Optical fiber sensor for berberine based on fluorescence quenching of 2-(4-diphenylyl)-6-phenylbenzoxazole

A new optical fiber sensor was prepared for the determination of berberine in aqueous solution using a micrometer-sized flow-cell and a bifurcated optical fiber. The sensing is based on fluorescence quenching of 2-(4-diphenylyl)-6-phenylbenzoxazole (PBBO) in the PVC membrane. This process is accompanied by non-fluorescent ground-state complex formation. With this sensor, berberine can be determined in sample solutions from 2.42 × 10^–5 mol L^–1 to 6.04 × 10^–7 mol L^–1. Satisfactory reproducibility, reversibility, and short response times of less than 1 min are realized. The sensor also shows good selectivity over some common pharmaceutical species and alkali and alkali-earth metal salts, and can be used for the direct assay of berberine in commercial tablets. The results are in correspondence with those obtained by the pharmacopoeia method. Received: 4 April 1997 / Revised: 8 August 1997 / Accepted: 12 August 1997     

Chelating resin preconcentration and Hg(II) assisted elution followed by differential-pulse anodic stripping voltammetry for the determination of Cd, Cu, and Pb in seawater

A new and efficient Hg(II) back-elution method for the desorption of Cd, Cu, and Pb from Chelex-100 chelating resin was developed. A smaller eluent volume and shorter elution time can be achieved using an Hg(II) containing eluent rather than pure nitric acid. Owing to the remaining Hg(II) ion in the effluent, a mercury thin-film electrode is formed in-situ during the anodic stripping voltammetric determination without any further addition of Hg(II). The results indicate that all the analytes in seawater matrix can be completely adsorbed on Chelex-100 resin from the sample at pH 6.5, and subsequently eluted from the resin with an acid solution of 5 × 10^–4 mol/L Hg²⁺ + 1 mol/L HClO₄. The detection limits obtained from the differential-pulse anodic (μg L^–1 to ng L^–1) stripping voltammetry are at sub-ppb to ppt (μg L^–1 to ng L^–1) levels permitting to determine Cd, Cu and Pb traces in seawater. The analytical reliability was confirmed by the analysis of the certified reference material CASS-II (open ocean seawater). Received: 22 April 1997 / Revised: 5 August 1997 / Accepted: 7 August 1997     

Determination of N-acetyl-L-cysteine by flow-injection potentiometry

A multi-purpose tubular flow-through sensor was constructed with an AgI-based membrane. The membrane was prepared by pressing silver salts (AgI, Ag₂S) and powdered Teflon. This membrane was incorporated in the tubular flow-through sensor body. A 2-mm diameter hole was drilled through the center of the tubular sensor and the membrane, thus determining the active sensor volume of about 4 μL. The tubular sensor with reference electrode was placed into a complex flow-injection system and used for the flow-through determination of N-acetyl-L-cysteine, (NAC), in perchloric acid medium, pH = l. Linear dependence was established between the recorded signal height and the concentration of NAC in the injected sample. The recorded change in potential for a decade change in concentration, 62 mV {p (NAC)}^–1, in the concentration range from 1 × 10^–4 to 1 × 10^–1 mol L^–1, was based on the formation mechanism of the sparingly soluble deposit between silver and NAC on the surface of the sensitive part of the membrane. Received: 7 July 1997 / Revised: 22 September 1997 / Accepted: 26 September 1997     

The europium/samarium-2-benzoyl-indane-1,3-dione-cetyltrimethylammonium bromide fluorescence system and its analytical application

The fluorescence system of the Eu³⁺ (or/and Sm³⁺)-2-benzoyl-indane-1,3-dione(BID)-cetyltrimethylammonium bromide (CTMAB) was investigated at excitation wavelengths of 350 nm, and emission wavelengths at 612 nm for europium and 565, 606 and 650 nm for samarium, respectively. The system was used for the determination of Eu and Sm in rare earth oxides and of BID in water. Europium or/and samarium could be determined in the range of 1.0 × 10^–9–1.0 × 10^–5 mol/L and 2.0 × 10^–8–5.0 × 10^–5 mol/L, respectively; 1.0 × 10^–6–2.0 × 10^–5 mol/L of BID could also be determined. Received: 5 March 1997 / Revised: 6 August 1997 / Accepted: 20 September 1997     

The europium/samarium-2-benzoyl-indane-1,3-dione-cetyltrimethylammonium bromide fluorescence system and its analytical application

