Magdalena Maj-Zurawska

Fully automated potentiometric determination of ionized magnesium in blood serum

Abstract Basic observations leading to the successful application of magnesium ion-selective electrodes for the automated determination of ionized magnesium in undiluted serum are discussed. The principles of the method are described and the first numerical results are reported.

Interpretation of the selectivity and detection limit of liquid ion-exchanger electrodes.

An equation has been derived which describes the e.m.f. of a liquid ion-exchanger membrane electrode in conditions of low concentration levels of the primary and interfering ions. The equation is based on the assumption that if the external solution contains no excess of ions which may exchange with the organic phase, then the concentration of the exchanger at the interface decreases, and this is responsible for formation of a diffusion layer inside the membrane. Therefore the potential response depends on the initial concentration of the ion-exchanger in the membrane phase, on the thicknesses of the diffusion layer on both sides of the interface, and on the diffusion coefficients of the species in both phases. This equation explains the non-Nernstian behaviour of the electrode in the presence of interferents, as well as the variation of the conditional selectivity coefficients. The parameters mentioned also influence the detection limit of an electrode. The electrode behaviour has been tested in unstirred solutions and in solutions stirred at different rates. Through its influence on the diffusion layer thickness, the stirring also influences the electrode potential and the characteristics of the electrode.

Behaviour of iodide-selective electrodes at low concentrations of iodide

Abstract The influence of adsorption of iodide ions and redox processes on the response mechanism of solid-state iodide-selectives electrode is discussed. Such effects can cause non-Nernstian calibration curves for iodide concentrations below 10 -5 M. A semi-empirical equation is given which explains the electrode response under the conditions normally applied.

Carbonate ion selective electrodes with trifluoroacetophenone derivatives in potentiometric clinical analyser

Properties of six derivatives of 1-trifluoroacetylbenzene: [4-(n-butyl)- (1), 4-(n-hexadecyl)- (2), 4-dodecyloxy- (3), 4-(n-dodecylsulfonyl)- (4), N,N-dioctyl-4-trifluoroacetylbenzamide (5), octyl-p-trifluoroacetylbenzoate (6)] as neutral carriers for carbonate ion were examined and compared. The sensitivity towards carbonate ion was for (3) pH dependent. This eliminates (3) from practical applications in clinical analysis. When measuring CO(3)(2-) within the physiological range of human blood using as carriers compounds 1 and 2 the interference of chloride must be taken into account. In the case of carriers 4, 5, 6 this effect is negligible. Electrodes with membranes containing as carriers 2, 4, 5 and 6 were tested in an automatic potentiometric clinical analyser Microlyte 6, KONE. To avoid contamination by atmospheric CO(2) of three aqueous standards (TES, NaCl, NaHCO(3)), pH was adjusted by coulomeric generation of H(+) or OH(-) in a system devoid of carbon dioxide. Recovery of HCO(3)(-) calculated from measured CO(3)(2-) and pH, was investigated in a series of aqueous solutions and spiked bovine serum samples. The correlation between added and recovered concentration of HCO(3)(-) was linear with the intercept close to 0 and slope equal to 1 in aqueous solutions for all ligands and in bovine serum samples only in the case of ligand (2).

Effect of the platinum surface on the potential of nitrate-selective electrodes without internal solution

Abstract The effects of oxidation or reduction of the platinum contact used in nitrate-selective electrodes without internal reference solutions were studied by using voltammetric curves. A correlation was found between the state of the platinum surface and the standard potential of the ion-selective electrode under potentiometric conditions.

Carbonate ion-selective electrode with reduced interference from salicylate

A carbonate ion-selective electrode for determination of total carbon dioxide species such as carbon dioxide, bicarbonate and carbonate with reduced interference from salicylate is described. Derivatives of trifluoroacetophenone were used as neutral carriers for carbonate. A polymer-free liquid carbonate-selective membrane with a cellophane outer membrane was found to give a carbonate-selective electrode with a negligible response to salicylate. The electrical contact was obtained by insertion of a silver/silver chloride electrode directly into the liquid membrane. The electrode does not require any aqueous filling solution and is therefore maintenance-free. The response to carbon dioxide species was found to be highly reproducible with a response time of 1-2 min at total carbon dioxide concentrations in the range from 5 to 50 mM. The lifetime of the electrode was at least 3 months. The electrode is regarded as very promising for clinical analysis of carbon dioxide species in body fluids such as plasma and serum.

