Triiodide PVC Membrane Electrode Based on a Charge-Transfer Complex of Iodine with 2,4,6,8- Tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane

A novel triiodide ion-selective electrode based on a charge-transfer complex of iodine with 2,4,6,8-tetraphenyl-2,4,6,8-tetraazabicyclo[3.3.0]octane as membrane carrier was prepared. The electrode has a linear dynamic range between 5.0 × 10(-)(2) and 3.5 × 10(-)(6) M, with a near-Nernstian slope of 54.7 ± 0.8 mV decade(-)(1) and a detection limit of 2.0 × 10(-)(6) M. The potentiometric response is independent of the pH of the solution in the pH range 4.0-10.5. The electrode possesses the advantages of low resistance, short conditioning time, fast response, and, especially, very good selectivities over a wide variety of other anions. The electrode can be used for at least 10 months without any considerable divergence in potentials. It was used as an indicator electrode in potentiometric titration of triiodide ions.     

Ion-Pair Extraction of Metalloporphyrins into Acetonitrile for Determination of Copper(II)

Equilibrium study of ion-pair extraction of a cationic water-soluble porphyrin [5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin, H(2)tmpyp(4+)] and its metalloporphyrins (MP) into the acetonitrile layer, separated by addition of sodium chloride (4.00 mol dm(-)(3)) to a 1:1 (v/v) acetonitrile-water mixed solvent, was carried out to develop a new and useful method for the determination of a subnanogram amount of copper(II). M denotes Zn(2+), Cu(2+), Co(3+), Fe(3+), and Mn(3+), and P(2)(-) is porphyrinate ion. The extraction and dissociation constants of the ion-pair complexes, defined by K(ex) = [MP(ClO(4))(4)](org)[MP(4+)](aq)(-)(1)[ClO(4)(-)](aq)(-)(4), K(dis,1) = [MP(ClO(4))(3)(+)](org)[ClO(4)(-)](org)[MP(ClO(4))(4)](org)(-)(1), and K(dis,2) = [MP(ClO(4))(2)(2+)](org)[ClO(4)(-)](org)[MP(ClO(4))(3)(+)](org)(-)(1), were determined by taking into account the partition constant of sodium perchlorate (K(D) = 1.82 ± 0.01). The equilibrium constants were found to be K(ex)K(dis,1) = (7.2 ± 1.3) × 10(4), (6.4 ± 0.9) × 10(4), (1.35 ± 0.13) × 10(5), (4.8 ± 0.6) × 10(3), (1.23 ± 0.05) × 10(4), and (1.42 ± 0.07) × 10(3) at 25 °C for the free base porphyrin (H(2)tmpyp(4+)) and the metalloporphyrins of zinc(II), copper(II), cobalt(III), iron(III), and manganese(III), respectively. The K(dis,2) values were (2.9 ± 1.4) × 10(-)(2), (3.1 ± 1.1) × 10(-)(2), (8.0 ± 4.9) × 10(-)(3), and (5.1 ± 2.2) × 10(-)(2) for the free base porphyrins and the metalloporphyrins of zinc(II), copper(II), and cobalt(III), respectively. The results were developed for determination of a trace amount of copper(II) (3 × 10(-)(8)-4 × 10(-)(6) mol dm(-)(3)) in natural water samples using H(2)tmpyp(4+) with a molar absorptivity of 3.1 × 10(5) mol(-)(1) dm(3) cm(-)(1) at a precision of 1.3% (RSD). The determination of copper(II) was not interfered by the presence of 10(-)(4) mol dm(-)(3) of Mn(2+), Co(2+), Ni(2+), Hg(2+), Cd(2+), Ag(+), Cr(3+), V(5+), Al(3+), Mg(2+), Ca(2+), Br(-), I(-), SCN(-), and S(2)O(3)(2)(-) and 10(-)(5) mol dm(-)(3) of Fe(3+), Zn(2+), and Pd(2+).     