The fluorescence system of the Eu³⁺ (or/and Sm³⁺)-2-benzoyl-indane-1,3-dione(BID)-cetyltrimethylammonium bromide (CTMAB) was investigated at excitation wavelengths of 350 nm, and emission wavelengths at 612 nm for europium and 565, 606 and 650 nm for samarium, respectively. The system was used for the determination of Eu and Sm in rare earth oxides and of BID in water. Europium or/and samarium could be determined in the range of 1.0 × 10^–9–1.0 × 10^–5 mol/L and 2.0 × 10^–8–5.0 × 10^–5 mol/L, respectively; 1.0 × 10^–6–2.0 × 10^–5 mol/L of BID could also be determined. Received: 5 March 1997 / Revised: 6 August 1997 / Accepted: 20 September 1997     

Flow injection chemiluminescence determination of medazepam

A new flow injection chemiluminescence method for the assay of medazepam is explored. The method involves the use of permanganate in sulfuric acid for the oxidation of medazepam with the emission of chemiluminescence detected by a photomultiplier tube. A simplex procedure was employed for optimising the conditions for high sensitivity detection, which were found to be 1.03 × 10^–3 mol L^–1 permanganate, 0.153 mol L^–1 sulfuric acid and 3.43 mL min^–1 flow rate. The linear calibration range was 3.7 × 10^–5 to 1.7 × 10^–3 mol L^–1. The detection limit (3σ) and the sample throughput were 1.85 × 10^–5 mol L^–1 and 100 per hour, respectively. The relative standard deviation for 5 replicate determinations of 1.9 × 10^–4 mol L^–1 medazepam was 0.15%. Common excipients (starch, glucose, maltose, lactose) used in pharmaceutical preparations had no effect. Received: 2 February 1998 / Revised: 20 May 1998 / Accepted: 25 May 1998     

Highly selective thiocyanate poly(vinyl chloride) membrane electrode based on a cadmium–Schiff’s base complex

A PVC membrane electrode based on a cadmium–salen (N,N′-bis-salicylidene-1,2ethylenediamine) complex as an anion carrier is described. The electrode has an anti-Hofmeister selectivity sequence with a preference for thiocyanate at pH 1.5–11.0. It has a linear response to thiocyanate from 1.0 × 10^–6 to 1.0 × 10^–1 mol L^–1 with a slope of 59.1 ± 0.2 mV per decade, and a detection limit of 7 × 10^–7 mol L^–1. This electrode has high selectivity for thiocyanate relative to many common organic and inorganic anions. The proposed sensor has a fast response time of approximately 15 s. It was applied to the determination of thiocyanate in a milk sample. Received: 1 December 2000 / Revised: 19 April 2001 / Accepted: 30 April 2001     

Flow injection-solid phase spectrofluorimetric determination of pyridoxine in presence of group B-vitamins

A flow-through optosensor has been prepared for the sensitive and selective determination of pyridoxine (vitamin B₆) in aqueous solutions. The sensor was developed in conjunction with a monochannel flow-injection analysis system with fluorimetric detection using Sephadex SP-C25 resin as an active sorbent substrate. This method of determination is carried out without any derivatization. The wavelengths of excitation and emission were 295 and 385 nm, respectively. When a HCl (10^–3 mol L^–1) / NaCl (3 × 10^–2 mol L^–1) solution is used as carrier solution, the sensor responds linearly in the measuring range of 5–200, 10–400 and 50–1800 ng mL^–1 with detection limits of 0.33, 0.67, and 5.70 ng mL^–1 for 2000, 1000 and 200 μL of sample volume, respectively. The relative standard deviation for ten independent determinations is less than 0.75% for 0.2 and 1.0 mL of sample volumes used, and 1.31% for 2.0 mL of sample volume used. The method was satisfactorily applied to the determination of vitamin B₆ in pharmaceutical preparations. Received: 4 June 1998 / Revised: 16 July 1998 / Accepted: 6 August 1998     

Covalent modification of glassy carbon electrodes with glycine for voltammetric separation of dopamine and ascorbic acid

