Lars Risinger

A cesium-selective electrode prepared from a crystalline synthetic zeolite of the mordenite type

Abstract Cesium-selective electrodes were prepared from a synthetic zeolite molecular sieve of the mordenite type. The membrane was made from zeolite crystals embedded in an epoxy resin. The response towards cesium ions -was linear from about 3 × 10-5 to at least 0.1 mol l-1, with almost Nernstian slope. The usable pH range was 3.5–9 for 0.01 mol l-1 cesium solutions. The selectivity order was Cs > Ag, K > Na > Li for univalent ions. The response for divalent ions was not Nernstian, Cs > Ba > Ca > Cu. About two weeks after the hydration, there was a degradation of electrode performance indicated by increased detection limit and decreased slope.

A post-column enzyme reactor for detection of oxidized cholesterols in h.p.l.c. separations

Abstract An enzyme reactor is used as a post-column detector after the h.p.l.e. separation of a mixture of cholesterol and some auto-oxidation products. The enzyme reactor contains cholesterol oxidase immobilized on controlled-pore glass. A method for purification of the enzyme is described. The properties and the solvent dependence of the reactor are discussed. A post-column system for dilution of the eluent with aqueous buffer is described.

Optimization of flow-through cells for dialysis and other membrane separations in flow-injection analysis using a laminar flow model

Abstract The mass transfer in the liquid channels of flow-through dialysers can be described fairly accurately by a laminar flow model. The mass transfer resistance in the liquid channels was of the same magnitude as that of a cellulose acetate membrane for the most effective dialysis cell. Thin channels are therefore of importance both for efficiency and for low dispersion in flow injection. A cell with a pressed membrane support provided the lowest dispersion and pressure dependence. A computer-based model provides a means for the determination of membrane permeabilities.

Preconcentration of copper(II) on immobilized 8-quinolinol in a flow-injection system with an ion-selective electrode detector

Abstract A column containing 8-quinolinol, immobilized on porous glass, is used for preconcentration and medium exchange in a flow-injection system with a copper ion-selective electrode detector. The metal ions are bound to the chelating ion exchanger while the anions and inert sample components pass to waste without contacting the electrode. Acid is then injected to elute the ions into a neutralizing buffer passing the electrode. Matrix effects are thus reduced because all measurements are made in the same buffer. The detection limits are 10 −7 and 3 × 10 −8 M copper(II) for sample volumes of 5 and 25 ml, respectively. The maximum throughput is 12 and 5 samples h −1 for the two stated injection volumes.

Removal of humic acid and surfactant interferences in trace metal determinations by differential-pulse anodic stripping voltammetry with use of adsorption and chelate ion-exchanger columns in a flow-injection system

Abstract A flow-injection differential-pulse anodic stripping voltammetry (d.p.a.s.v.) method is modified so that interferences from humic acids or surfactants are eliminated. The injected, slightly acidic sample is passed through a silica anion-exchanger column to remove compoundswith a strong tendency to adsorb to the electrode. The sample then passes to a chelate ion-exchange column containing immobilized 8-quinolinol. The metal ions are retained and later eluted with acid into the voltammetric cell. The results show that the interferences from up to 500 mg 1 –1 humic acid or at least 50 mg 1 -1 Triton X-100 can be removed and that the metal ion can be determined in a range similar to that for normal d.p.a.s.v. methods. The complete cycle time for a determination was 12 min.

Determination of Zinc(II) ions with a reactor containing immobilized carboxypeptidase a in a flow system

Abstract Zinc(II) was determined in the range 2–20 × 10−7 M through its activation of immobilized, metal-free carboxypeptidase A. The activity was assessed by injection of hippuryl- l -phenylalanine and the decomposition product was determined by high-performance liquid chromatography. Cadmium(II) and mercury(II) depressed the zinc response of the method although there was excess of metal-free enzyme in the reactor. Regeneration was made by pumping a 1,10-phenathroline solution through the reactor between successive samples.

Trace enrichment of aluminium ions on immobilized desferrioxamine

Abstract Aluminium was quantitatively preconcentrated in a flow-injection system using a column containing desferrioxamine immobilized on porous glass. The metal ions were eluted with 2 M nitric acid for determination by atomic absorption spectrometry. It was necessary to add a soluble chelate former to prevent precipitation of aluminium hydroxides from the standard solutions. Computer modelling with known equilibrium constants indicated that a buffer with lactate as a ligand was adequate. The pH range was restricted to 5.5–6.0 for quantitative uptake of aluminium ions becauxse of the competition of protons with the binding sites of desferrioxamine in the acidic region and precipitation of hydroxides in the alkaline region. The ligand-exchange kinetics were also of importance and the column was therefore kept at 50°C. The immobilized desferrioxamine column showed good stability and could be used continuously over a period of at least 2 months. Aluminum from peritoneal dialysis solutions was preconcentrated on immobilized desferrioxamine and determined by graphite furnace atomic absorption spectrometry.

Effect of sample viscosity on the behavior of thin-channel flow-through dialyzers in flow-injection analysis

Abstract Viscous samples will cause pressure changes which affect the transfer of solutes through an on-line dialyzer. Pressure changes can cause pre-peaks in the recorded signal due to membrane movements. The flow-rate in the receiver channel increases temporarily as the pressure builds up. A pressure differential of 30 kPa forced 1% of the liquid (at 0.4 ml min -1 through the membrane to the side with the lower pressure. This hydrodynamic flow will influence the diffusionally driven transfer of solutes and may cause analytical errors. The design of a flow-injection analysis (FIA) system with a flow-through dialyzer is discussed. The membrane should be stiff, with a low solvent transport rate to minimize fluctuations of the response due to changes of the differential pressure when a viscous sample is passing the dialyzer. The system should be made with stiff components and without flow restrictors after the dialyzer on the sample side.

Effect of whole blood and plasma on the permeability of glucose through different cellulose and cellulose acetate membranes

Abstract Model experiments were made with a flow-through dialysis cell to study how glucose in whole blood, plasma and aqueous standards is transferred through different membranes. Detection was via a glucose dehydrogenase reaction and NADH monitoring with a chemically modified electrode. The detector response was linear with the glucose concentration in all cases. The non-Newtonian properties of whole blood seem to be of great importance for the response. Theoretical models are proposed both for the effect of plasma viscosity and the red cell volume fraction (haematocrit) dependence on the glucose transfer rate. The models could only partially explain the differences between measurements in aqueous solutions and body fluids. Better agreement and less haematocrit dependence were obtained with low-permeability membranes.

Deoxygenation of supporting electrolytes in stripping voltammetry by glocuse and co-immobilized glocuse oxidase and catalase in a flow system

Abstract Carrier solutions for stripping voltammetry in flow systems are deoxygenated by reaction with glucose added to the carrier. The reaction was catalyzed by glocuse oxidase and catalase co-immobilized in an enzyme reactor which was inserted before the injector. The oxygenated was removed at least as efficiently as with nitrogen purging and the voltametric behaviour of cadmium(II), lead(II) and zinc(II) was unaffected by the glucose/gluconic acid system. A particular advantage is the rapid start-up compared to the lengthy purging of carrier solution when nitrogen degassing is used. The enzyme reactor made from porous glass was effective for several months.