Flow injection sample processing with nickel oxide electrode amperometric detection of amino acids separated by ion-exchange chromatography

A new automatic post-column sampler has been designed and used for interfacing the column outlet to a flow injection system. Segments of the effluent from the column are injected into the flow injection apparatus at regular intervals. Separation and detection of several amino acids are described. A nickel oxide electrode detector yields characteristic, useful signals for concentrations as low as 10^-6 M. Samples containing 20–35 μg of total amino acids in 1 ml were chromatographed.     

Flow injection potentiometry by a new coated graphite ion-selective electrode for the determination of Pb(2+)

A new graphite coated electrode for the determination of Pb²⁺ based on a recently synthesized ionophore 1-hydroxy-2-{2-[2-(2-hydroxy-ethoxy)-ethoxy]-ethoxy methyl}-anthracene-9,10-dione (L) has been developed. The electrode was used in flow injection potentiometry by a home-made flow cell. Under both the batch and flow conditions, the electrode revealed a near Nernstian response over a wide lead ion concentration range (10⁻⁶ to 10⁻¹ M) and very low limit of detection. In flow injection potentiometry, excellent reproducibility (RSD%=0.49%), very high sampling rate (170 injections h⁻¹) and stable baseline was observed in the presence of 10⁻³ M KCl as ionic strength adjuster. The electrode showed high sensitivity and good selectivity for Pb²⁺ over a wide variety of alkali, alkali earth and transition metal ions and the electrode can be used for at least 3 months without any considerable change in potential response. The proposed sensor was successfully applied to the direct determination of lead in real samples and also used for the titrimetric determination of phosphate ions by both batch and flow injection potentiometry.     

Flow injection,amperometric determination of ethanol in wines after solid-phase extraction

Ethanol in wines was determined by flow injection analysis with an amperometric detector using an oxidized nickel wire. Solid-phase extraction with a strong anion exchanger was used to remove interferences such as organic acids from the matrix, and the residue of the extraction was injected directly into the FIA system. The recoveries of ethanol from wines spiked with standards ranged from 101% to 103%. The response of the nickel electrode to ethanol is dependent on the applied potential and the pH of the carrier. The optimal conditions for the detection of ethanol were an applied potential of +0.60 V (vs. Ag/AgCl) in a carrier of 100 mM sodium hydroxide solution. The electrode exhibited a linear response from 10⁻⁵ to 10⁻³ M, with a detection limit of 1 × 10⁻⁶ M. The method was demonstrated by the determination of ethanol in wines.     

Voltammetric and flow-injection determination of carbohydrates using composite electrodes based on carbon nanotubes and Nickel(II) hexacyanoferrate

The catalytic activities of oxo and hydroxo forms of nickel(II) on the surface of a nickel deposit and an inorganic film of nickel(II) hexacyanoferrate(III) (NiHCF) electrodeposited on a glassy-carbon electrode both unmodified and modified by multi-wall or single-wall (also functionalized) carbon nanotubes in the electrooxidation of glucose, sucrose, and maltose are compared. A more pronounced catalytic effect was obtained in the electrooxidation of these carbohydrates on an electrode modified with functionalized singlewall carbon nanotubes and a NiHCF film. Methods are proposed for the voltammetric and flow-injection determination of carbohydrates on this composite electrode. A linear dependence of the analytical signal on the analyte concentration was observed in the range from 5 × 10^?7 to 5 × 10^?2 M under stationary conditions and from 0.003 to 0.3 μmol under flow conditions.     

A New Poly (Vinyl Chloride) Matrix Membrane Electrode for Manual and Flow Injection Determination of Pilocarpine in Some Pharmaceutical Preparations

Abstract

A novel pilocarpinium ion selective membrane electrode is prepared, characterized and used in pharmaceutical analysis. The electrode system incorporates a PVC membrane with pilocarpine-reineckatc ion pair complex as an electroactive material. The electrode exhibits a fast near-Nernstian response for 10^?1-4x10^?5M pilocarpine over the pH range 4-6.5. The electrode displays a good selectivity for pilocarpine with respect to a number of foreign cations. Pilocarpine in various pharmaccutical preparations is determined either by direct potentiometry or potentiometric titration with NaTPB using the proposed sensor. Pilocarpinereineckate membrane is also used in a flow through sandwich cell as a detector for flow injection determination of pilocarpine. Results with an average recovery of 99% and a relative standard deviation of ± 0.3% are obtained. The data compare favorably with those obtained by the standard US Pharmacopeia method.     

