Flow-injection chemiluminescence determination of tetracyclines with in situ electrogenerated bromine as the oxidant

By designing a novel flow-through electrolytic cell (FEC), bromine was produced near to the surface of the platinum electrode by electrochemical oxidation of acidic KBr. The fast and weak chemiluminescence signal produced by the chemical reaction of the electrogenerated bromine with H₂O₂ was greatly enhanced by tetracyclines Based on these observations, a new, sensitive and simple electrogenerated chemiluminescence (ECL) method for the determination of tetracyclines was developed. Under the optimum experimental conditions, the calibration graphs are linear over the range 3.0×10⁻⁸ to 5.0×10⁻⁵ g ml⁻¹ for tetracycline, 2.0×10⁻⁷ to 2.4×10⁻⁵ g ml⁻¹ for oxytetracycline and 1.0×10⁻⁷ to 5.0×10⁻⁵ g ml⁻¹ for chlortetracycline. The limits of detection (S/N=3) are 1.0×10⁻⁸ g ml⁻¹ for tetracycline, 7.0×10⁻⁸ g ml⁻¹ for oxytetracycline and 1.5×10⁻⁷ g ml⁻¹ for chlortetracycline. For the determination 5.0×10⁻⁷ g ml⁻¹ tetracycline, the relative standard deviation was <5%. The proposed method was used to determine tetracyclines in pharmaceutical formulations.     

The electrogenerated chemiluminescent behavior of hemin and its catalytic activity for the electrogenerated chemiluminescence of lucigenin

The electrogenerated chemiluminescent (ECL) behavior of hemin at a platinum electrode in the alkaline solution has been investigated in detail. Under the optimum conditions the linear response range of hemin is 1.0×10⁻⁵–1.0×10⁻⁸ g ml⁻¹, the detection limit was 1.0×10⁻⁸ g ml⁻¹, and the relative standard derivation for 1×10⁻⁷ g ml⁻¹ hemin was 2.8%. It has been also found that hemin would catalyze the ECL of lucigenin at a platinum electrode in a neutral solution in the presence of hydrogen peroxide, the catalytic ECL intensity was linear with the concentration of hemin in the range of 1.0×10⁻¹⁴–1.0×10⁻¹⁰ g ml⁻¹. IgG labeled with hemin was used to examine the ECL catalytic activity of hemin after conjugating to protein, and the results showed that hemin retained ECL catalytic activity when conjugated to protein.     

Voltammetric study of the interaction of ciprofloxacin-copper with nucleic acids and the determination of nucleic acids

The behavior of the ciprofloxacin (CPFX) complex with copper, Cu(II)L₂, at a mercury electrode has been investigated in borax–boric acid buffer. The adsorption phenomena were observed by linear sweep voltammetry. The mechanism of the electrode reaction was found to be reduction of Cu(II)L₂ adsorbed on the surface of the electrode by an irreversible charge transfer to metal amalgam, Cu(0)(Hg). In the presence of DNA, the formation of the electrochemically non-active complexes Cu(II)L₂–DNA results in the decrease of the equilibrium concentration of Cu(II)L₂ and its peak current. Under the optimum conditions, the decrease of the peak current is proportional to DNA concentration. The linear ranges are 6.67×10⁻⁸ to 1.20×10⁻⁵ g ml⁻¹ for calf thymus DNA (ctDNA), 3.30×10⁻⁸ to 2.33×10⁻⁶ g ml⁻¹ for fish sperm DNA (fsDNA) and 1.0×10⁻⁸ to 1.2×10⁻⁶ g ml⁻¹ for yeast RNA. The detection limits are 5.00×10⁻⁹, 3.00×10⁻⁹ and 2.50×10⁻⁹ g ml⁻¹, respectively. This method exhibits good recovery and high sensitivity.     

