HPLC online reductive scanning voltammetric detection of diquat, paraquat and difenzoquat with mercury electrodes

To establish an electrochemical HPLC detection system which is suitable for the voltammetric characterisation of unknown contaminants and food components at working potentials lower than –1 V, a modified flow-through cell for the use of a hanging mercury drop electrode (HMDE) is described. The introduction of silanised glass capillaries and a new flow-channel design provide a high HMDE lifetime, which is recommended in HPLC detection. As test system the herbicides diquat, paraquat and difenzoquat and ethylviologen as internal standard were measured using differential pulse voltammetry (DPV) detection to improve selectivity. Spiked water samples were analysed with voltammetric and UV-detection and results agreed well.     

Determination of palladium in deoxo catalysts by differential pulse voltammetry

A differential pulse voltammetric (DPV) method is proposed for the determination of palladium in deoxo catalysts, which are used in recombination units of the cover gas system of nuclear reactors. The electrode system consists of hanging mercury drop electrode (HMDE)/ platinum auxillary/ Ag-AgCl reference electrode. The supporting electrolyte consists of 0.02 M diammonium tartrate + 0.08 M ammonia-ammonium chloride buffer at pH 9.0. Fe, Cu, Pb, Ni, Co and Zn did not interfere. The interference by Cd could be eliminated using 0.005 M EDTA. The relative standard deviation of the proposed method was about 4.0% at ∼ 0.5% palladium content in the catalyst. The method is simple, rapid and free from any possible interferences. Received: 8 December 1998 / Revised: 11 January 1999 / Accepted: 18 January 1999     

镓(Ⅲ)-茜素红体系的吸附伏安法

本文对镓(Ⅲ)与茜素红络合物在悬汞电极上的吸附伏安法作了研究,确定了电极过程.在HAc-NH_4Ac(pH4.06)缓冲溶液中茜素红存在下,用1.5阶微分吸附伏安法测定镓,其线性范围是1×10~(-10)～1×10~(-7)mol/L,其检测限为1×10~(-10)mol/L,并用此法测定了中草药中的痕量镓.     

Catalytic determination of ultra trace amounts of vanadium with detection by linear sweep voltammetry

Vanadium has a very strong catalytic effect on the bromate oxidation of gallocyanine at pH of 4.0. The oxidation product of gallocyanine exhibit a voltammetric wave at + 0.10 V vs. Ag/AgCl reference electrode in 0.10 mol/l KNO₃ medium. The linear scan voltammetric behaviour of the reaction products at HMDE has been studied and selected as the indicator component for the reaction. A detection limit of 0.05 ng/ml and calibration graph from 0.30–200.00 ng/ml vanadium were obtained. Vanadium in water and gasoline samples was determined by this method, with satisfactory results. Received: 26 September 1996 / Revised: 6 November 1996 / Accepted: 15 November 1996     

Selective voltammetric determination of chromium (VI) with DTPA and nitrate

A voltammetric determination of Cr(VI) in a flow system is described based on the selective accumulation of the reduction product of Cr(VI) on an HMDE, its complexation with DTPA and subsequent reduction of the complex in presence of nitrate. The calibration graphs were linear up to 100 and 5 nmol/L for deposition times 120 and 600 s, respectively. The relative standard deviation was 2.8% (n = 5) for Cr(VI) concentrations of 5 × 10^–8 mol/L. The detection limits (3 σ) for Cr(VI) were 1.0 and 0.12 nmol/L at deposition times of 120 and 600 s, respectively. Typical interferences derived from real water samples are discussed. The method has been applied for the determination of Cr(VI) in spiked natural water samples. Received: 18 June 1998 / Revised: 28 September 1998 / Accepted: 5 October 1998     

Catalytic determination of ultra trace amounts of vanadium with detection by linear sweep voltammetry

Vanadium has a very strong catalytic effect on the bromate oxidation of gallocyanine at pH of 4.0. The oxidation product of gallocyanine exhibit a voltammetric wave at + 0.10 V vs. Ag/AgCl reference electrode in 0.10 mol/l KNO₃ medium. The linear scan voltammetric behaviour of the reaction products at HMDE has been studied and selected as the indicator component for the reaction. A detection limit of 0.05 ng/ml and calibration graph from 0.30–200.00 ng/ml vanadium were obtained. Vanadium in water and gasoline samples was determined by this method, with satisfactory results.     