Determination of iridium in natural waters by clean chemical extraction and negative thermal ionization mass spectrometry

Methods for the precise, routine measurement of Ir in seawater, riverwater, and estuarine water using isotope dilution negative thermal ionization mass spectrometry (ID-NTIMS) have been developed. After equilibration with a (191)Ir-enriched spike, Ir is separated from solution by coprecipitation with ferric hydroxide, followed by anion exchange chromatography using a reductive elution technique. UV irradiation is employed for the decomposition of trace organics, which interfere with negative ion production. IrO(2)(-) ions are produced in the mass spectrometer by heating the sample on a Ni-wire filament in the presence of Ba(OH)(2). Detection efficiencies ranged from 0.1% to 0.3%. We have used these procedures to determine the concentrations of Ir in 4 kg samples from the Pacific Ocean, the Atlantic Ocean, the Baltic Sea, and the rivers supplying the Baltic. Our chemical procedures introduce a total blank of ～2 × 10(8) atoms per sample. The distribution of Ir in the oceans is fairly uniform, averaging ～4 × 10(8) atoms kg(-)(1). The concentrations in the rivers supplying the Baltic Sea range from (17.4 ± 0.9) × 10(8) for a pristine river to (92.9 ± 2.2) × 10(8) atoms kg(-)(1) for a polluted river. The distribution, speciation, and transport of Ir in natural waters can now be subjected to intensive study.     

Mercury/homocysteine ligation-induced ON/OFF-switching of a T-T mismatch-based oligonucleotide molecular beacon

A molecular beacon (MB) with stem-loop (hairpin) DNA structure and with attached fluorophore-quencher pair at the ends of the strand has been applied to study the interactions of Hg(2+) ions with a thymine-thymine (T-T) mismatch in Watson-Crick base-pairs and the ligative disassembly of MB·Hg(2+) complex by Hg(2+) sequestration with small biomolecule ligands. In this work, a five base-pair stem with configuration 5'-GGTGG...CCTCC-3' for self-hybridization of MB has been utilized. In this configuration, the four GC base-pair binding energy is not sufficient to hybridize fully at intermediate temperatures and to form a hairpin MB conformation. The T-T mismatch built-in into the stem area can effectively bind Hg(2+) ions creating a bridge, T-Hg-T. We have found that the T-Hg-T bridge strongly enhances the ability of MB to hybridize, as evidenced by an unusually large MB melting temperature shift observed on bridge formation, ΔT(m) = +15.1 ± 0.5 °C, for 100 nM MB in MOPS buffer. The observed ΔT(m) is the largest of the ΔT(m) found for other MBs and dsDNA structures. By fitting the parameters of the proposed model of reversible MB interactions to the experimental data, we have determined the T-Hg-T bridge formation constant at 25 °C, K(1) = 8.92 ± 0.42 × 10(17) M(-1) from mercury(II) titration data and K(1) = 1.04 ± 0.51 × 10(18) M(-1) from the bridge disassembly data; ΔG° = -24.53 ± 0.13 kcal/mol. We have found that the biomarker of oxidative stress and cardiovascular disease, homocysteine (Hcys), can sequester Hg(2+) ions from the T-Hg-T complex and withdraw Hg(2+) ions from MB in the form of stable Hg(Hcys)(2)H(2) complexes. Both the model fitting and independent (1)H NMR results on the thymidine-Hg-Hcys system indicate also the high importance of 1:1 complexes. The high value of K(1) for T-Hg-T bridge formation enables analytical determinations of low concentrations of Hg(2+) (limit of detection LOD = 19 nM or 3.8 ppb, based on 3σ method) and Hcys (LOD = 23 nM, 3σ method). The conditional stability constants for Hg(Hcys)H(2)(2+) and Hg(Hcys)(2)H(2) at 52 °C have been determined, β(112) = 5.37 ± 0.3 × 10(46) M(-3), β(122) = 3.80 ± 0.6 × 10(68) M(-4), respectively.     

Determination of the second-order susceptibility of ammonium dihydrogen phosphate and α-quartz at 633 and 1064 nm

The second-order susceptibility d(36) of ammonium dihydrogen phosphate (ADP) was determined from phase-matched second-harmonic generation (SHG) at two wavelengths. A cw single-mode He-Ne laser (λ= 633 nm) and a cw single-mode Nd:YAG laser (λ= 1064 nm) were used as fundamental beam sources. The results were d(36)(ADP, 633 nm) =(1.31 ± 0.05) ×10(-9) esu = 0.55 ± 0.02 pm/V and d(36)(ADP, 1064 nm) = (1.10 ± 0.06) × 10(-9) esu = 0.46 ± 0.03 pm/V. The d(11) values of α-quartz were determined relative to d(36)(ADP) to be d(11)(α-quartz, 633 nm) = (7.4 ± 0.3) × 10(-10) esu = 0.31 ± 0.01 pm/V and d(11)(α-quartz, 1064 nm) = (7.1 ± 0.3) × 10(-10) esu = 0.30 ± 0.01 pm/V by the use of the Maker fringe method. The Miller's delta ofADP and α-quartz is in good agreement at the two wavelengths.     

