Tore Ramstad

Electrosorptive Detection Based on Double-Layer Capacitance for Selective Anion Monitoring in Ion Chromatography. 1. Mercury Electrodes.

Abstract A novel scheme is presented for the detection of selected anions in ion chromatography, based on changes in differential double-layer capacitance, Δ C d , for a metal electrode at a suitable electrode potential induced by specific anion adsorption. The detector in this liquid chromatography/double-layer capacitance (LC/DLC) arrangement is a hanging or dropping mercury electrode in a large-volume wall-jet configuration. The anions observed to be detected readily by this approach include chloride, bromide, iodide, azide, thiocyanate, and thiosulfate. The relationship between Δ C d and the analyte concentration can be arranged to be approximately linear at least over the range 1–100 mg l −1 . An attractive feature of the technique is its insensitivity to ionic concentration gradients in the flowing stream. Another virtue of LC/DLC is its selectivity, although it is less sensitive for the detection of complexing anions than amperometric detection based on mercury oxidation. Dual detection schemes are also devised, involving monitoring double-layer capacitance at two potentials or combined with amperometric detection.

Electrosorptive detection in liquid chromatography of simple charged organic compounds

SummaryThe applicability of electrosorptive (ES) detection in liquid chromatography to charged organic species, specifically amines, alkanolamines, and alkylsulfonates is demonstrated. Separations can be carried out using ion-pair chromatography, and detection acieved by measuring changes in differential double-layer capacitance (DLC), ΔCd, brought about by analyte adsorption at potentials either well negative or positive of the potential of zero charge (pzc), depending on whether cationic or anionic forms, respectively, are being determined. The ΔCd-analyte concentration response curves are linear over a limited range. A welcome feature of the technique is that organic modifiers may be used in the mobile phase without deleterious consequence to the detection. Attempts were also made to detect these charged species as ion pairs by DLC at potentials close to the pzc. This approach, however, appears to have less practical utility.

Determination of impurities in α-cyclodextrin by gradient high-performance liquid chromatography with pulsed amperometric detection

Abstract A gradient normal-phase high-performance liquid chromatography (NP-HPLC) method has been developed for the determination of impurities in α-cyclodextrin. The method is selective for six linear sugars and several late-eluting unidentified impurities, as well as for α-, β- and γ-cyclodextrins. The HPLC system utilizes mobile phases consisting of acetonitrile–water (A, 64:36; B, 35:65), a 5-μm YMC Polyamine II column (250×4.6 mm I.D.), and pulsed amperometric detection. Experiments were conducted to determine the optimal mobile phase concentrations, establish the precision of the method, and demonstrate linearity and determine relative response factors for select sugars. The method was applied to select lots of commercial-grade α-cyclodextrin from two different manufacturers. The method is novel in that it offers gradient elution of strongly-retained impurities at ambient column temperatures and pulsed amperometric detection.

Electrosorptive detection based on double-layer capacitance for selected ion monitoring in ion chromatography: II. Silver

Abstract A scheme is presented for the detection of selected anions in ion chromatography based on changes in differential double-layer capacitance, Δ C d , induced by specific anion adsorption on a solid metal electrode. The detector in the liquid chromatography-double-layer capacitance (LC-DLC) arrangement described here is a silver electrode configured in a large-volume wall-jet geometry. The aqueous-phase anions most readily determined by this approach are chloride, bromide, iodide, azide and thiocyanate; additional ions, including iodate, periodate, bromate, thiosulfate and sulfide, are also determinable. Detection limits are sub ppm in most cases. The measurement scheme developed includes provision for faradaic current measurement, thereby providing for simultaneous capacitive and amperometric detection. The benefits of electrode rotation in capacitance detection are also discussed. It is shown that DLC detection is suitable for use with either low-capacity ion chromatography columns or conventional ion exchange columns.

Electrochemical investigations of platinum-methyluracil-blue and the related binuclear complex [(NH3)2Pt(MeU)2Pt(NH3)2](NO3)2