Catalytic Oxidation of Dopamine at a Nickel Hexacyanoferrate Surface Modified Graphite Wax Composite Electrode Coated with Nafion

A chemically modified electrode was successfully fabricated by means of depositing a thin layer of nickel hexacyanoferrate (NiHCF) on an amine adsorbed graphite paraffin wax composite electrode using a new approach. The electrode was further coated with Nafion. The electrochemical characteristics of the modified electrode were studied using cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The modified electrode catalyzed dopamine (DA) oxidation in the concentration range of 1.5×10^?6 to 1.2×10^?3 M without the interference from ascorbic acid (AA). A detection limit of 4.9×10^?7 M was obtained for DA in the presence of AA with a correlation coefficient of 0.9972 based on S/N=3. Flow injection analysis was used for the determination of dopamine with excellent reproducible results. The analytical utility of the sensor was evaluated for detection of DA in urine.     

Determination of total primary amines in seawater and plant nectar with flow injection sample processing and fluorescence detection

Flow injection sample processing is used with fluorescence detection for the determination of total primary amines in seawater and nectar. The effects of carrifer stream flow rate and dispersion tube length on sensitivity and sampling rates were studied. Relative responses of several amino acids and other primary amines were determined using two dispersion tube lengths. Linear calibration curves were obtained over the ranges 0–10^-6 M and O–1O^-5 M glycine. Precisions of better than 2% at 10^-6 M and a detection limit of 1 × lO^-8 M glycine were obtained. Applications to the analysis of seawater from the vicinity of a baited lobster trap and diluted nectar samples from Erythrina sp. are described.     

An electrochemical sensor for dipyrone determination based on nickel-salen film modified electrode

An amperometric dipyrone sensor based on a polymeric nickel-salen (salen = N,N´-ethylenebis(salicydeneiminato)) film coated platinum electrode was developed. The sensor was constructed by electropolymerization of nickel-salen complex at a platinum electrode in acetonitrile/tetrabuthylamonium perchlorate by cyclic voltammetry. After cycling the modified electrode in a 0.50 mol L^-1 KCl solution, the estimated surface concentration was found to be equal to 1.29 x 10^-9 mol cm^-2. This is a typical behavior of an electrode surface immobilized with a redox couple that can usually be considered as a reversible single-electron reduction/oxidation of the nickel(II)/nickel(III) couple. A plot of the anodic current versus the dipyrone concentration for chronoamperometry (potential fixed = +0.50 V) at the sensor was linear in the 4.7 x 10^-6 to 1.1 x 10^-4 mol L^-1 concentration range and the concentration limit was 1.2 x 10^-6 mol L^-1. The proposed electrode is useful for the quality control and routine analysis of dipyrone in pharmaceutical formulations.     

Fabrication of a Nitrate Selective Electrode for Determination of Nitrate in Fertilizers by Using Flow Injection Analysis System

A nitrate ion selective electrode (nitrate-ISE) based on pyrrole modification on a common pencil lead was proposed. The lab-made nitrate-ISE was easily constructed by pyrrole polymerization with nitrate ion as the dopant using cyclic voltammetry. The fabricated nitrate-ISE was then coupled with flow injection analysis (FIA) for an automatic system. The flow potentiometry provided working range of 1x10^-4 to 4x10^-3 mol L^-1 of nitrate and allowed sample throughput up to 50 samples h^-1. Linear regression analysis showed good agreement (r²=0.9961) with Nernstian response. This system was applied to determine nitrate-nitrogen (nitrate-N) in fertilizer samples.     

Application of ion-selective electrodes in environmental analysis : Determination of Acid and Fluoride concentrations in Rain-water with a Flow-Injection System

A flow-injection method is described for the measurement of acid and fluoride concentrations. The conditions were optimized to ensure small sample and reagent consumption, low detection limit and the highest rate of analysis allowed by the potentiometric sensor. With a microcapillary pH-sensitive glass electrode, 20-μl sample volumes and 1.0–1.5 ml min^?1 carrier flow rates, strong acids were determined at concentrations as low as 10^?5 M (0.2 nmol of acid in 20 μ1). The relative standard deviation was about 1% at 10?4 M strong acid concentration at an injection rate of 500–550 h^?1. With a flow- through fluoride-selective electrode, 250-μl sample volumes and a 1 ml min^?1 carrier flow rates, fluoride concentrations as low as 10^?7 M were measured (ca. 0.5 ng of fluoride in 250 μ1). The injection rate was 40 h^?1 at concentrations below 10^?6 M, but 60 h^?1 above 10^?5 M. The methods were used successfully for determining the acid and fluoride concentrations in rain-waters.     