On-line coupling of flow field-flow fractionation and multiangle laser light scattering for the characterization of macromolecules in aqueous solution as illustrated by sulfonated polystyrene samples

Seven sulfonated polystyrene standards (18?000-3?000?000 g/mol), taken as model substances for macromolecular polyelectrolytes, were dissolved in aqueous 0.1 M sodium nitrate solution and characterized by multiangle laser light scattering coupled on-line to flow field-flow fractionation. The distributions of molar mass and root mean square radius and the diffusion coefficients were obtained for each sample using a constant field of force for separation. Relationships between molar mass and root mean square radius [?R(G)(2)?(z)(0.5) = (2.71 × 10(-)(2))M(w)(0.56)] or diffusion coefficient [D = (7.10 × 10(-)(8))M(w)(-)(0.68)] were calculated. To investigate the static analytical range of this novel hyphenated technique a mixture of all seven samples was fractionated applying a programmed field. The relationship obtained between root mean square radius and molar mass was used to calculate a Mark-Houwink equation [[η]calcd = (2.99 × 10(-)(2))M(w)(0.68)]. To verify this result, the intrinsic viscosities for all samples were measured at low shear rate and found to be in good agreement [[η]calcd = (2.77 × 10(-)(2))M(w)(0.67)].     

An ion-selective electrode for acetate based on a urea-functionalized porphyrin as a hydrogen-bonding ionophore

An ion-selective electrode for acetate based on (α,α,α,α)-5,10,15,20-tetrakis[2-(4-fluorophenylureylene)phenyl]porphyrin as an ionophore that has no metal center and forms hydrogen bonds to the analyte is described. At pH 7.0 (0.1 M HEPES-NaOH buffer), the electrode based on this ionophore and cationic sites (50 mol % relative to the ionophore) responds to acetate in a linear range from 1.58 × 10(-)(4) to 1.58 × 10(-)(2) M with a slope of -54.8 ± 0.8 mV/decade and a detection limit of (3.06 ± 1.15) × 10(-)(5) M. Selectivity coefficients determined with the separate solution method (SSM) indicate that interferences of hydrophobic inorganic anions are relatively small (log[Formula: see text] (SSM): NO(3)(-), +0.68; SCN(-), +0.60; NO(2)(-), +0.22; I(-), +0.20; ClO(4)(-), +0.12; Br(-), -0.13). Responses to anions that are good hydrogen bond acceptors, i.e., Cl(-), HSO(3)(-), and HCO(3)(-), were Nernstian and were weaker than the response to acetate (log[Formula: see text] (SSM): -0.54, -0.56, and -1.34, respectively). Negligibly small responses were observed for very hydrophilic anions, i.e., F(-), SO(4)(2)(-), and H(2)PO(4)(-)/HPO(4)(2)(-). While aliphatic carboxylates such as formate, propanoate, pyruvate, and lactate gave Nernstian responses similar to acetate, interferences of salicylate and benzoate were considerably decreased in comparison with electrodes based on cationic sites only. Concentrations of acetic acid in vinegar samples were determined by direct potentiometry and agreed with values determined by a standard enzymatic method.     

Preconcentration and Voltammetric Determination of Mercury(II) at a Chemically Modified Glassy Carbon Electrode

In this work, the organic compound 2-mercaptobenzimidazole was covalently bound on the surface of a glassy carbon rod, via silanization, yielding a material capable of selectively complexing Hg(2+) ions. This material was applied as an electrode for voltammetric determination of mercury(II) following its nonelectrolytic preconcentration. After exchanging the medium, the voltammetric measurements were carried out by anodic stripping in the differential pulse mode (pulse amplitude, 50 mV; scan rate, 1.25 mV s(-)(1)) using 10(-)(2) mol L(-)(1) NaSCN solution as supporting electrolyte. An anodic stripping peak was obtained at 0.06 V (vs SCE) by scanning the potential from -0.3 to +0.3 V. After a 5 min preconcentration period in a pH 4.0 Hg(2+) solution, this electrode shows increasing voltammetric response in the range 0.1-2.2 μg mL(-)(1), with a relative standard deviation of 5% and a practical detection limit of 0.1 μg mL(-)(1) (5.0 × 10(-)(7) mol dm(-)(3)). Compared with the conventional stripping approach, this chemically modified glassy carbon electrode procedure presented good discrimination against interference from Cu(II) in up to 10-fold molar excess.     