Determination of ketorolac in human serum by square wave adsorptive stripping voltammetry

The adsorption behavior of ketorolac on a hanging mercury drop electrode (HMDE) was explored by square-wave and cyclic voltammetry. The square wave voltammetric response of ketorolac depends on the parameters of the square wave voltammetry excitation signal as well as on the pH of the medium and the accumulation time. The drug was accumulated at HMDE and a well-defined peak was obtained at -1.41 V versus. Ag/AgCl (saturated KCl) in acetate buffer of pH 5.0. A square-wave adsorptive stripping voltammetric method for the quantitative determination of ketorolac was developed. The linear concentration range was 1x10(-10)-1x10(-8) when using 300 s accumulation at -0.8 V. The detection limit of ketorolac was 1.0x10 (-11)M . The precision was excellent with relative standard deviation of 3.85% at concentration of 5x10 (-8)M after 60 s accumulation time. Applicability to serum samples was illustrated. A detection limit of 14 ng per ml of serum was obtained.     

Selective voltammetric determination of chromium (VI) with DTPA and nitrate

A voltammetric determination of Cr(VI) in a flow system is described based on the selective accumulation of the reduction product of Cr(VI) on an HMDE, its complexation with DTPA and subsequent reduction of the complex in presence of nitrate. The calibration graphs were linear up to 100 and 5 nmol/L for deposition times 120 and 600 s, respectively. The relative standard deviation was 2.8% (n = 5) for Cr(VI) concentrations of 5 × 10^–8 mol/L. The detection limits (3 σ) for Cr(VI) were 1.0 and 0.12 nmol/L at deposition times of 120 and 600 s, respectively. Typical interferences derived from real water samples are discussed. The method has been applied for the determination of Cr(VI) in spiked natural water samples.     

Determination of the pesticide Chlorpyrifos by cathodic adsorptive stripping voltammetry

The adsorption behavior and differential pulse cathodic adsorptive stripping voltammetry of the pesticide Chlorpyrifos (CP) were investigated at the hanging mercury drop electrode (HMDE). The pesticide was accumulated at the HMDE and a well-defined stripping peak was obtained at –1.2 V vs Ag/AgCl electrode at pH 7.50. A voltammetric procedure was developed for the trace determination of Chlorpyrifos using differential pulse cathodic adsorptive stripping voltammetry (DP-CASV). The optimum working conditions for the determination of the compound were established. The peak current was linear over the concentration range 9.90 × 10^–8– 5.96 × 10^–7 mol/L of Chlorpyrifos. The influence of diverse ions and some other pesticides was investigated. The analysis of Chlorpyrifos in commercial formulations and treated waste water was carried out satisfactorily Received: 10 July 1997 / Revised: 1 April 1998 / Accepted: 6 April 1998     

Linear sweep voltammetric studies on the interaction of brilliant cresyl blue with nucleic acids and its analytical application

In this paper, the interaction of brilliant cresyl blue (BCB) with nucleic acids was studied and further applied for the microdetermination of nucleic acids. In aqueous Britton-Robinson (B-R) buffer solution, BCB can be easily reduced on the hanging mercury drop electrode (HMDE) and had a sensitive voltammetric reduction peak at -0.09 V (vs. SCE). The reduction peak current of BCB could be greatly decreased by the addition of DNA. The results of voltammetric measurements had indicated that a binding reaction was occurred between BCB and DNA and a new supramolecular complex was formed, which resulted in the decrease of the diffusion coefficient of the reaction solution and the decrease of the reduction peak current correspondingly. The conditions of interaction and the electrochemical detection were carefully investigated. Under the selected conditions, the calibration curves for the detection of fish sperm (fs)DNA, calf thymus (ct)DNA and yeast (y)RNA were established. The linear range of this assay was 1.0-30.0 microg/mL for fsDNA, 1.0-45.0 microg/mL for ctDNA and 1.0-25.0 microg/mL for yRNA, respectively. The detection limits were 0.38 microg/mL fsDNA, 0.43 microg/mL ctDNA, 0.64 microg/mL yRNA. The interaction parameters such as the equilibrium constant and the binding number were calculated by electrochemical method. The results showed that the 2:3 type of complex was formed in the fsDNA-BCB complex with the binding constant as 2.51 x 10(7). The proposed method was further applied to the synthetic samples determination with satisfactory results.     

Nonlinear phenomena at mercury Hg electrode/room-temperature ionic liquid (RTIL) interfaces: polarographic streaming maxima and current oscillation

The polarographic streaming maxima and cyclic voltammetric anodic current oscillation (CVACO) at a hanging mercury drop electrode (HMDE) in room-temperature ionic liquid (RTIL) have been studied for the first time using cyclic voltammetric, potential step chronoamperometric and pulse voltammetric techniques. The reversible redox reaction of the 2,1,3-benzothiadiazole (BTD)/BTD*- (an anion radical of BTD) couple with a formal potential (E0') of -1.36 V versus Ag/AgCl/NaCl(saturated) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) RTIL was typically employed for this purpose. A maximum was observed at the rising part of the normal pulse voltammogram for the reduction of BTD to BTD*- as well as of the reversed pulse voltammogram for the reoxidation of BTD*- to BTD at the HMDE. The conditions of the initiation and control of the CVACO at the HMDE in EMIBF4 were extensively investigated. Generally, the CVACO was enhanced by increasing the concentration of BTD at a given potential scan rate (upsilon) and was attenuated by increasing upsilon. An electrocapillary curve was measured using a dropping mercury electrode in EMIBF4, and the potential of zero charge was determined to be -0.23 V. On the basis of the modern theory of the polarographic streaming maxima of the first kind, the observed streaming maxima and CVACO phenomena are successfully explained to originate from the macroscopic instability at the electrode/solution interface wherein the oscillating mode creates the CVACO.     