Glycine was covalently grafted on to a glassy carbon electrode (GCE) by amine cation radical formation in electrooxidation of the amino-containing compound. X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry proved the immobilization of glycine on the GCE. The modified electrode reduced the overpotentials of dopamine (DA) and ascorbic acid (AA) by approximately 0.15 V and 0.23 V, respectively, and resolved the overlapping voltammetric response of DA and AA into two well-defined voltammetric peaks in cyclic voltammetry (CV) or differential pulse voltammetry (DPV), unlike the unmodified GCE; this can be used for the simultaneous determination of these species in a mixture. The differential pulse peak current was linearly dependent on DA and AA concentration in the range 5 × 10^–6– 8 × 10^–4 mol L^–1 and 6 × 10^–5– 4 × 10^–3 mol L^–1, with correlation coefficients of 0.996 and 0.994, respectively. The detection limits (3δ) for DA and AA were 1.8 × 10^–6 mol L^–1 and 2.1 × 10^–5 mol L^–1, respectively. The modified electrode is very sensitive, selective and stable, and has been applied to the determination of DA and AA simultaneously in samples with satisfactory results. Received: 30 October 2000 / Revised: 9 March 2001 / Accepted: 13 March 2001     

Voltammetric determination of cadmium(II) using a chemically modified electrode

An 1-(pyridylazo)-2-naphthol modified glassy carbon electrode has been investigated as sensor for the measurement of trace levels of Cd²⁺. Cd²⁺ is deposited on the surface of a PAN modified glassy carbon electrode at –1.10 V (vs. SCE) via forming Cd²⁺–PAN and subsequent reduction at the electrode. In the following step, Cd-PAN is oxidized, and voltammograms are recorded by scanning the potential in a positive direction. Calibration plots were found to be linear in the range 2 × 10^–8 mol/L to 8 × 10^–7 mol/L. The detection limit was 5 × 10^–10 mol/L, and the coefficient of variation, determined on one single electrode at a concentration of 5 × 10^–7 mol/L, was calculated to be 3.2% (n = 5). Using this new kind of modified electrode, trace levels of Cd(II) in water samples were determined; the average recovery was calculated to be 98.78%. Received: 17 August 2000 / Revised: 19 December 2000 / Accepted: 27 December 2000     

Spectrofluorometric determination of hesperidin by manual and flow-injection methods

A reliable and highly sensitive method for the determination of hesperidin is described. It involves the formation of a highly fluorescent complex between hesperidin and aluminium (III) in a micellar medium. There is a linear relationship between fluorescence intensity (λ_em = 496 nm, λ_ex = 391 nm) and hesperidin concentration over the range 5 × 10^–7– 2 × 10^–5 mol L^–1. The detection limit is 79 μg L^–1. The method can easily be adapted to a flow system using a three-channel manifold, the peak height being proportional to the hesperidin concentration over the range 1 × 10^–6– 1 × 10^–4 mol L^–1. Manual and flow-injection procedures have been successfully applied to the determination of hesperidin in orange peel and orange juice. Received: 21 October 1998 / Revised: 16 December 1998 / Accepted: 25 December 1998     

Synthesis and application of 4-(N,N-dimethylaminosulfonyl)-7-N-methylhydrazino-2,1,3-benzoxadiazole (MDBDH) as a new derivatizing agent for aldehydes

In a five step synthesis, 4-(N,N-dimethylaminosulfonyl)-7-N-methylhydrazino-2,1,3-benzoxadiazole (MDBDH) was prepared in high yields as a stable new derivatizing agent for carbonyl compounds. Reagent and derivatives have not been described in literature before. Major advantage of this substance compared with similar reagents is its improved solubility in polar solvents, e.g. methanol and ethanol. MDBDH reacts with aldehydes in the presence of an acidic catalyst under formation of the corresponding hydrazones. These are separated by means of reversed-phase liquid chromatography and detected UV/vis spectroscopically at wavelengths around 450 nm, depending on the individual hydrazone. MDBDH reacts with oxidizers as nitrogen dioxide and nitrite to only one product, 4-(N,N-dimethylaminosulfonyl)-7-methylamino-2,1,3-benzoxadiazole (MDBDA), which can easily be separated from the hydrazones of lower aldehydes. Due to large molar absorptivities and absorption maxima at wavelengths > 430 nm, limits of detection range from 4 × 10^–8 to 6 × 10^–8 mol · L^–1, and limits of quantification range from 1 × 10^–7 to 2 × 10^–7 mol · L^–1 for the individual hydrazones. The method was applied to the determination of aldehydes in automobile exhaust. Received: 31 July 2000 / Revised: 9 October 2000 / Accepted: 18 October 2000     