Analysis of the magnetic force generated at a hemispherical microelectrode

An analytical expression is presented for the magnetic force generated during steady-state voltammetry at a hemispherical microelectrode immersed in a uniform magnetic field. Diffusion of electrogenerated ions through the magnetic field results in a magnetic force that induces convective solution flow near the electrode surface. The magnetic force per unit volume,[Formula: see text] (i.e., force density), is shown to decrease as r(-)(2), where r is the distance away from the center of the hemispherical electrode. A consequence of the inverse square dependence of[Formula: see text] on r is that the magnetic force is confined to a microscopic solution volume near the electrode surface (e.g., ～2 × 10(-)(9) L for a 12.5-μm-radius hemispherical electrode). The net magnetic force acting on the diffusion layer volume,[Formula: see text] , is computed as a function of magnetic field strength and orientation and used in an approximate analysis of experimental data obtained at an inlaid 12.5-μm-radius Pt microdisk electrode. Enhancements in voltammetric currents are shown to result from magnetic forces as small as 2 × 10(-)(11) N.     

Strontium-selective membrane electrodes based on some recently synthesized benzo-substituted macrocyclic diamides

Eight different recently synthesized macrocyclic diamides were studied to characterize their abilities as strontium ion carriers in PVC membrane electrodes. The electrode based on 1,13-diaza-2,3;11,12-dibenzo-4,7,10-trioxacyclopentadecane-14,15-dione exhibits a Nernstian response for Sr(2+) ions over a wide concentration range (1.0 × 10(-)(1)-3.2 × 10(-)(5) M) with a limit of detection of 8.0 × 10(-)(6) M (0.7 ppm). The response time of the sensor is ～10 s, and the membrane can be used for more than three months without observing any deviation. The electrode revealed comparatively good selectivities with respect to many alkali, alkaline earth, and transition metal ions. It was used as an indicator electrode in potentiometric titration of carbonate ions with a strontium ion solution.     

Oxidation Reaction between Periodate and Polyhydroxyl Compounds and Its Application to Chemiluminescence

The oxidation reaction between periodate and polyhydroxyl compounds was studied. A strong chemiluminescent (CL) emission was observed when the reaction took place in a strong alkaline solution without any special CL reagent. However, in acidic or neutral solution, it was hard to record the CL with our instrument. It was interesting to find that in the presence of carbonate the CL signal was enhanced significantly. When O(2) gas and N(2) gas were blown into the reagent solutions, both background and CL signals of the sample were enhanced by O(2) and decreased by N(2). The spectral distribution of the CL emission showed two main bands (λ = 436-446 and 471-478 nm). Based on the studies of the spectra of CL, fluorescence and UV-visible, a possible CL mechanism was proposed. In strongly alkaline solution, periodate reacts with the dissolved oxygen to produce superoxide radical ions. A microamount of singlet oxygen ((1)O(2)*) could be produced from the superoxide radicals. A part of the superoxide radicals acts on carbonates and/or bicarbonates leading to the generation of carbonate radicals. Recombination of carbonate radicals may generate excited triplet dimers of two CO(2) molecules ((CO(2))(2)*). Mixing of periodate with carbonate generated were very few (1)O(2)* and (CO(2))(2)*. These two emitters contribute to the CL background. The addition of polyhydroxyl compounds or H(2)O(2) caused enhancement of the CL signal. It may be due to the production of (1)O(2)* during the oxidized decomposition of the analytes in periodate solution. This reaction system has been established as a flow injection analysis for H(2)O(2), pyrogallol, and α-thioglycerol and their detection limits were 5 × 10(-)(9), 5 × 10(-)(9), and 1 × 10(-)(8) M, respectively. Considering the effective reaction ions, IO(4)(-), CO(3)(2)(-), and OH(-) could be immobilized on a strongly basic anion-exchange resin. A highly sensitive flow CL sensor for H(2)O(2), pyrogallol, and α-thioglycerol was also prepared.     