Silica-modified electrode for the selective detection of mercury

Pure silica particles were dispersed within carbon paste and the resulting modified electrode was applied to the selective voltammetric detection of mercury(II) species after their accumulation at open circuit. The remarkable selectivity observed between pH 4 and 7 was attributed to the intrinsic adsorption mechanism which involves a condensation reaction between mercury(II) hydroxide and hydroxyl groups on the silica surface, leading to the formation of an inner-sphere-type surface complex. After optimization with respect to the electrode composition, the detection medium, and the voltammetric scan mode, a linear response was obtained in the concentration range between 2 × 10⁻⁷ M to 1 × 10⁻⁵ M, by applying anodic stripping square wave voltammetry. Various silica samples were used and their sorption behavior was discussed in relation to their specific surface area and porosity. The effect of chloride and pH on the accumulation of mercury(II) on silica was also investigated. Received: 4 September 1999 / Accepted: 5 January 2000     

Cathodic adsorptive stripping voltammetric detection of tRNA by labelling with osmium tetroxide

This communication reports about adsorptive stripping voltammetric determination of baker’s yeast tRNA that was modified with the complex osmium tetroxide bipyridine. The uracil and cytosine bases are able to react with [OsO₄(bipy)] within 2 h. We observed a 42-fold higher sensitivity for the catalytic Os-peak on the hanging mercury drop electrode (HMDE) compared with an [OsO₄(bipy)]-modified 20-base ssDNA. We found a 0.27 μg/l detection limit for [OsO₄(bipy)]-tRNA. A linear calibration function was observed up to 3 μg/l. The effect of accumulation potential was very small. These findings possibly indicate a strong adsorption on the HMDE. Such labelling of tRNA with [OsO₄(bipy)] holds great promise for future biosensor application regarding the detection of other RNA species.     

Speciation of trace amounts of aluminium in percolating water of forest soil by online coupling HPLC-ICP-MS

Procedures were developed for the speciation of trace amounts of aluminium present in percolating water of forest soil by online coupling of different chromatographic separation methods to an ICP-MS detection system. Inorganic and organic aluminium species were fractionated on a cation exchange column IONPAC CG12 (10-32). Phytotoxic polymeric aluminium hydroxides, as e.g. Al₁₃ (AlO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺), were determined using pyrocatechol violet (PCV) as a species dependant complexing reagent prior to the cation exchange step. Size fractionation of the organic aluminium species was obtained by size exclusion chromatography using the columns Superdex-75-HR 10/30 and Superdex-Peptide-HR 10/30. Validation of the speciation procedures proved that online coupling HPLC to the element selective and sensitive ICP-MS detection system leads to low detection limits of 0.3–0.6 μg/L and high precision and reproducibility (1.2–3.5%) of the speciation procedures. Speciation data determined for aluminium in a percolating water of the Zierenberg catchment are given. Received: 20 November 1998 / Revised: 28 January 1999 / Accepted: 3 February 1999     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Cathodic adsorptive stripping voltammetry of 6-thiopurine in absence and presence of some metal ions

Summary Cathodic stripping voltammetry of an adsorbed 6-thiopurine at HMDE was investigated in solutions of varying pH. A rapid and sensitive differential pulse voltammetric method was selected for its trace determination. A method has also been developed for the determination of 6-thiopurine in presence of Cu(II), due to the strong adsorption of the Cu-6-thiopurine complex at the surface of the HMDE and subsequent reduction of the surface-bound complex. A detection limit of 9.9×10^–9 mol/l was achieved in presence of Cu(II) and the slope of the straight line was seven times the slope in absence of Cu(II). Cathodic adsorptive stripping (CAS) voltammetry of 6-thiopurine in presence of Ni(II), Pb(II), Cd(II), and Fe(III) was also investigated. The influence of several operational parameters has been considered. Statistical analysis of the calibration curve data is included.     

