Sweat testing for cystic fibrosis: errors associated with the in-situ sweat test using chloride ion selective electrodes.

The in-situ sweat test is prone to errors from various sources. This paper examines errors due to evaporation, absorption of water into the skin, and pressure of the electrode on the skin. Only evaporation caused serious errors. The accuracy and precision when measuring small chloride/sweat samples on the skin are not significantly worse than when measuring bulk solutions.

Sweat testing for cystic fibrosis: characteristics of a combination chloride ion-selective electrode.

Filling solution leakage, reproducibility, stability, interferences and temperature effects have been studied in relation to the use of the Orion 96-17-01 combination chloride ion-selective electrode in sweat testing for cystic fibrosis. While stability and reproducibility is good, certain antiseptic systems and especially Hibitaine (chlorhexidine gluconate) interfere with the electrode and therefore need to be absent during sweat testing. Temperature conditions during calibration and sweat testing need to be controlled in view of the extent of response variations for even modest temperature differences between sensor-skin interface and electrode stem.

Development and publication of work with selective ion-sensitive electrodes.

Features concerning development, response, selectivity and applications of selective ion-sensitive electrodes are reviewed. Recommendations are made concerning investigations involving the development and applications of electrodes and attention given to the manner of presenting data for publication.

A chlorate ion-selective electrode based on a poly(vinyl chloride)-matrix membrane.

An electrode has been prepared, consisting of a PVC membrane containing Corning 477316 nitrate liquid ion-exchanger in the chlorate form, which responds to chlorate ions. It has a faster response (1-2 sec) and lower limit of detection (3 x 10(-5)M) than the nitrate electrode for chlorate determination. Selectivity coefficients for the electrode towards several other ions have been measured.

Radiotracer studies on ion-selective electrode membranes containing metal complexes of a nonylphenoxypoly(ethyleneoxy)ethanol in poly(vinyl chloride) matrices.

Radiotracer studies with (133)Ba, (45)Ca and (36)Cl are reported for PVC matrix membranes containing 2-nitrophenyl phenyl ether and the tetraphenylborate of a barium (or calcium) complex with a nonylphenoxypoly(ethyleneoxy)ethanol (NP), Antarox CO880. The results show that there is very limited permeation of radioactive barium and calcium ions through the membrane systems. However, the continued uptake (with time) of radioactive barium ions by the membranes (the uptake of calcium ion is less) suggests that in relation to selective electrode response the stabilization of ions by the NP ligand within the membrane is important in addition to the simple availability of membrane pathways for primary-ion transport through the membrane. Permselectivity to counter-ions is indicated by the non-permeation of radioactive chloride ions.

Applied potential and radiotracer studies on poly(vinyl chloride) matrix ion-selective electrode membranes.

This paper reports the effect of applied potentials on PVC matrix membranes containing (i) the barium ion-sensitive barium-Antarox C0880 complex and 2-nitrophenyl phenyl ether solvent mediator, and (ii) the calcium ion-sensitive Orion 92-20-02 phosphate-based calcium liquid ion-exchanger. Platinum electrodes were placed in solutions on each side of the membranes. The barium ion-sensitive membranes are unable to maintain stable current flows but the calcium ion-sensitive membranes are characterized by stable current flows over prolonged periods even after successive polarity reversals. Results are presented, from radiotracer experiments for permeation of ions through the membranes with and without an applied potential. No evidence was found for significant permeation of barium-133 ions through the barium ion-sensing membranes into an initially inactive solution, but it was found that barium-133 ions were incorporated into the membranes after removal of the applied potential. Permeation of sodium-22 ions through the calcium ion-sensing membranes occurred only to a limited extent in the presence of an applied potential and not at all in its absence, confirming electrode selectivity trends for calcium and sodium. Calcium-45 ions did not permeate the calcium ion-sensing membranes into an inactive counter-solution against the potential gradient, but on reversal of the polarity, permeation occurred to a far greater extent than in the absence of an applied potential. These differences in behaviour are compatible with the more complicated membrane pathways of the barium ion-sensing membranes, imposed by the complexing of barium ions by the ethyleneoxy units of Antarox C0880 in a tight helical conformation. The calcium ion-sensing membranes are much less constrained, thus permitting more facile replacement of the calcium ions in the membrane by ions from solution.

Radiotracer studies on ion-selective membranes based on poly(vinyl chloride) matrices.