Insight to ternary complexes of co-adsorption of norfloxacin and Cu(II) onto montmorillonite at different pH using EXAFS

Co-adsorption of norfloxacin (Nor) and Cu(II) on montmorillonite at pH 4.5, 7.0 and 9.0 was studied by integrated batch adsorption experiments and extended X-ray absorption fine structure (EXAFS) spectroscopy. Under such pH conditions the dominant species of Nor are cation (Nor(+)), zwitterion (Nor(±)), and anion (Nor(-)), respectively. Results indicated that Nor sorption decreased with an increase of solution pH. The presence of Cu(II) slightly suppressed the Nor(+) sorption at pH 4.5, while increased Nor(±) and Nor(-)sorption on montmorillonite at pH 7.0 and 9.0, respectively. In contrast, Nor increased Cu(II) adsorption at pH 4.5, but had little effect on the adsorption of Cu(II) on montmorillonite at pH 7.0 and 9.0. Spectroscopic results showed that, at pH 4.5, Nor(+) was sorbed on montmorillonite by the formation of outer-sphere montmorillonite-Nor-Cu(II) ternary surface complex. At pH 7.0, montmorillonite-Nor-Cu(II) and montmorillonite-Cu(II)-Nor ternary surface complexes co-exist. At pH 9.0, montmorillonite-Cu(II)-Nor ternary surface complex was likely formed, which was different to Cu(II)(Nor)(2) precipitate of the solution.     

Determination of the diffusion coefficient of hydrogen in aqueous solution using single and double potential step chronoamperometry at a disk ultramicroelectrode

An assessment is made of single and double potential step chronoamperometry (SPSC and DPSC, respectively) at Pt disk ultramicroelectrodes (UMEs) as methods for determining the value of the diffusion coefficient of hydrogen in aqueous solutions. In SPSC, measured currents for the oxidation of dissolved hydrogen (at concentrations close to saturated solution values) comprise a significant contribution, at short to moderate times, from the oxidative desorption of adsorbed hydrogen as well as the diffusion-controlled oxidation of the solution species. Provided that the electrode is preconditioned using a well-defined potential cycling procedure, the behavior for the oxidative desorption step alone can be established in an Ar-saturated solution. The chronoamperometric characteristics for the solution diffusion-controlled process may then be determined, from which the diffusion coefficient of hydrogen can be measured. In DPSC, a locally supersaturated solution of hydrogen is created transiently through the diffusion-controlled reduction of a known concentration of protons in an initial potential step. Hydrogen is subsequently collected back through oxidation to protons; the current flowing depends on the diffusion coefficients of the two species and the duration of the forward step. Under these conditions, the contribution from surface electrochemical processes to the forward and reverse chronoamperommograms is shown to be negligible. By solving the mass transport problem for DPSC with arbitrary diffusion coefficients of the redox species, the diffusion coefficient of hydrogen is readily determined. Both methods yield a consistent value for the diffusion coefficient of hydrogen, D(H)((2)), in 0.1 mol dm(-)(3) KNO(3) of 5.0 × 10(-)(5) cm(2) s(-)(1).     

Oxidation-state speciation of

The analytical utility of chemically modified microelectrodes for oxidation-state speciation of redox couples by cyclic voltammetry has been explored. [Re(I)(DMPE)3]+/[Re(II)(DMPE)3]2+, where DMPE = 1,2-bis(dimethylphosphino)ethane, was studied at carbon-fiber microelectrodes of approximately 5 microm in radius coated with Nafion-entrapped solgel-derived silica (Nafion-silica) composite. The results are compared with cyclic voltammetry of [Fe(CN)6]3-/[Fe(CN)6]4- at bare carbon-fiber microelectrodes. At both microelectrodes, the cathodic and anodic limiting currents are linearly proportional to the concentrations of the reducible and oxidizable species of a redox couple, respectively. The shape of the cyclic voltammogram and the magnitude of the steady-state limiting current are not affected by the potential at which the scan starts. Speciation of both forms of a redox couple could be achieved voltammetrically at the microelectrodes. However, a considerably slower scan rate was required to achieve steady state at the modified electrode because of the smaller diffusion coefficients of [Re(I)(DMPE)3]+ and [Re(II)(DMPE)3]2+ in the Nafion-silica composite. The detection limit at the modified electrode was considerably lower (5 x 10(-9) M for [Re(I)(DMPE)3]+) than at the bare electrode (6 x 10(-5) M for [Fe(CN)6]3- and [Fe(CN)6]4-) because of the substantial preconcentration of [Re(I)(DMPE)3]+ by the Nafion-silica composite.     