Preparation of multiwall carbon nanotubes film modified electrode and its application to simultaneous determination of oxidizable amino acids in ion chromatography

A multiwall carbon nanotubes (MWNTs) film modified electrode was prepared and used as an amperometric sensor for the simultaneous determination of oxidizable amino acids including cysteine, tryptophane and tyrosine. The electrochemical behaviors of these amino acids at this modified electrode were studied by cyclic voltammetry (CV). The results indicated that the MWNTs chemically modified electrode (CME) exhibited efficient electrocatalytic activity towards the oxidation of these amino acids with relatively high sensitivity, stability and long-life. Following separation by ion chromatography (IC) with 2.0×10⁻³ mol l⁻¹ citric acid buffer solution (pH 6.5) as eluent, cysteine, tryptophane and tyrosine could be determined by the MWNTs CME successfully. Under the optimal conditions, the detection limits were 7.0×10⁻⁷ mol l⁻¹ for cysteine, 2.0×10⁻⁷ mol l⁻¹ for tryptophane and 3.5×10⁻⁷ mol l⁻¹ for tyrosine at the signal-to-noise of 3, respectively. The method was applied successfully to the determination of these substances in plasma.     

Spectrophotometric determination of nitrite based on its catalytic effect on the oxidation of carminic acid by bromate

A highly sensitive and selective method is described for the determination of trace amounts of nitrite based on its effect on the oxidation of carminic acid with bromate. The reaction was monitored spectrophotometrically by measuring the decrease in absorbance of carminic acid at 490 nm after 3 min of mixing the reagents. The optimum reaction conditions were 1.8×10⁻¹ mol l⁻¹ H₂SO₄, 3.8×10⁻³ mol l⁻¹ KBrO₃, and 1.2×10⁻⁴ mol l⁻¹ carminic acid at 30°C. By using the recommended procedure, the calibration graph was linear from 0.2 to 14 ng ml⁻¹ of nitrite; the detection limit was 0.04 ng ml⁻¹; the R.S.D. for six replicate determinations of 6 ng ml⁻¹ was 1.7%. The method is mostly free from interference, especially from large amounts of nitrate and ammonium ions. The proposed method was applied to the determination of nitrite in rain and river water.     

Electron transfer in organometallic clusters : XI. Redox chemistry of M3(CO)12(M = Ru, Os) and PPh3 derivatives; mechanism of catalysed nucleophilic substitution

The generality of a two-electron reduction process involving an mechanism has been established for M₃(CO)₁₂ and M₃(CO)₁₂−n(PPh₃)_n (M = Ru, Os) clusters in all solvents. Detailed coulometric and spectral studies in CH₂Cl₂ provide strong evidence for the formation of an ‘opened’ M₃(CO)₁₂²⁻ species the triangulo radical anions M₃(CO)₁₂^−· having a half-life of < 10⁻⁶ s in CH₂Cl₂. However, the electrochemical response is sensitive to the presence of water and is concentration dependent. An electrochemical response for “opened” M₃(CO)₁₂²⁻ is only detected at low concentrations < 5 × 10⁻⁴ mol dm⁻³ and under drybox conditions. The electroactive species ground at higher concentrations and in the presence of water M₃(CO)₁₁²⁻ and M₆(CO)₁₈²⁻ were confirmed by a study of the electrochemistry of these anions in CH₂Cl₂; HM₃(CO)₁₁⁻ is not a product. The couple [M₆(CO)₁₈]^−/2− is chemically reversible under certain conditions but oxidation of HM₃(CO)₁₁⁻ is chemically irreversible. Different electrochemical behaviour for Ru₃(CO)₁₂ is found when [PPN][X] (X = OAc⁻, Cl⁻) salts are supporting electrolytes. In these solutions formation of the ultimate electroactive species [μ-C(O)XRu₃(CO)₁₀]⁻ at the electrode is stopped under CO or at low temperatures but Ru₃(CO)₁₂^−· is still trapped by reversible attack by X presumably as [η¹-C(O)XRu₃(CO)₁₁]⁻. It is shown that electrode-initiated electron catalysed substitution of M₃(CO)₁₂ only takes place on the electrochemical timescale when M = Ru, but it is slow, inefficient and non-selective, whereas BPK-initiated nucleophilic substitution of Ru₃(CO)₁₂ is only specific and fast in ether solvents particulary THF. Metal---metal bond cleavage is the most important influence on the rate and specificity of catalytic substitution by electron or [PPN]-initiation. The redox chemistry of M₃(CO)₁₂ clusters (M = Fe, Ru, Os) is a consequence of the relative rates of metal---metal bond dissociation, metal-metal bond strength and ligand dissociation and in many aspects resembles their photochemistry.     