Development of a voltammetric method for the determination of iron(III) in Zn-Fe alloy galvanic baths

A square wave voltammetric method whith a static mercury drop electrode (SMDE) was developed for the quantitative determination of iron (III) in Zn-Fe alloy galvanic baths. Real alloy bath samples were analyzed by the standard addition method and recovery tests were carried out. 0.50 mol L^–1 sodium citrate (pH 6.0) or 0.20 mol L^–1 oxalic acid (pH 4.0) were applied as supporting electrolytes resulting in both cases in a peak potential of about –0.20 V vs. Ag|AgCl (saturated KCl). The iron (III) concentration in the alloy bath was 9.0 × 10^–4 mol L^–1. A good correlation (r = 0.9999) was achieved between the iron (III) concentration and the peak current in the electrolytes studied, with linear response ranges from 1.0 × 10^–6 to 1.2 × 10^–4 mol L^–1. Interference levels for some metals such as copper (II), lead (II), chromium (III) and manganese (II) that can hinder the Zn-Fe alloy deposition were evaluated; only copper (II) interferes seriously. Received: 4 April 2000 / Revised: 19 June 2000 / Accepted: 22 June 2000     

Amperometric method for the determination of myoglobin based on the electrochemical reduction on the glassy carbon electrode

An irreversible reduction peak of oxymyoglobin (MbO₂) was observed on the bare glassy carbon electrode (GCE) in acetate buffer solution under atmospheric conditions. It is the reduction of bonded oxygen in Mb, but not the heme Fe(III)/Fe(II) redox couple that underwent electrochemical reaction on the electrode. The peak current achieved a maximum value in acetate buffer solution of pH 4.0. The peak potential was pH dependent, suggesting that the proton was involved in the electrochemical reaction. Furthermore, the peak current was linearly related to the concentration of myoglobin in the range of 2.5 × 10^–8∼ 1.0 × 10^–6 mol · L^–1 with a detection limit of 5 × 10^–9 mol · L^–1. Received: 20 March 1998 / Revised: 24 June 1998 / Accepted: 1 July 1998     

Synthesis of glycosylated porphyrins as neutral ionophores for a berberine-sensitive electrode

For preparing a berberine-sensitive electrode, 5,10,15,20-tetrakis[2-(2,3,4,6-tetraacetyl-β-D-glucopyranosyl)-1-O-phenyl]porphyrin (T(o-glu)PPH₂) was synthesized from the reaction of pyrrole with ortho-acetylglycosylated benzaldehyde by Lindsay’s method. The electrode based on T(o-glu)PPH₂ with an optimized membrane composition exhibits Nernstian response to berberine in the concentration range 2.4 × 10^–7–5.0 × 10^–3 mol L^–1, with a pH range from 3.9 to 10.2, and a fast response time of 30 s. The electrode shows fair selectivity towards berberine with respect to common co-existing species. T(o-glu)PPH₂ shows better potentiometric response characteristics comparing to chloro[5,10,15,20-tetrakis[2-(2,3,4,6-tetraacetyl-β-D-glucopyranosyl)-1-O-phenyl]-porphinato]-manganese (MnT(o-glu)PPCl) and better selectivity towards berberine than tetraphenylporphyrin (TPPH₂). The effect of the composition of the electrode membrane has been studied and the experimental conditions optimized. The contents of berberine in pharmaceutical tablets were determined by direct potentiometry and the results agreed with values obtained by the pharmacopoeia method. Received: 17 July 2000 / Revised: 18 September 2000 / Accepted: 23 October 2000     

Platinum electrode coated with a bentonite–carbon composite as an environmental sensor for detection of lead

A Pt wire coated with a bentonite–carbon composite in a poly(vinyl chloride) membrane was used for detection of lead. The sensor has a Nernstian slope of 29.42±0.50 mV per decade over a wide range of concentration, 1.0×10⁻⁷ to 1.0×10⁻³ mol L⁻¹ Pb(NO₃)₂. The detection limit is 5.0×10⁻⁸ mol L⁻¹ Pb(NO₃)₂ and the electrode is applicable in the pH range 3.0–6.7. It has a response time of approximately 10 s and can be used at least for three months. The electrode has good selectivity relative to nineteen other metal ions. The practical analytical utility of the electrode is demonstrated by measurement of Pb(II) in industrial waste and river water samples.     