Radiotracer studies with (45)Ca, (89)Sr and (133)Ba have provided evidence that the permeation of magnesium, strontium and barium ions through PVC membranes containing Orion 92-20-02 liquid ion-exchanger is inhibited by their low affinity for the liquid ion-exchanger sites. Experiments with (7)Be indicate a strong affinity of the membrane for beryllium ions with corresponding inhibition of permeation. When acid is present in the solution on one side of the membrane, preferential permeation by protons may lead to transport of ions against their concentration gradient in order to maintain the balance of charge.

An evaluation of solvent mediators for ion-selective electrode membranes based on calcium bis(dialkylphosphate) sensors trapped in poly(vinyl chloride) matrices.

This study on several alkan-1-ols, di-n-octylphenyl phosphorate and di-n-nonyl phthalate as solvent materials in ion-selective electrode membranes based on calcium bis(dialkylphosphate) sensors trapped in a PVC matrix indicates that the best electrodes are made with di-n-octylphenyl phosphonate. These are selective for calcium. The liquid alcohols readily exude from the PVC matrix, and solid alcohols are completely unsuitable for membrane fabrication. However, despite a tendency to be exuded, decan-1-ol gives acceptable electrodes responsive to bivalent cations if calcium bis (di-2-ethylhexylphosphate) is used as sensor. Several selectivity coefficients, k(caj), greater than unity were found, but though such values for j = Na or K do not involve complete loss of selectivity towards calcium, a value of K(CaCu) = 13 for a membrane with di-n-nonyl phthalate as mediator and calcium bis(di-n-decylphosphate) as sensor indicates greater selectivity for copper than for calcium.

Radiotracer studies on calcium ion-selective electrode membranes based on poly(vinyl chloride) matrices.

Radiotracer studies with (45)Ca and (36)Cl demonstrate that PVC matrix membranes containing Orion 92-20-02 liquid calcium ion-exchanger are permselective to counter-cations. Diffusion coefficients are quoted for the migration of (45)Ca between pairs of calcium solutions and are discussed in terms of solution concentration, membrane thickness and concentration level of sensor in the membrane. Migration of calcium ions from calcium chloride solution to a Group (II) metal chloride solution through a PVC membrane containing calcium liquid ion-exchanger is discussed in terms of solvent extraction and electrode selectivity coefficient parameters. Thus, magnesium, strontium and barium ions appear to inhibit migration through the membrane by their low affinity for the membrane liquid ion-exchanger sites, while the inhibition by beryllium ions is attributed to site blockage by the strong affinity of dialkylphosphate sites for beryllium.

Interference studies on a PVC-matrix membrane calcium ion-selective electrode based on calcium bis-di[4-(1,1,3,3-tetramethylbutyl)phenyl] phosphate as sensor and tri-n-pentyl phosphate as solvent mediator.

Interference potentials, E(M), defined as the difference between the observed potential and the potential calculated assuming that there is no interference, have been determined, and their fit to the equation E(M) = -(RT/F) 1n [1 + K(M)a(B)/(a(Ca))1 2 (z(B)) discussed for a PVC-matrix membrane calcium ion-selective electrode based on calcium bis-di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate as sensor with tri-n-pentyl phosphate as solvent mediator in mixed solutions of CaCl(2)BCl(x), where B = Li, Na, K, Mg, Zn or Cu, at total ionic strengths of 0.2, 0.4 and 0.6. Generally, K(M) for the alkali metal ions is considerably less for this electrode than for a comparison electrode based on Orion 92-20-02 liquid ion-exchanger in PVC. Occasionally, the data do not fit the equation. Thus, particularly for the bivalent ions, although the plots of exp[-E(M)F/RT] vs. a(B)/(a(Ca))1 2 (z(B)) are usually linear, they frequently do not intercept the ordinate at 1.00 and/or may give negative slopes and hence negative K(M) values.

Effect of chloride ions on the behaviour of the orion copper(II) ion-selective electrode.

The normal shiny surface of a copper(II) ion-selective electrode tarnishes when exposed to chloride ions. Polishing with fine emery cloth easily removes this dull surface layer and fully restores the proper potential response characteristics of the electrode. No such loss of character is evident with a non-chloride based reference electrode except in the presence of added chloride ions. The electrode also seems less affected in premixed solutions of copper(II) and chloride. the chloride ions then being largely bound as copper(II) chloro-complexes.

