Arthur K. Covington

A miniature flow-through cell with a four-function chemfet integrated circuit for simultaneous measurements of potassium, hydrogen, calcium and sodium ions

Abstract A miniature, flow-through cell with a four-functin ChemFET integrated-circuit is described for the simultaneous measurement of K + , H + , Ca 2+ and Na + in aqueous solutions. Depletion-mode field-effect devices with ion-implanted n − channels are used, in conjunction with ionophore-doped, polymeric electroactive gates. A polyimide/photopolymer-based technique is employed for selective encapsulation of the devices, and an efficient, V-type flow-through cell cap, suitable for application in blood analysis, is described. The ChemFET cell has a dead-space of 30 μl and a 100% response time of 20 s ata sample flow rate of 24.3 μl s −1 , hence the effective dead space is approximately 486 μl. the useful lifetime of the cell is typically 4–12 weeks. Electronic circuitry is described for multiplexing the individual channels rapidly and accurately, and for linearizing the resultant analogue output signal.

Solvent properties of PVC membranes

Abstract The solvent properties of PVC/dioctyl sebacate membranes have been explored by (i) measuring the dielectric constant of membranes as a function of composition and (ii) measuring the conductivity of membranes containing 1 mM NaBPh4 as a function of composition. Our measurements indicate that in pure liquid plasticizer contact ion pairs are readily formed, whereas the introduction of PVC causes the ion pairs (greatly reduced in number) to be solvent separated.

IFCC guideline for sampling, measuring and reporting ionized magnesium in plasma.

Abstract Analyzers with ion-selective electrodes (ISEs) for ionized magnesium (iMg) should yield comparable and unbiased results for iMg. This IFCC guideline on sampling, measuring and reporting iMg in plasma provides a prerequisite to achieve this goal [in this document, “plasma” refers to circulating plasma and the forms in which it is sampled, namely the plasma phase of anticoagulated whole blood (or “blood”), plasma separated from blood cells, or serum]. The guideline recommends measuring and reporting ionized magnesium as a substance concentration relative to the substance concentration of magnesium in primary aqueous calibrants with magnesium, sodium, and calcium chloride of physiological ionic strength. The recommended name is “the concentration of ionized magnesium in plasma”. Based on this guideline, results will be approximately 3% higher than the true substance concentration and 4% lower than the true molality in plasma. Calcium ions interfere with all current magnesium ion-selective electrodes (Mg-ISEs), and thus it is necessary to determine both ions simultaneously in each sample and correct the result for Ca2+ interference. Binding of Mg in plasma is pH-dependent. Therefore, pH should be measured simultaneously with iMg to allow adjustment of the result to pH 7.4. The concentration of iMg in plasma may be physiologically and clinically more relevant than the concentration of total magnesium. Furthermore, blood-gas analyzers or instruments for point-of-care testing are able to measure plasma iMg using whole blood (with intact blood cells) as the sample, minimizing turn-around time compared to serum and plasma, which require removal of blood cells. Clin Chem Lab Med 2008;46:21–6.

Approaches to the problems of solvation in pure solvents and preferential solvation in mixed solvents

Of paramount importance to the understanding of the behaviour of solutes in solvent media is the problem of solute-solvent interaction (solvation), or, in solvent mixtures, of solvent-sorting in the solvation sphere of the solute (preferential solvation). Different techniques and theoretical approaches have been developed but the relation of these to classical thermodynamics has rarely been critically discussed. Theories of solute-solvent interaction can be broadly divided into continuum models and models whioh acknowledge the molecular nature of the solvent. In the latter, the arrangement of the solvent is often treated in terms of discrete structure only in. the solvation sphere, or, more generally, through distribution functions which can be obtained in principle from X-ray and neutron scattering experiments. Unfortunately theoretical approaches based on the nature and extent of intermolecular forces have not yet reached the state of development, in spite of the power of modern high speed computers, that they can be of. real us in interpreting the properties of systems of experimental and technological importance. By highlighting the interrelations between the.different approaches, the possible origin of some recently reported discrepancies in the interpretation of systems studied by two or more experimental methods may become apparent.

Recommendations for measurement of and conventions for reporting sodium and potassium by ion-selective electrodes in undiluted serum, plasma or whole blood

Abstract Ion-selective electrodes (ISEs) respond to ion-activity and therefore do not sense substance concentration directly. However, it is recognized that sodium and potassium in plasma will continue to be expressed for clinical purposes in terms of substance concentration (mmol/l). A convention is proposed whereby for routine clinical purposes results of ISE measurements of sodium and potassium in undiluted plasma should be reported in terms of substance concentration (mmol/l). In specimens with normal concentrations of plasma water, total CO2, lipids, protein and pH, the values will concur with the total substance concentration as determined for example by flame atomic emission spectrometry (FAES) or ISE measurements on diluted samples. In specimens with abnormal concentrations of plasma water, the results will differ. However, under these circumstances, measurements of sodium and potassium by ISE in the undiluted sample will more appropriately reflect the activity of sodium and potassium and are therefore clinically more relevant than the determination in diluted samples. Detailed recommendations are made about practical procedures to achieve this. The recommended name for this quantity is the substance concentration of ionized sodium or ionized potassium in plasma, as opposed to total sodium or total potassium determined by, e.g. FAES, or ISE measurements on diluted samples.