Highly selective and sensitive copper(II) membrane coated graphite electrode based on a recently synthesized Schiff’s base

A PVC membrane electrode based on bis-2-thiophenal propanediamine (TPDA) coated directly on graphite is described. The electrode exhibits a Nernstian response for Cu²⁺ over a very wide concentration range (1.0×10⁻¹ to 6.0×10⁻⁸ M) with a detection limit of 3.0×10⁻⁸ M (2.56 ng ml⁻¹). It has a fast response time and can be used for at least 2 months without any major deviation. The proposed sensor revealed very good selectivities for Cu²⁺ over a wide variety of other metal ions and could be used in the pH range of 3.0–7.0. It was successfully used for direct determination of copper in black tea and as an indicator electrode in potentiometric titration of copper ion.     

Catalytic flow injection determination of vanadium by oxidation of N-(3-sulfopropyl)-3,3',5,5'-tetramethylbenzidine using bromate

A catalytic flow-injection (FI) method was developed for the determination of 10⁻⁹ mol l⁻¹ levels of vanadium(IV, V). The method is based on the catalytic effect of vanadium(V) on oxidation of N-(3-sulfopropyl)-3,3′,5,5′-tetramethylbenzidine (TMBZ·PS) using bromate as oxidant to form a yellow dye (λ_max=460 nm). The use of 5-sulfosalicylic acid (SSA) as an activator enhanced the sensitivity of the method. The calibration graphs with a working range 0.05–8.0 ng ml⁻¹ were obtained for vanadium(V). Vanadium(IV) was also determined, being oxidized by bromate. The detection limit (signal/noise, S/N=3) was 0.01 ng ml⁻¹ (ca. 2×10⁻¹⁰ mol l⁻¹) vanadium. The relative standard deviations (R.S.D.) for 15 determinations of 0.5 ng ml⁻¹ vanadium, and for ten determinations of 0.1 and 1.0 ng ml⁻¹ vanadium were 0.41, 2.6 and 0.25%, respectively, with a sampling rate of 15 samples h⁻¹. The proposed method was successfully applied to the determination of vanadium in natural waters.     

Amperometric detection of amino acids in a flow-injection system with a nickel(II)-modified electrode with an Eastman-AQ polymer film

Amperometic flow measurements were made at +0.55 V (vs. Ag/AgCl) in 0.1 mol l⁻¹ KOH electrolyte with an Ni(II) chemically modified electrode (CME) with an Eastman-AQ polymer film. The use and characteristics of a Ni(II)-containing crystalline and polymer-modified electrode obtained by a double coating step as a detector for amino acids in a flow-injection system using reversed-phase liquid chromatography are described. The detection of these analytes is based on the higher oxidation state of nickel (NiOOH) controlled by the applied potential. The electroanalytical parameters and the detection current for a series of amines and amino acids were investigated. The use of such a CME in the flow-injection technique was found to be suitable in a solution at low pH. The linear range for glycine is 5 × 10⁻⁶-0.1 mol l⁻¹ with a detection limit of 1.0×10⁻⁶ mol l⁻¹. A 1 × 10⁻⁴ mol l⁻¹ mixture of serine and tyrosine was also detected after separation on an Nucleosil C₁₈ column.     

Flow-injection chemiluminescence determination of chlortetracycline using on-line electrogenerated [Cu(HIO(6))(2)](5-) as the oxidant

A novel chemiluminescence (CL) flow system for the determination of chlortetracycline is described. It is based on the direct CL reaction of chlortetracycline and [Cu(HIO₆)₂]⁵⁻ in KOH medium. The unstable [Cu(HIO₆)₂]⁵⁻ was on-line electrogenerated by constant-current electrolysis. The CL intensity was linear with chlortetracycline concentration in the range of 0.1–100 μg ml⁻¹. The determination limit was 5.3×10⁻⁸ g ml⁻¹. The whole process could be completed in 1 min. The proposed method is suitable for automatic and continuous analysis, and has been applied satisfactorily to analysis of chlortetracycline in biological fluid.     