PVC ion-selective electrodes based on calcium bis-[dialkyl- and di-(4-alkylphenyl) phosphates] and mixed solvent mediators.

PVC calcium ion-selective electrodes based on either calcium bis-di(n-decyl)phosphate or calcium bis-di[4-(1,1,3,3-tetramethylbutyl)phenyl]phosphate with various solvent mediators (alone or in pairs) have been evaluated with particular respect to interference from Na(+), K(+), Mg(2+), Sr(2+), Ba(2+), Mn(2+), Cu(2+), Ni(2+) and Zn(2+) ions. PVC calcium ion-selective electrodes based on calcium bis-di[4-(n-octyl)phenyl]phosphate with varied amounts of decan-l-ol plus di-n-octyl phenylphosphonate showed a continuous gradation in selectivity coefficients on going from a high fraction of decan-l-ol to a high fraction of the second mediator. Thus, k(pot)(Ca,Mg) changed from 1.6 for an electrode based exclusively on decan-l-ol to 4.9 x 10(-4) for one based completely on di-n-octyl phenylphosphonate. The corresponding k(pot)(Ca,Na) values were 7.0 x 10(-2) and 1.1 x 10(-3).

Mechanistic studies of ion-selective electrodes.

A knowledge of the chemical composition of blood, urine and other body fluids is a daily requirement for departments of biochemistry. Electronic circuitry and computers to process the data are readily available, but satisfactory transducers to convert chemical composition into electrical signals are frequently the weakest link in the chain of measurement. This review is concerned with one group of transducers: ion-selective electrodes. Since the commercial success of the calcium and fluoride ions-selective electrodes in the mid-sixties, a range of other electrodes has become available. Their use has already conferred considerable benefits upon medicine, e.g. rapid, low-cost, multiple assay of major blood components; diagnostic surveys of chloride sweat levels relating to cystic fibrosis; and monitoring blood fluorides during and after halothane administration. Their adoption for indirectly sensing enzymes and associated substrates is particularly noteworthy. Recent advances in electronics, coupled with flow injection schemes based on ion-selective electrodes, have facilitated the management of hundreds of samples daily. However, developments in the mechanistic knowledge of these sensors have not matched the increase in their application, although definite progress can be reported; for example with regard to the origin of the potential signals induced by ion activities in solution. Numerous techniques have been devised to unravel mechanistic problems, among which radioisotope tracer and impedance measurements may be cited as especially valuable. Selectivity performance, particularly in complex biological media, and undesirable features such as protein poisoning, need further research. Organic chemists are now better placed to synthesize new designs of acyclic and cyclic molecules as mobile site, ion-selective, sensor materials which, with appropriate mediator solvents, provide improved sensor cocktails. This design feature is well illustrated by the continuing guest for a lithium ion-selective electrode compatible with the high levels of sodium interference in blood.

Utilization of supplemental iron by premature infants fed fortified human milk.

We measured red blood cell iron incorporation (RBC-inc) in 13 human milk-fed premature infants (birthweight 1037 +/- 289 g, gestational age 27 +/- 2 wk, weight at start of study 1571 +/- 426 g) who were receiving full tube-feedings of human milk fortified with a commercial human milk fortifier (FortHM). The relative RBC-inc of supplemental iron (2 mg/kg/d of ferrous sulfate) was assessed using 57Fe sulfate mixed directly into a 24-h volume of FortHM, and 54Fe sulfate given as a bolus between two FortHM feedings the next day. RBC-inc was similar between the two methods of supplemental iron administration (4.7 +/- 2.5% vs 4.6 +/- 1.5%, respectively). Although these values are lower than RBC-inc expected from iron native to human milk, the relatively large amount of iron in the supplements contributed most of the iron incorporated into RBC by the infants. There was a significant positive correlation between the reticulocyte count and RBC-inc. As the high nutrient (especially calcium) content of the FortHM did not interfere with iron utilization, adding iron directly to FortHM, or incorporating it into commercial fortifiers, may be a practical method to provide iron to premature infants.

Studies on bis(crown ether)-based ion-selective electrodes for the potentiometric determination of sodium and potassium in serum.