Polarographic determination of pilocarpine using the catalysed pre-wave of nickel(II)

On the basis of a detailed study of the pilocarpine-induced nickel(II) pre-wave using various polarographic techniques, an electrode process mechanism is proposed in which the formation of a catalytic complex between aquo-nickel(II) and veronalate-nickel(II) on the one hand and unprotonated pilocarpine adsorbed on the electrode surface on the other is followed by the reduction of nickel(II) in the complex and the release of the catalytic ligand. The pre-peak recorded by differential-pulse polarography in the system 1 × 10^?3 M Ni(II)-1 × 10^?2 M sodium veronal, nitric acid (pH 8.5) (with ionic strength maintained at 0.2 with sodium nitrate) can be used for quantitative determination of pilocarpine at concentrations in the range 2.5 × 10^?7-8 × 10^?6 M.     

Electrocatalytic oxidation of formaldehyde and methanol on Ni(OH)2/Ni electrode

A nickel hydroxide-modified nickel electrode (Ni(OH)₂/Ni) was successfully prepared by the cyclic voltammetry (CV) method and the electrocatalytic properties of the electrode for formaldehyde and methanol oxidation have been investigated respectively. The Ni(OH)₂/Ni electrode exhibits high electrocatalytic activity in the reaction. A new method has been developed for formaldehyde determination at the nickel hydroxide-modified nickel electrode and the experimental parameters were optimized. The oxidation peak current is linearly proportional to the concentration of formaldehyde in the range of 7.0 × 10^?5 to 1.6 × 10^?2 M with a detection limit of 2.0 × 10^?5 M. Recoveries of artificial samples are between 93.3 and 103.5%. The effect of scan rate and methanol concentration on the electrochemical behavior of methanol were investigated respectively.     

Stopped-flow linear sweep voltammetry at the reticulated vitreous carbon electrode in a flow injection system : Determination of dopamine in the presence of ascorbic acid

A flow injection system was used to introduce a sample in to a flow-through electrochemical cell based on a reticulated vitreous carbon (r.v.c.) electrode. Products of the initial electrolysis are trapped in the porous electrode by stopping the flow. Linear sweep voltammetry under stopped-flow conditions forms the basis of a determination of a reversibly electrolyzed species in the presence of an irreversibly electrolyzed species. The determination of dopamine in the presence of ascorbic acid was used as an illustrative example. Response varied linearly with dopamine concentration between 8 X 10^?7 and 3 X 10^?5 M and the detection limit (S/N = 3) was 5 X 10^?7 M dopamine.     

An enzyme reactor electrode for urea determinations.

An enzyme reactor electrode system for the determination of urea is described. A buffer is pumped through an enzyme reactor (0.4 ml) containing urease immobilized with glutaraldehyde to glass. The effluent is mixed with sodium hydroxide pumped through a second channel and fed through an ammonia gas electrode. Samples are introduced via a third flow channel and mixed with the buffer. The conversion of urea to ammonia is quantitative for sample concentrations of less than 0.03 M for a flow rate of 40 ml h^-1. The reactor electrode shows a Nernstian slope of 57 mV/decade for 5·10^-5–3·10^-2 M urea. The response is independent of variations in the flow rate, enzyme activity or temperature of the reactor.     

A tetra-coordinate nickel(II) complex as neutral carrier for nitrate-selective PVC membrane electrode

The potentiometric response properties and applications of a tetra-coordinate nickel(II) complex with relatively high selectivity toward nitrate ion are described. The nickel(II) complex of 5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradeca-4,6,11,13-tetraene was used as a neutral carrier into plasticized poly(vinyl chloride) (PVC) membrane. The influence of several variables was investigated in order to optimize the potentiometric response and selectivity of the electrode. The resulting membrane electrode incorporating 31.0% PVC, 61.0% dioctyl phthalate (DOP) as plasticizer, 3% methyltrioctylammonium chloride (MTOAC) as a cationic additive and 5% carrier (all w/w) demonstrates a Nernstian response slope of −59.6 mV per decade over the concentration range of 5×10⁻⁶-1×10⁻¹ M NO₃⁻. The electrode exhibits a fast response time (≤10 s), a detection limit of 2.5×10⁻⁶ M, and can be used over a wide pH range of 4-12. The electrode shows improved selectivity in comparison to most of the previously reported nitrate-selective electrodes. It was successfully applied to the determination of nitrate ion in natural water samples.     