Measuring quaternary ammonium cleaning agents with ion selective electrodes

Data for coated-wire, ion selective electrodes (ISEs) are presented for cationic surfactant ions found in common cleaners including benzyldimethyltetradecylammonium, benzyldimethyldodecylammonium, and benzyldimethylhexadecylammonium. The ion exchangers dinonylnaphthalene sulfonic acid, tetraphenyborate, and tetrakis(4-chlorophenyl)borate are examined, showing dinonylnaphthalene sulfonic acid to be the favored species. The ISEs exhibit approximately Nernstian behavior down to the 10⁻⁶ M limit of detection with lifetimes in excess of 50 days when used continuously, and a shelf life of over 100 days. Reaching the upper detection limit at the critical micelle concentration requires use of polymeric-membrane reference electrodes including a new membrane cocktail, which allow response measurements of an order of magnitude higher than the traditional fritted-glass reference electrode. The surfactant ISEs show excellent selectivity over the common metal ions Na⁺, K⁺, Mg²⁺, Ca²⁺, and Cu²⁺ with selectivity coefficients less than 10^−5.3. The ISEs are also selective over the lower molecular weight quaternary ammonium ions tetradecyltrimethylammonium, dodecyltrimethylammonium, benzyldimethyl(2-hydroxyethyl)ammonium, and tetrabutylammonium with selectivity coefficients ranging from 10^−1.7 to 10^−5.5. Use of a single electrode to determine accurately the total cationic surfactant concentration in common cleaning solutions is accomplished with information about concentration dependent interferences and a modified Nikolsky–Eisenman model. Finally, quaternary ammonium surfactants have a deleterious effect on the measurements of pH and common ions like K⁺, Mg²⁺ and Ca²⁺ with polymeric ISEs. This makes it critical to include surfactant electrodes in a detector array when cleaning agents are present.     

Amperometric biosensors based on the immobilization of oxidases in a Prussian blue film by electrochemical codeposition

Prussian blue has been formed by cyclic voltammetry onto the basal pyrolytic graphite surface to prepare a chemically modified electrode which provides excellent electrocatalysis for both oxidation and reduction of hydrogen peroxide. It is found for the first time that glucose oxidase or -amino oxidase can be incorporated into a Prussian blue film during its electrochemical growth process. Two amperometric biosensors were fabricated by electrochemical codeposition, and the resulting sensors were protected by coverage with a thin film of Nafion. The influence of various experimental conditions was examined for optimum analytical performance. The glucose sensor responds rapidly to substrates with a detection limit of 2 × 10⁻⁶ M and a linear concentration range of 0.01–3 mM. There was no interference from 2 mM ascorbic acid or uric acid. Another ( -amino acid) sensor gave a detection limit of 3 × 10⁻⁵ M -alanine, injected with a linear concentration range of 7.0 × 10⁻⁵-1.4 × 10⁻² M. Glucose and -amino acid sensors remain relatively stable for 20 and 15 days, respectively. There is no obvious interference from anion electroactive species due to a low operating potential and excellent permselectivity of Nafion.     

Electrocatalytic response of norepinephrine at a thiolactic acid self-assembled gold electrode

The thiolactic acid (TLA) self-assembled monolayer modified gold electrode (TLA/Au) is demonstrated to catalyze the electrochemical response of norepinephrine (NE) by cyclic voltammetry. A pair of well-defined redox waves were obtained and the calculated standard rate constant (k_s) is 5.11×10⁻³ cm s⁻¹ at the self-assembled electrode. The electrode reaction is a pseudo-reversible process. The peak current and the concentration of NE are a linear relationship in the range of 4.0×10⁻⁵–2.0×10⁻³ mol l⁻¹. The detection limit is 2.0×10⁻⁶ mol l⁻¹. By ac impedance spectroscopy the apparent electron transfer rate constant (k_app) of Fe(CN)³⁻/Fe(CN)⁴⁻ at the TLA/Au electrode was obtained as 2.5×10⁻⁵ cm s⁻¹.     

Simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional ion-selective electrodes in series

An on-line automated system for the simultaneous flow injection determination of calcium and fluoride in natural and borehole water with conventional calcium-selective and fluoride-selective membrane electrodes as sensors in series is described. Samples (30 μl) are injected into a TISAB II (pH=5.50) carrier solution as an ionic strength adjustment buffer. The sample-buffer zone formed is first channeled to a fluoride-selective membrane electrode and then via the calcium-selective membrane electrode to the reference electrodes. The system is suitable for the simultaneous on-site monitoring of calcium (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 1.94×10⁻⁶ mol l⁻¹ recovery 99.22%, RSD<0.5%) and fluoride (linear range 10⁻⁵–10⁻² mol l⁻¹ detection limit 4.83×10⁻⁶ mol l⁻¹ recovery 98.63%, RSD=0.3%) at a sampling rate of 60 samples h⁻¹.     

A novel thin-layer amperometric detector based on chemically modified ring-disc electrode and its application for simultaneous measurements of nitric oxide and nitrite in rat brain combined with in vivo microdialysis

A novel thin-layer amperometric detector (TLAD) based on chemically modified ring-disc electrode and its application for simultaneous measurements of nitric oxide (NO) and nitrite (NO₂⁻) in rat brain were demonstrated in this work. The ring-disc electrode was simultaneously sensitive to nitric oxide (NO) and nitrite (NO₂⁻) by modifying its inner disc with electropolymerized film of cobalt(II) tetraaminophthalocyanine (polyCoTAPc)/Nafion and its outer ring with poly(vinylpyridine) (PVP), respectively. The ring-disc electrode was used to constitute a novel TLAD in radial flow cell for simultaneous measurements of NO and NO₂⁻ in rat brain combined with techniques of high performance liquid chromatography (HPLC) and in vivo microdialysis. It was found that the basal concentration of NO in the caudate nucleus of rat brain is lower than 1.0×10⁻⁷ mol l⁻¹, NO₂⁻ concentration is 5.0×10⁻⁷ mol l⁻¹ and NO exists in brain maybe mainly in the form of its decomposed product.     

PVC membrane based potentiometric sensors for uranium determination

Two novel uranyl PVC matrix membrane sensors responsive to uranyl ion are described. The first sensor incorporates tris(2-ethylhexyl)phosphate (TEHP) as both electroactive material and plasticizer and sodium tetraphenylborate (NaTPB) as an ion discriminator. The sensor displays a rapid and linear response for UO₂²⁺ ions over the concentration range 1×10⁻¹–2×10⁻⁵ mol l⁻¹ UO₂²⁺ with a cationic slope of 25.0±0.2 mV decade⁻¹. The working pH range is 2.8–3.6 and the life span is 4 weeks. The second sensor contains O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uronium hexafluorophosphate (TPTU) as a sensing material, sodium tetraphenylborate as an ion discriminator and dioctyl phenylphosphonate (DOPP) as a plasticizer. Linear and stable response for 1×10⁻¹–5×10⁻⁵ mol l⁻¹ UO₂²⁺ with near-Nernstian slope of 27.5±0.2 mV decade⁻¹ are obtained. The working pH range is 2.5–3.5 and the life span of the sensor is 6 weeks. Interference from many inorganic cations is negligible for both sensors. However, interference caused by some ions (e.g. Th⁴⁺, Cu²⁺, Fe³⁺) is eliminated by a prior ion exchange or solvent extraction step. Direct potentiometric determination of as little as 5 μg ml⁻¹ uranium in aqueous solutions shows an average recovery of 97.2±1.3%. Application for the determination of uranium at levels of 0.01–1 wt.% in naturally occurring and certified ores gives results with good correlation with data obtained by X-ray fluorescence.     