Bis(crown ether)-based ion-selective electrodes for sodium and potassium are described, based on the bis[(12-crown-4)-2-ylmethyl]-2-dodecyl-2-methyl malonate sensor(I) for sodium and the bis[(benzo-15-crown-5)-15-ylmethyl] pimelate sensor(II) for potassium. The best results were obtained when the sensors were used in association with 2-nitrophenyl octyl ether as plasticising solvent mediator and potassium tetrakis(4-chlorophenyl)borate as anion excluder in poly(vinyl chloride) matrices. Electrode slopes were near-Nernstian, with detection limits of less than 10(-5) M. The electrode features are compared with those of a sodium glass membrane electrode, for sensor I, and with a valinomycin-based potassium electrode, for sensor II. The electrodes are also discussed in relation to others reported for sensors I and II and are shown to be superior. However, although the electrodes described offer promising alternatives to glass electrodes for sodium and valinomycin electrodes for potassium, data for sodium and potassium measurements in blood serum indicate a need for further research in order to improve the correlation with flame photometric measurements.

Multi-enzyme electrodes for the determination of starch by flow injection.

A tri-enzyme electrode has been developed for determining starch in a flowing stream based on amperometric monitoring of hydrogen peroxide at a potential of +600 mV versus a silver--silver chloride reference electrode. The nylon-based starch-sensing membranes (over a platinum electrode) were prepared from an enzyme cocktail containing various ratios of amyloglucosidase (AMG), mutarotase (MUT) and glucose oxidase (GO). The best starch-sensing membrane (to give the type A electrode) was made from an enzyme cocktail of AMG--MUT--GO (2000 + 100 + 100 U; where 1 U = 16.67 nanokatals), i.e., containing a high level of AMG. In this system, starch samples were first incubated with soluble alpha-amylase (alpha-AMY) (1000 U) for 1 h at room temperature prior to analysis with the tri-enzyme electrode. Attempts were also made to immobilize alpha-AMY on to nylon net, either alone or as a component of a four-membrane starch-enzyme electrode but starch signals were weak compared with those generated by starch pre-treated with soluble alpha-AMY. This system, associated with the type A starch electrode, not only exhibited a calibration of wide linear range (1 x 10(-4)-0.1% m/v starch) but also showed promising operational properties. It has excellent thermal stability over the range 30-70 degrees C.

Sweat testing for cystic fibrosis. Diagnostic screening with a combination chloride ion-selective electrode.

Screening of sweat chloride in newborn infants with the Orion Skin Chloride Measuring System and incorporating some procedural innovations is described. The results indicate that while diagnostic screening for cystic fibrosis can be readily undertaken, the test with the chloride ion-selective electrode is best left at least to the second day of life or later because of insufficient sweating in very young babies. Measurements on 2 babies with cystic fibrosis are also discussed.

Feeding tolerance in premature infants fed fortified human milk.

BACKGROUND: To evaluate feeding tolerance in premature infants immediately after the addition of human milk fortifier (HMF) to their expressed human milk diet. METHODS: Data on milk intake, feeding tolerance, and related assessments and growth milestones from a prospective study of feeding strategies in premature infants were analyzed. The database was searched for the first day HMF was added to the feeding of infants receiving human milk exclusively. The following assessments were tabulated for the 5 days before and the 5 days after the addition of HMF: milk intake, the number of episodes of abdominal distension, gastric residual volume (GRV) more than 2 ml/kg and more than 50% of the volume fed in the prior 3 hours. bile-stained gastric residual, emesis or regurgitation, blood in the stool, the number of abdominal radiographs, the number of episodes of apnea and bradycardia, changes in findings in the clinical examination, and the number of hours feeding was withheld. The time to achieve full tube feeding, complete oral feeding, and hospital discharge were recorded. RESULTS: Seventy-six exclusively human milk-fed premature infants (birth weight, 1065+/-18 g; gestational age, 27+/-0.1 weeks; mean +/- SEM) who received HMF beginning 22+/-0.8 days of age were evaluated. There were significant increases in milk intake and in the number of episodes of GRV more than 2 ml/kg and emesis after the addition of HMF. There were no differences in the number of hours feeding was withheld or any other assessment after the addition of HMF. Infants with increases in GRV more than 2 ml/kg and/or emesis after the addition of HMF were not more likely to be delayed in the time to achieve full tube feeding, complete oral feeding, or hospital discharge than infants who did not experience these events. CONCLUSION: These data suggest that, when all feeding and related assessments and the time to achieve important growth milestones are considered, the addition of HMF does not adversely affect the outcome of the premature infant.