Electrochemiluminescent metallopolymer-nanoparticle composites: nanoparticle size effects

Metallopolymer-gold nanocomposites have been synthesized in which the metal complex-Au nanoparticle (NP) mole ratio is systematically varied by mixing solutions of 4-(dimethylamino) pyridine protected gold nanoparticles and a [Ru(bpy)(2)PVP(10)](2+) metallopolymer; bpy is 2,2'-bipyridyl and PVP is poly-(4-vinylpyridine). The impact of changing the gold nanoparticle diameter ranging from 4.0 ± 0.5 to 12.5 ± 1 nm has been investigated. The photo induced emission of the metallopolymer undergoes static quenching by the metal nanoparticles irrespective of their size. When the volume ratio of Au NP-Ru is 1, the quenching efficiency increases from 38% to 93% on going from 4.0 ± 0.5 to 12.5 ± 1 nm diameter nanoparticles while the radius of the quenching sphere remains unaffected at 75 ± 5 ?. The conductivity of thin films is initially unaffected by nanoparticle incorporation until a percolation threshold is reached at a mole ratio of 4.95 × 10(-2) after which the conductivity increases before reaching a maximum. For thin films of the nanocomposites on electrodes, the electrochemiluminescence intensity of the nanocomposite initially increases as nanoparticles are added before decreasing for the highest loadings. The electrochemiluminescence intensity increases with increasing nanoparticle diameter. The electrochemiluminescence (ECL) emission intensity of the nanocomposite formed using 12.5 nm particles at mole ratios between 5 × 10(-3) and 10 × 10(-3) is approximately 7-fold higher than that found for the parent metallopolymer. The application of these materials for low cost ECL-based point of care devices is discussed.     

Poly(vinyl) chloride membrane copper-selective electrode based on 1-phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione

1-Phenyl-2-(2-hydroxyphenylhydrazo)butane-1,3-dione (H(2)L) was used as an effective ionophore for copper-selective poly(vinyl) chloride (PVC) membrane electrodes. Optimization of the composition of the membrane and of the conditions of the analysis was performed, and under the optimized conditions the electrode has a detection limit of 6.30×10(-7) M Cu(II) at pH 4.0 with response time 10s and displays a linear EMF versus log[Cu(2+)] response over the concentration range 2.0×10(-6) to 5.0×10(-3) M Cu(II) with a Nernstian slope of 28.80±0.11 mV/decade over the pH range of 3.0-8.0. The sensor is stable for 9 weeks and exhibits good selectivity with respect to alkali, alkali earth and transition metal ions (e.g. Na(+), K(+), Ba(2+), Ca(2+), Zn(2+), Cd(2+), Co(2+), Mn(2+), Ni(2+), Fe(2+), Al(3+)) in the 3.0-8.0 pH range. It was successfully applied for the direct determination of copper(II) in zinc, aluminum and nickel based alloys, in soils polluted by oil, and as an indicator electrode for potentiometric titration of copper ions with EDTA.     

Measurement of SDS Micelle-Peptide Association Using (1)H NMR Chemical Shift Analysis and Pulsed-Field Gradient NMR Spectroscopy

The binding of two simple tripeptides, glycyl-histidyl-glycine (GHG) and phenylalanyl-histidyl-phenylalanine (FHF) with SDS micelles was examined using (1)H NMR chemical shift analysis and self-diffusion coefficients measured with pulsed-field gradient NMR spectroscopy. The presence of GHG or FHF did not appear to significantly affect the critical micelle concentration (cmc) or the average size of the SDS micelles formed. The chemical shifts of several of the GHG resonances change as a function of SDS concentration, indicating an interaction between the peptide and the micelles. In addition, the concentration-dependent decrease observed for the GHG diffusion coefficients suggests association of the peptide with SDS micelles. The free and micelle-associated GHG are in fast exchange on both the (1)H chemical shift and diffusion time scales. The equilibrium constant for the binding of GHG to SDS micelles was determined from the analysis of the concentration dependence of the histidine C2 and C4 resonances to be 17 ± 5 and 24 ± 6 M(-)(1), respectively. The precision of the equilibrium constants obtained by analysis of the chemical shift data is limited by the small chemical shift changes observed. Analysis of the concentration dependence of the diffusion coefficients produced an equilibrium constant of 17 ± 1 M(-)(1). The more hydrophobic peptide, FHF is strongly associated with the SDS micelles. Because the fraction of free FHF is small in these solutions, it was not possible to determine a formation constant for the interaction of FHF with the SDS micelles by analysis of either the (1)H chemical shift or diffusion coefficient data. The cmc of SDS in 0.10 M Na(2)C(2)O(4) buffer was determined to be 5.4 ± 0.1 mM by analysis of the SDS diffusion coefficients in the absence of the peptides. The SDS cmc could also be extracted from the GHG and FHF diffusion coefficients measured as a function of the SDS concentration. The cmc determined from the GHG diffusion data, 5.7 ± 0.2 mM, is in good agreement with the value determined from analysis of the SDS diffusion coefficients in the 5.0 mM GHG solution, 5.2 ± 0.1 mM. The smaller cmc determined from the FHF diffusion data, 4.1 ± 0.1 mM, may reflect some association of the SDS with the peptide prior to micelle formation in bulk solution.     