Role of axial ligation on potentiometric response of Co(III) tetraphenylporphyrin-doped polymeric membranes to nitrite ions

Two types of Co(III) tetraphenylporphyrins, Co(III)TPPX (I) and Co(III)(N)TPPX (II), where X = C1⁻ or NO⁻₂ and N = C₅H₅N or C₆H₅CH₂C₅H₄N, are used as ionophores to prepare nitrite responsive polymeric membrane electrodes. The influence of the initial axial ligand (X⁻ and N) on the operative ionophore mechanism of these metalloporphyrins within the solvent polymeric membranes is examined. Results from potentiometric and electrodialysis experiments suggest that in the presence of nitrite in the test sample and internal solution, both types of Co (III) porphyrins studied (I and II) act as neutral carriers and that the addition of lipophilic cationic sites (e.g., tridodecylmethylammonium ions (TDMA⁺)) to the organic membrane is essential to improve the selectivity and long term stability of sensors prepared with these species. Membranes formulated with (I) or (II) in the nitrite form along with TDMACl in plasticized PVC films exhibit the following selectivity sequence: SCN⁻ > NO⁻₂ ˜ C1O⁻₄ > Sal⁻ > NO⁻₃ > Br⁻ > C1⁻. Membrane electrodes with added lipophilic cationic sites are shown to exhibit rapid, fully reversible and Nernstian response towards nitrite ions in the concentration range of 10⁻¹–10⁻⁵ M, with good long term stability.     

Electrochemical study and flow-injection amperometric detection of trace NO(2)(-) at CuPtCl(6) chemically modified electrode

A thin film of mixed-valent CuPtCl₆ is deposited on a glassy carbon electrode by continuous cyclic scanning in a solution containing 3×10⁻³ M CuCl₂+3×10⁻³ M K₂PtCl₆+1 M KCl in the potential range from 700 to −800 mV. The cyclic voltammetry is used to study the electrochemical behaviors of nitrite on CuPtCl₆/GC modified electrode and the electrode displays a good catalytic activity toward the oxidation of nitrite. The effects of the film thickness, pH, the electrode stability and precision have been evaluated. Experiments in flow-injection analysis are performed to characterize the electrode as an amperometric sensor for the detection of nitrite. The modified electrode shows a wide dynamic range, quite a low detection limit and short response time. The linear relationship between the flow-injection peak currents and the concentrations of nitrite is at a range of 1×10⁻⁷–2×10⁻³ M with a detection limit of 5×10⁻⁸ M.     

Determination of potassium ions in biodiesel using a nickel(II) hexacyanoferrate-modified electrode

In this work, nickel hexacyanoferrate-modified electrode was developed to determine potassium ions in biodiesel by potentiometry. The modified electrodes exhibit a linear response to potassium ions in the concentration range of 4.0 × 10⁻⁵ to 1.0 × 10⁻² mol L⁻¹, with a detection limit of 1.9 × 10⁻⁵ mol L⁻¹, and a near-Nernstian slope (53–55 mV per decade) at 25 °C. The method developed in this work was compared with flame photometry and the potassium concentration found in biodiesel showed that the modified electrode method gives results similar to those obtained by flame photometry.     

Fingerprint patterns from laser-induced Azido photochemistry of spin-labeled photoaffinity ATP analogs in matrix-assisted laser desorption/ionization mass spectrometry

The photochemical reaction of azide derivatives induced by ultraviolet (UV) laser in matrix-assisted laser desorption/ionization mass spectrometry (MALDI) is reported. A novel synthesized class of azide aromatic derivatives, spin-labeled photoaffinity non-nucleoside adenosine triphosphate (ATP) analogs which are useful probes in study of muscle contraction mechanism, is used in this investigation. In the negative ion MALDI spectra of these ATP analogs, “fingerprint” peaks corresponding to [M − 10 − 1]⁻, [M − 12 − 1]⁻, [M − 16 − 1]⁻, [M − 26 − 1]⁻, [M − 28 − 1]⁻, [M − 41 − 1]⁻, and [M − 42 − 1]⁻ were observed with relative intensities depending on the MALDI matrix. Only the [M − 16 − 1]⁻ is present in the similar mass spectra of the analog in which the azido group is replaced by a hydrogen. A model is suggested for the photochemical reactions of azide derivatives under UV laser irradiation. The photoreaction fingerprint information is diagnostically useful in characterization of azido compounds, especially for spin-labeled photoaffinity non-nucleoside ATP analogs.