Comparison of different flow-through adsorptive stripping voltammetric methods for the determination of molybdenum(VI)

Four different complexing reagents namely chloranilic acid, oxine, tropolone, and cupferron were applied for AdSV determinations of molybdenum. The parameters for the determination using a flow-through cell with a hanging mercury drop (HMDE) as working electrode were examined systematically for all four systems and evaluated. Cyclic voltammograms were recorded to examine the electrode reaction, alternating current (AC) voltammetry was used to determine adsorption processes. The comparison includes sensitivity, detection limit, linear concentration range, the susceptibility to interference by organic compounds or foreign ions, and the applicability to sea and tap water samples. The interpretation of the electrode reaction mechanism for the reduction of the Mo cupferron complex published from Jiao et al. [10] was improved. Received: 12 February 1999 / Revised: 10 May 1999 / Accepted: 14 May 1999     

Determination of total nitrogen in food by flow injection analysis (FIA) with a potentiometric differential detection system

A flow injection set-up based on potentiometric detection and gas diffusion device for the determination of total nitrogen in food is described. The detection system consisted of two ammonium-sensitive electrodes placed sequentially and each alternately operating as reference electrode. Tubular electrodes without an inner reference solution were prepared with a PVC membrane composed of nonactin in Tris (2-ethylhexyl) phosphate and potassium tetrakis (4-chlorophenyl) borate to reduce the membrane resistance. The food sample digests were inserted into the system, and the ammonium present was converted into ammonia gas. The gas diffused through a gas-permeable membrane to a buffer acceptor stream with a pH that ensured transformation to the ammonium cation, which was potentiometrically detected. Good agreement between FIA results and those provided by the reference procedure was obtained, with relative deviation errors below 5%. Using the proposed system, low reagent consumption is possible, a sampling rate of about 30 samples/h was achieved, as well as a good reproducibility for consecutive injections of the same sample (variation coefficient < 2%). Received: 8 October 1998 / Revised: 7 January 1999 / Accepted: 12 January 1999     

Determination of allura red in some food samples by adsorptive stripping voltammetry

Square wave (SW) voltammetry was used to explore the adsorption properties of the food additive dye Allura Red on a hanging mercury drop electrode (HMDE). By using the adsorptive stripping voltammetric approach, we developed a sensitive electroanalytical method for the determination of this azo dye. A well-developed voltammetric peak probably related to the cathodic reduction of the azo moiety was obtained in pH 9 Britton-Robinson (B-R) buffer at 613 mV. Cyclic voltammetric studies indicated that the reduction process was irreversible and primarily controlled by adsorption. The adsorptive voltammetric signal was evaluated with respect to various experimental conditions; the optimized values were supporting electrolyte, B-R buffer; pH 11; accumulation time, 180 s; accumulation potential, 0.0 V; scan rate, 900 mV/s; pulse amplitude, 75 mV; and SW frequency, 90 Hz. Adsorptive voltammetric peak current showed a linear response for Allura Red in the concentration range of 2.5 x 10(-8) to 2.0 x 10(-7) mol/L (r = 0.998). The limit of detection was 8.5 x 10(-9) mol/L (4.2 ng/mL), the precision in terms of relative standard deviation was 1.3%, and the mean recovery was 102%. Possible interferences by several substances usually present in food products such as food additive azo dyes (E110, E102), gelatin, natural and artificial sweeteners, preservatives, and antioxidants were also evaluated. The proposed electrochemical procedure was successfully applied to the determination of this food dye in commercially available candy and a soft drink. The results were compared by statistical evaluation with those obtained by a reference spectrophotometric method.     

Molecularly imprinted polymer-based solid-phase extraction combined with molecularly imprinted polymer-based sensor for detection of uric acid

The development of an easy-to-use, rapid, robust and inexpensive technique is required which can measure the basal concentration of uric acid (UA) lower than 1.0 x 10(-7)M ( approximately 0.017 mgL(-1)) in biological samples to attend the problem of hypouricemia. In the present work an artificial receptor for UA, silica gel-bonded molecularly imprinted polymer (MIP), was used as a sorbent for molecularly imprinted solid-phase extraction (MISPE) in column chromatography. The use of a sensor based on a MIP-modified hanging mercury drop electrode (HMDE), as reported from our laboratory, could estimate UA with detection limit as low as 0.024 mgL(-1) under the optimized conditions of differential pulse, cathodic stripping voltammetric (DPCSV) measurement. However, in the current investigation, with the use of the combination of MISPE followed by detection with a MIP-based HMDE sensor, the minimum detectable concentration could go down to 0.0008 mgL(-1) (RSD=0.63%, S/N=3). The same MIP receptor for both MISPE and the corresponding sensor was able to enhance the preconcentration of analyte substantially so as to attain the desired level of sensitivity; and that to without any interference (cross-reactivity) from other structurally related analogues including the major interferent like ascorbic acid prevalent in the aqueous environment of biological samples.