锂在人造石墨、中间相炭微球及无定形碳中的扩散系数

采用X-射线衍射及恒电位阶跃计时电流法测定了不同炭材料的结构及锂在这些炭材料中的扩散系数。发现炭电极的放电程度与结构对锂在炭电极中的扩散系数有重要影响。随着放电程度的增加,锂在MCMB电极中的扩散系数由4.43×10-9cm2/s减少到5.24×10-10cm2/s,在50%的放电程度下,锂在蔗糖热解炭、树脂热解炭、人造石墨及MCMB中的扩散系数分别为1.4×10-10cm2/s,5.75×10-10cm2/s,1.24×10-9cm2/s,2.1×10-9cm2/s。与无定形碳如蔗糖热解炭及树脂热解炭比较,锂在石墨化炭如人造石墨、MCMB中的扩散要容易得多。     

Cyclic voltammetric detection in capillary electrophoresis with application to metal ions

Fast cyclic voltammetry (CV) was evaluated over sweep rates of 20-1000 V/s at Au disk electrodes (25 and 10 μm) for end-capillary detection in capillary electrophoresis with metal ions as test analytes; some studies were also done with 25-μm Pt disk electrodes. The waveform applied to the electrode consisted of a preconcentration period (55-330 ms) followed by cyclic voltammetry (2-100 ms). Maximum signal-to-noise was obtained with the integrated CV current as the analytical signal, and this was linearly proportional to sweep rate; maximum response was obtained at sweep rates of >100 V/s for 10-μm electrodes and >200 V/s for 25-μm electrodes; sweep rates of >400 V/s caused peak tailing due to trapping of the analyte at the electrode. With this CV detection approach, comigrating analytes could be identified and determined. Reproducibilities for six analytes over the range 1.0 × 10(-)(7)-1.0 × 10(-)(5) mol/L were 2%-5%, and calibration curves were linear, with response factors in the range of 2%-6%. Detection limits (2 × peak-to-peak baseline noise) were in the range of 5 × 10(-)(9)-4 × 10(-)(8) mol/L, which are 1-2 orders of magnitude better than results obtained previously with square-wave pulsed amperometric detection of metal ions.     

Determination of boron by flow injection analysis using a conductivity detector

A flow injection method for the determination of boron using a conductivity detector has been described. Boric acid injected into the flow system reacts with mannitol (0.3 M) in the mobile phase and an equivalent amount of H(+) is liberated in the stream. The increase in the conductance of the mobile phase due to the liberated H(+) has been equated to the boron concentration in the sample. A linear calibration for light- and heavy-water samples containing 0-20 μg/mL boron was obtained. Boron concentrations in the samples of light and heavy water and lithium pentaborate solution have been measured. The interferences due to various ions such as Na(+), Li(+), Cu(2+), Ni(2+), Co(2+), Fe(3+), Al(3+), SO(4)(2-), NO(3)(-), F(-), and Cl(-) could be eliminated by adopting a two-step sample pretreatment procedure. In the first step, all the anions were converted to Cl(-) by treating the sample solution with a strong anion-exchange resin. In the second step, the solution obtained from the first step was passed through a silver-guard cartridge to remove interfering cations and Cl(-). The relative standard deviation was ±0.25% for the determination of 1 μg of boron in light water, and the limit of detection was 0.01 μg present in an injection volume of 100 μL. The corresponding values for heavy water were ±0.38% and 0.1 μg, respectively.