Cobalt(II) ion-selective electrode with solid contact

A new all plastic sensor for Co²⁺ ions based on 2-amino-5 (hydroxynaphtyloazo-1′)-1,3,4 thiadiazole (ATIDAN) as ionophore was prepared. The electrode exhibits a low detection limit of 1.5 × 10⁻⁶ mol L⁻¹ and almost theoretical Nernstian slope in the activity range 4.0 × 10⁻⁶–1 × 10⁻¹ mol L⁻¹ of cobalt ions. The response time of the sensor is less than 10 s and it can be used over a period of 6 months without any measurable divergence in potential. The proposed sensor shows a fairly good selectivity for Co(II) over other metal ions. The electrode was successfully applied for determination of Co²⁺ in real samples and as an indicator electrode in potentiometric titration of Co²⁺ ions with EDTA.      

Optosensing of hydrochloric acid based on the fluorescence quenching of a flavone copolymer

An optical fiber sensor for the continuous determination of hydrochloric acid is presented. It is based on the fluorescence quenching of a flavone containing copolymer membrane by hydrochloric acid. The quenching efficiency is greatly enhanced in the presence of Fe(III). This enhancement is attributed to the primary inner filter effect, as well as the formation of a complex between the 4′-N,N-dimethylaminoflavone group in the copolymer and the Fe(III) species extracted from hydrochloric acid solution. The optical response is linear and reversible for 0.10–6.00 mol L^–1 HCl with a response time of the order of a second. The standard deviations for repeated alternative measurements of 0.20 and 2.00 mol L^–1 hydrochloric acid are 0.32% and 0.46% (n = 10), respectively, indicating a good reproducibility. Because of the covalently bonding of the dye to polymer, the sensor exhibits also a good stability. Selectivity has also been evaluated for some potential interferents. The sensor in conjunction with a flow-injection system can be used for on-line determination of hydrochloric acid. Received: 15 January 1998 / Revised: 14 April 1998 / Accepted: 18 April 1998     

Vermiculite clay mineral as an effective carbon paste electrode modifier for the preconcentration and voltammetric determination of Hg(II) and Ag(I) ions

A vermiculite modified carbon paste electrode (VMCPE) was employed for the in situ preconcentration of traces of Hg(II) and Ag(I) via an ion-exchange route. Heavy metal ions were accumulated in Britton-Robinson (BR) buffer pH 7 for Hg(II) and pH 6 for Ag(I), and afterwards reduced at –0.7 V vs. Ag/AgCl in the separate measurement solution (BR buffer pH 5 + 0.05 mol/L NaNO₃) prior to the anodic stripping square-wave voltammetric (ASSWV) detection. For Hg(II) ions, at 15 min accumulation, a linear range from 1.0 × 10^–7 to 8.0 × 10^–6 mol/L was obtained, with a 5.7 × 10^–8 mol/L limit of detection. The VMCPE response was linear for Ag(I) ions in the concentration range from 2.0 × 10^–7 to 8.0 × 10^–6 mol/L, at 10 min accumulation with a corresponding limit of detection of 6.3 × 10^–8 mol/L. The relative standard deviation of the analytical procedure including accumulation from a 5 × 10^–7 mol/L solution of Hg (15 min) or Ag(I) (10 min), electrolysis, ASSWV detection, regeneration and activation of the VMCPE, was 4% (n = 6). The optimisation of the parameters for the application of the VMCPE in combination with ASSWV detection is presented and discussed. Received: 10 July 1997 / Revised: 31 October 1997 / Accepted: 3 November 1997     

Flow-injection analysis based on a membrane separation module and a bulk acoustic wave impedance sensor – determination of the volatile acidity of fermentation products

A novel flow-injection analysis (FIA) system has been developed for the rapid determination of the volatile acidity of some fermentation products like vinegars and juices. The proposed method is based on the diffusion of volatile acids, mainly acetic acid, across a PTFE gas-permeable membrane from an acid stream into an alkaline stream, and the acids trapped in the acceptor solution are determined online by a bulk acoustic wave impedance sensor based on changes in the conductivity of the solution. It exhibited a linear frequency response up to 10 mmol · L^–1 acetic acid with a detection limit of 50 μmol · L^–1, and the precision was better than 1% (RSD) at a through-put of 72 h^–1. The effects of operating voltage for the detector, cell constant of the electrode, composition of acceptor stream, flow rates and temperature on the FIA performance were also investigated. Received: 2 June 1997 / Revised: 7 July 1997 / Accepted: 12 July 1997