Ion-selective electrode control based on coulometrically determined stability constants of biologically important calcium and magnesium complexes

Abstract Coulometric titration was used for the determination of stability constants of calcium and magnesium complexes with ligands that may be present in biological fluids. The experimental conditions were the same as in clinical analysers. On the basis of these constants and the content of ionized calcium or magnesium measured with ion-selective electrodes, the total concentrations of both metals may be calculated. The results obtained were in good agreement with expectation.

Determination of ionized magnesium in erythrocytes using a potentiometric analyzer

Abstract A simple method for the determination of ionized magnesium in erythrocytes using a potentiometric analyzer Microlyte 6 (Kone, Finland) was investigated. A set of aqueous solutions containing electrolytes at the same concentration as in erythrocytes in Tris/TES buffer (pH 7.2) was used as standards. The erythrocyte cell membranes were destroyed using three different methods: (A) HCl solution of pH 3.5, (B) dry ice–acetone mixture, (C) ultrasound. The result did not differ significantly between methods B and C (Student’s test, α=0.01). The results were compared with those obtained using the 31 P nuclear magnetic resonance method ( 31 P NMR). The dissociation constant of MgATP was calculated from the 31 P NMR results. The results obtained from potentiometry and 31 P NMR did not differ significantly (Student’s test, α=0.01). Total magnesium concentration was determined using atomic absorption spectrometry (AAS).

Selectivity coefficients of ion-selective magnesium electrodes used for simultaneous determination of magnesium and calcium ions.

Membrane ion-selective magnesium electrodes are commonly used to determine ionized magnesium concentration in blood serum and intracellular fluid by potentiometric clinical analyzers. The selectivity of these electrodes against calcium ion is typically insufficient to avoid calcium interference in blood serum analysis. For this reason the selectivity coefficient for calcium ion has to be studied to make possible any mathematical corrections for calcium ion influence. Existing methods relate to the thermodynamic concept of ISE response which suggest a single constant value of the selectivity coefficient and slope that are stable over the concentration ranges of calcium and magnesium ions in the samples. Unfortunately, this rarely happens, and we rather observe dependences on solution and membrane composition, readout time, matrices (anticoagulant, vial coats) that justify usage of apparent selectivities and slopes. To get the practical insight into the response of magnesium ion-selective electrodes a novel method for estimating the selectivity coefficients and the slope of the electrode characteristics is proposed. This method is an effective starting point for selecting electrodes and designing transient signal software in a potentiometric clinical analyzer. The method allows obtaining the ionized magnesium concentration in blood serum with minimal possible error by addressing the assessed targets, i.e. apparent selectivity and slope. The method is based on computer simulation and on the Nicolsky-Eisenman equation. Usually only a few iterations are needed to obtain stable congruent results. The method presented is particularly useful in conditions where is not possible to obtain calibration curve, which is typical for clinical analyzer where at most three point calibration is performed.

Ionized magnesium in plasma and erythrocytes for the assessment of low magnesium status in alcohol dependent patients

BACKGROUND Studies on the homeostasis of magnesium in alcohol-dependent patients have often been characterized by low hypomagnesemia detection rates. This may be due to the fact that the content of magnesium in blood serum constitutes only 1% of the average magnesium level within the human body. However, the concentration of ionized magnesium is more physiologically important and makes up 67% of the total magnesium within a human organism. There are no data concerning the determination of the ionized fraction of magnesium in patients addicted to alcohol and its influence on mental health status. METHODS This study included 100 alcohol-dependent patients and 50 healthy subjects. The free magnesium fraction was determined using the potentiometric method by means of using ion-selective electrodes. The total magnesium level was determined by using a biochemical Indiko Plus analyzer. In this study, different psychometric scales were applied. RESULTS Our results confirm the usefulness of ionized magnesium concentrations in erythrocytes and plasma as a diagnostic parameter of low magnesium status in alcohol-dependent patients. CONCLUSIONS The lower the concentration of ionized magnesium, the worse the quality of life an alcohol-dependent person might experience. In the case of total magnesium, no such correlation was determined.