modulation in the diffusion zone in front of the electrode, thus altering the solubility of the polymer chains. This non-manual sensor preparation protocol could be successfully used for the modification of a novel indirectly heated electrode. The non-isothermal operating mode allows working at the optimum temperature of the enzyme sensors without any thermal distortion of the bulk solution. Increased surface temperature of the sensor thus accelerates transport as well as kinetic processes, resulting in an enhanced amperometric signal.In the presence of interfering compounds such as ascorbic acid, the proposed technique allows use of the diverging thermal impact on the sensing process, for different electrochemically active compounds, for a deconvolution of the amperometric signal at different electrode temperatures. A calculation method for determination of glucose in the presence of one interfering compound is presented as a basis for a calculative interference elimination.     

ATR-SEIRAS study of the adsorption of acetate anions at chemically deposited silver thin film electrodes

The adsorption of acetate anions at silver thin film electrodes has been studied by in-situ infrared spectroscopy experiments with a Kretschmann internal reflection configuration. Stable silver thin films were chemically deposited on germanium substrates. Ex-situ STM images show mean grain sizes ranging from ca. 20 to 90 nm for deposition times between 2 and 20 min, respectively. The thickness of the silver film, measured by AFM, is typically around 10 nm for a deposition time of 10 min and increases up to 50 nm for a deposition time of 20 min. Roughness factors around 2.3 have been obtained for the silver films from the charge involved in lead underpotential deposition (UPD). A noticeable enhancement of the infrared absorption of adsorbed species (SEIRA effect) is observed when the silver films are used as electrodes under internal total reflection conditions. Maximum intensities of the adsorbate bands were observed for a deposition time of 10 min and an angle of incidence around 65 degrees . The potential-dependent infrared spectra of acetate and interfacial water are consistent with previously proposed models involving the existence of weakly hydrogen-bonded water molecules at potentials below the potential of zero charge and the reorientation of water molecules at potentials above the potential of zero charge. Results reported in this work suggest a weak interaction between acetate and water molecules adsorbed at the silver thin film electrodes.     

Minimization of interference effects from iodine-consuming samples in the determination of water with the karl fischer reagent in a flow-injection system

A principal feature of the flow-injection approach for determination of water with the Karl Fischer reagent is the small influence of side reactions. This is confirmed by the determination of water in an iodine-consuming sample, penicillin (fugacillin).     

Chloride interference with non-stoichiometric copper sulphide copper(II)-selective electrodes : Part 2. Exact determination of the chloride interference region

A general equation for calculating the limit of the ligand interference region with the non-stoichiometric copper sulphide single-crystal electrode is described. The critical ligand concentration can be computed for a preselected maximal error in copper(II) ion determinations. The effect of the composition of the electrode material is discussed. The experimental results for the chloride interference show good agreement with theoretical predictions.     

Molybdenum disulphide/cellulose acetate nanofiber composite on screen printed electrodes for detecting cardiac troponin by electrical impedance spectroscopy

Cellulose - Elevated levels of Troponin I, a cardiac biomarker, is indicative of an Acute Myocardial Infarction. However, current immunosensing methods to detect the presence of Troponin I such as...     

Membrane electrodes for the determination of glutathione

Four glutathione (GSH)-selective electrodes were developed with different techniques and in different polymeric matrices. Precipitation-based technique with bathophenanthroline-ferrous as cationic exchanger in polyvinyl chloride (PVC) matrix was used for sensor 1 fabrication. β-Cyclodextrin (β-CD)-based technique with either tetrakis(4-chlorophenyl)borate (TpClPB) or bathophenanthroline-ferrous as fixed anionic and cationic sites in PVC matrix was used for fabrication of sensors 2 and 3, respectively.β-CD-based technique with TpClPB as fixed anionic site in polyurethane (Tecoflex) matrix was used for sensor 4 fabrication. Linear responses of 1 × 10⁻⁵ to 1 × 10⁻⁴ M and 1 × 10⁻⁶ to 1 × 10⁻³ M with slopes of 37.5 and 32.0 mV/decade within pH 7-8 were obtained by using electrodes 1 and 3, respectively. On the other hand, linear responses of 1 × 10⁻⁵ to 1 × 10⁻² and 1 × 10⁻⁵ to 1 × 10⁻³ M with slopes of 47.9 and 54.3 mV/decade within pH 5-6 were obtained by using electrodes 2 and 4, respectively. The percentage recoveries for determination of GSH by the four proposed GSH-selective electrodes were 100 ± 1, 100.5 ± 0.7, 100 ± 1 and 99.0 ± 0.8% for sensors 1, 2, 3 and 4, respectively. Determination of GSH in capsules by the proposed electrodes revealed their applicability for determination of GSH in its pharmaceutical formulations. Also, they were used to determine GSH selectively in presence of its oxidized form (GSSG). Sensor 4 was successfully applied for determination of glutathione in plasma with average recovery of 100.4 ± 1.11%. The proposed method was compared with a reported one. No significant difference for both accuracy and precision was observed.     

Chloride interference with non-stoichiometric copper sulphide copper(II)-selective electrodes: Part 3. New equation for the electrode response

A general equation for the electrode potential response under metastable equilibrium conditions is derived for the non-stoichiometric copper sulphide electrodes immersed in solutions containing Cu(II) and Cu(I) and the interfering chloride ions. The theoretically predicted “anomalous” slopes (S_II = δ E/δ) log C_Cu(II) of the calibration curves, greater than 29.6 mV at 25° C, were confirmed experimentally in concentrated chloride solutions. The inapplicability of the Nicolsky equation for the electrodes at equilibrium is discussed on the basis of the theory presented. The new relationship (Eqn. 13) can be adapted for other ion-selective electrodes for which the exchange reactions with the solution species are fast enough and mass transport is not inhibited.