3D-printed porous electrodes for advanced electrochemical flow reactors: A Ni/stainless steel electrode and its mass transport characteristics

Porous electrodes have shown high performance in industrial electrochemical processes and redox flow batteries for energy storage. These materials offer great advantages over planar electrodes in terms of larger surface area, superior space time yield and enhanced mass transport. In this work, a highly ordered porous stainless steel structure was manufactured by 3D-printing and coated with nickel from an acidic bath by electrodeposition in a divided rectangular channel flow cell. Following the electrodeposition, the volumetric mass transport coefficient of this electrode was determined by the electrochemical reduction of 1.0×10⁻³ mol dm⁻³ of ferricyanide ions by linear sweep voltammetry and chronoamperometry. The convection diffusion characteristics are compared with other geometries to demonstrate the novelty and the advantages of 3D-printed porous electrodes in electrochemical flow reactors. Robust porous electrodes with tailored surface area, composition, volumetric porosity and flow properties are possible.     

Determination of ethanol by air-stream separation with flow injection and electrochemical detection at a nickel oxide electrode

Ethanol is separated by an air stream from a 1-ml sample with collection in 1 ml of 0.2 mol l^-1 sodium hydroxide. Measurement is by voltammetry at a tubular, catalytic nickel oxide electrode with 30-μl sample injected into a continuous flow stream. Relative standard deviation for repetitive measurements is 1.8% for synthetic samples. Serum samples extracted for 100 s and then measured yielded linear results for concentrations of 2 × 10^-4–5 × 10^-2 mol l^-1 ethanol (0.001–0.23%), with relative standard deviation of 2.0%. The time per determination was about 2 min.     

Methylene Blue Poly(Vinyl Chloride) Membrane Electrode Based on the Methylene Blue-Teteraphenylborate Ion-Pair Complex and Its Application to Pharmaceutical Analysis

Abstract

A methylene blue (MB) poly (vinyl chloride) membrane electrode based on MBtetraphenylborate (TPB) ion-pair complex is described. The linear response covers the range 1x10^?2 - 1x10^?6 mol dm^?3 MB solution, with a slope of 56.5±0.5 mv/decade (pH range 3.0-1 0.0). The detection limit is 7.75x 10^?7 mol dm^?3. The electrode shows stability, good reproducibility and fast response. Interferences from common inorganic cations and some organic bases are negligible. These characteristics of the electrode enabled it to be used successfully for the determination of MB in injection.     

Pulsed Amperometric Detection of Histamine at Glassy Carbon Electrodes Modified with Gold Nanoparticles

Gold nanocrystal‐modified glassy carbon electrodes (nAu‐GCE) were prepared and used for the determination of histamine by flow injection and high performance liquid chromatography using pulsed amperometric detection (PAD) as the detection mode. Experimental variables involved in the electrodeposition process of gold from a HAuCl₄ solution were optimized. A catalytic enhancement of the histamine voltammetric response was observed at the nAu‐GCE when compared with that obtained at a conventional Au disk electrode, as a consequence of the microdispersion of gold nanocrystals on the GC substrate. The morphological and electrochemical characteristics of the nAu‐GCE were evaluated by SEM and cyclic voltammetry. PAD using a very simple potential waveform consisting of an anodic potential (+700 mV for 500 ms) and a cathodic potential (?300 mV for 30 ms), was used to avoid the electrode surface fouling when histamine was detected under flowing conditions. Flow injection amperometric responses showed much higher I_p values and signal‐to‐noise ratios at the nAu‐GCE than at a conventional gold disk electrode. A limit of detection of 6×10^?7 mol L^?1 histamine was obtained. HPLC‐PAD at the nAu‐GCE was used for the determination of histamine in the presence of other biogenic amines and indole. Histamine was determined in sardine samples spiked at a 50 μg g^?1 concentration level, with good results. Furthermore, the chromatographic PAD method was also used for monitoring the formation of histamine during the decomposition process of sardine samples.     

Determination of aliphatic amines by high-performance liquid chromatography-amperometric detection after derivatization with naphthalene-2,3-dicarboxaldehyde

A simple and sensitive liquid chromatographic method has been developed for the determination of low molecular weight aliphatic amines after their pre-column derivatization with naphthalene-2,3-dicarboxaldehyde (NDA). Derivatization conditions, including the NDA concentration, reaction pH and reaction time have been investigated for method optimization. The chromatographic separation of five amines was performed on ABZ PLUS column using mobile phase of methanol-water (80:20, v/v) at a flow rate of 0.2 mL min⁻¹. The detection was carried out with a 6 mm glassy carbon electrode at the applied potential of 0.7 V versus Ag/AgCl reference electrode. The detection limits were between 23.3 and 34.4 nmol L⁻¹ of amines with a sample injection volume of 2 μL. The present method was applied for the determination of aliphatic amines in lake water. The recovery ranged 52.2-127.9%. The RSD in analytes retention time was less than 0.3% and 2.4% for intra- and inter-day analyses, respectively. The RSD in peak area was below 5.8% for both intra-day and inter-day analyses. The total analysis was completed within 20 min.