Abstract The redox behavior of the head-to-head bis(μ- (1-methyluracilato- N 3 , O 2 )-bis( cis -diammine platinum(II)) dinitrate, PtMeU, and platinum 1-methyluracil blue, PtMeUB, was studied by cyclic voltammetry (CV), rotating disk voltammetry (RDV), and controlled-potential coulometry (CPC). Redox titrimetry, electrochemistry/electron paramagnetic resonance spectroscopy (EPR), and liquid chromatography (LC) served as complementary techniques. The former reactant exhibits two-step electro-oxidation, consistent with the formation of a mixed-valence Pt(II, III) state en route to Pt(III, III). The latter also appears to oxidize to a uniform Pt(III) state. Although the oxidative-reductive electrochemistry of both reactants exhibits chemical reversibility, the heterogeneous electron-transfer kinetics are notably sluggish. The latter appears to be associated with the formation of an inhibiting film on the electrode surface. A slow conversion of PtMeU to a PtMeUB-like state was revealed by CV and LC. The complex, oligomeric nature of PtMeUB was revealed by means of gradient LC examination. Comparing oxidative and reductive electrolysis curves for PtMeUB yielded an average platinum oxidation state of 2.08. All observed behavior for PtMeUB, as well as for PtMeU, is accounted for by invoking +2 and +3 oxidation states for platinum; redox titrimetry using Ce(IV) revealed inconsequential oxidation of both of these systems beyond the III state. An estimate of molecular weight for the platinum blue was made by employing RDV in conjunction with the Einstein-Stokes equation.

The platinum benzole acid blues — a new class of blue platinum compounds

SummaryThe preparation of platinum blues from Pt(NH3)2(H2O)22+ and benzoic or phthalic acids is described. The compounds were characterised by e.p.r., u.v./vis. and x-ray photoelectron spectroscopy. Unlike pyrimidine blues, the platinum blues reported here appear to be non-ionic.

Electrosorptive Detection of Simple Organic Compounds in Liquid Chromatography.

Abstract The detection of a range of neutral organic compounds in liquid chromatography based on decreases in differential double-layer capacitance, Δ C d , at a mercury—aqueous interface caused by analyte specific adsorption is described. The arrangement employs alternating current phase-selective measurements using a large-volume wall-jet configuration. The low-molecular-weight organic solutes examined include aliphatic alcohols, diols, mono- and dicarboxylic acids, amines and alkanolamines. The differential capacitance measurements were conducted close to the potential of zero charge, where adsorption of such species is most extensive. Plots of Δ C d versus analyte concentration were generally sigmoidal, in accordance with expectations from the Frumkin adsorption isotherm. A potential-step coulometric method, where variations in the non-faradaic charge, Δ(Δ q ), are measured, was found to be a useful alternative detection scheme. Methods based on measuring transient capacitive currents associated with tensammetric adsorption—desorption behavior were also briefly investigated. A virtue of these double-layer capacitance detection schemes is that the magnitude of the Δ C d or Δ(Δ q ) response exhibits a clear sensitivity to the organic molecular structure, as anticipated in view of the known dependence of the adsorption thermodynamic upon solute hydrophobicity.

Thin-layer cell with a rapidly dropping mercury electrode for liquid chromatographic detection

SummaryA thin-layer cell with a horizontally positioned, rapidly dropping mercury electrode (TL-RDME) was developed for use in electrochemical detection in liquid chromatography. A specific application to electrosorptive detection based on the measurement of differential doublelayer capacitance, Cd, is detailed. The construction is of both Kel-F and acrylic materials, the latter chosen for ease of viewing. Interchangeable capillaries of varying internal diameter combined with gasket-determined channel thickness allows for a variation in drop times from below 10 ms to more than 200 ms. A drain solution flowing past the reference electrode, which is positioned upstream of the working electrode, provides a convenient electrolyte make-up stream. Typical Cd responses are compared to those obtained using a largevolume wall-jet cell/electrode geometry.

Localized unit responses in the optic tectum of carp

Abstract Temporal properties of single unit responses were correlated with the position of the recording electrode in the carp optic tectum. ON, OFF and ON-OFF visual units were encountered. Lesions corresponding to these units were found in all layers of the tectum with the exception of the periventricular gray (stratum griseum periventriculare). The greatest activity was encountered in the inner plexiform region (internal portion of the stratum plexiforme et fibrosum externum), where OFF units predominated. Based on the methods used, there is no apparent segregation of response types into tectal layers.

Pulse-enhanced electrosorptive detection of ions at silver in ion chromatography

Abstract Differential double-layer capacitance measurements applied to the silver-aqueous interface may serve as the basis of detection for surface-active ions in ion chromatography. There is a caveat to this approach, though, and that is that the electrode is subject to gradual fouling due to the accumulation of adsorbed matter. We report here on the incorporation of potential pulsing sequences to counter this damaging effect. Two approaches have been taken: 1) inclusion of a periodic potential cleaning pulse in the conventional, fixed-potential double-layer capacitance measurement scheme, and 2) utilization of a potential-pulse/coulometric measurement technique. the potential-pulse/coulometric procedure entails measurement of the change in charge required to charge the electrical double layer in response to specific adsorption of analyte. Its response-concentration behavior is similar to that observed for differential capacitance detection. the use of a combined, dual-potential (sequential) capacitive/a...