Mechanistic studies of the valinomycin-based potassium-selective electrode using AC impedance methods

Abstract Impedance measurements have been carried out on valinomycin-based, PVC matrix potassium-selective electrodes. The results show effects arising at high frequency from the bulk of the membrane, whereas the lower frequency part of the impedance spectrum reflects the charge transfer processes occurring at the membrane—solution interface. Exchange currents have been calculated for the exchange of both potassium and sodium ions across the interface and these do not reflect the selectivity ratio found for the membranes in emf measurements.

Use of ion-selective electrodes for blood-electrolyte analysis. Recommendations for nomenclature, definitions and conventions

Abstract This paper will familiarize the reader with the terms used to describe the behavior of ion-selective electrodes, particularly in relation to their use in clinical chemistry for determination of blood electrolyte cations. It serves as an introduction to a series of papers dealing with important cations in blood, namely calcium, sodium, and potassium. The detailed relationships between the ion activity determined by means of ion-selective electrode potentiometry in undiluted specimens, and the total substance concentration measured by flame atomic-emission spectrometry are described by flow chart and equations. Adoption of a convention for reporting results is recommended. The Working Group on Selective Electrodes has taken into account recent revisions of IUPAC recommendations on nomenclature and selectivity coefficient determinations for ion-selective electrodes, and benefited from the experience of a member of the WG, who was also involved in the IUPAC discussions. Nomenclature for determined quantities follows previous IUPAC/IFCC joint recommendations.

IFCC recommended reference method for the determination of the substance concentration of ionized calcium in undiluted serum, plasma or whole blood.

Abstract A reference method is described for the determination of the substance concentration of ionized calcium in plasma by which ionized calcium (free or unbound) may be reliably determined on the basis of calibration with aqueous solutions with known concentration of ionized calcium. The composition of the calibration solutions is chosen such that the activity coefficient of the calcium ion is assumed to be identical both in the calibration solutions and in “normal” plasma, i.e. by convention, the ionic strength (Im ) is 0.160 mol/kg. The convention is adopted of reporting ionized calcium measurements as concentration expressed as mmol/l. The proposed reference method for ionized calcium measurement in plasma is based on the use of a cell consisting of an external reference electrode with a saturated potassium chloride liquid/liquid junction in combination with a calcium ion-selective membrane electrode of defined construction and performance. Procedures for using the reference cell and a protocol for sample measurement are described. The preparation of the calibration solutions to be used are described in detail in Appendix A, secondary calibration solutions and check standards in Appendix B, and reference cell vessel design in Appendix C.

Comparative performance of 14-crown-4 derivatives as lithium-selective electrodes

A series of neutral ionophore-based lithium-selective liquid-membrane electrodes have been prepared and the electrode performance compared with similar electrodes based on the lithium ionophores ETH 1810-ortho-nitrophenyl octyl ether (oNPOE) and ETH 2137-bis(1-butylpentyl) adipate (BBPA). By using a diamide substituted 14-crown-4 macrocycle, selectivities for Li+ in the presence of Na+ of log kpotLi,Na = -3.25 and -2.92 were obtained for diisobutylamide-oNPOE and di-n-butylamide-oNPOE derivatives. The di-n-butylamide-oNPOE based electrode functioned satisfactorily in serum, exhibiting a fast response time (10-15 s), an acceptable lifetime of 50 d and minimal protein interference.

A pitzer mixed electrolyte solution theory approach to assignment of pH to standard buffer solutions

The IUPAC recommendations for the pH scale for aqueous solutions are based on the Bates-Guggenheim (B-G) convention (1961) for the single ion activity coefficient of the chloride ion in the standard buffer(s). This convention was adopted as a reasonable estimate based on the Debye-Huckel theory and is limited in its application to ionic strengths less than 0.1 mol-kg−1. This approach ignores the results of many workers over the years on the properties of mixed electrolyte solutions and their prediction on the basis of the theories of Harned, Scatchard, Guggenheim and more recently of Pitzer. The literature data of EMF measurements on appropriate weak acid systems have been reexamined to determine both the pKa values and values of appropriate Pitzer interaction coefficients. The latter are used to calculate single chloride ion activity coefficients for the chosen compositions of pH standard buffers, and compared with the B-G convention values. Calculations were made to check the consistency of the pH values with determined pKa values using the Pitzer treatment for all the required single ion activity coefficients. The overall aim was to remove the ionic strength restriction of the B-G convention and rationalize the approach to pH standardization for such diverse aqueous media as sea water, blood and acid-rain water.