Simultaneous Determination of Aromatic Amines by Liquid Chromatography Coupled with Carbon Nanotubes/Poly (3-methylthiophene) Modified Dual-Electrode

Liquid chromatography with amperometric detection (LC-AD) is developed and applied to simultaneously determine five aromatic amines. In the LC-AD, a new carbon nanotubes/poly(3-methylthiophene) modified dual-electrode is fabricated and then used as the working electrode. It is found that this chemically modified electrode (CME) exhibits efficiently electrocatalytic oxidation for aromatic amines with relatively high sensitivity, stability and long-life. Thus, lower detection in LC-AD can be achieved, which are 4.0 × 10^–8 mol L^–1 for aniline, 1.6 ×10^–7 mol L^–1 for 4-nitroaniline, 1.0 × 10^–7 mol L^–1 for 4-chloroaniline, 1.5 × 10^–7 mol L^–1 for 1-naphthylamine, 1.7 × 10^–7 mol L^–1 for 2-bromoaniline. The recoveries of the five analytes are also determined, which range between 0.95 and 1.05 for drinking water, 0.86 and 1.10 for the LiWa River water.     

A novel amperometric sensor and chromatographic detector for determination of parathion

A novel amperometric sensor and chromatographic detector for determination of parathion has been fabricated from a multi-wall carbon nano-tube (MWCNT)/Nafion film-modified glassy-carbon electrode (GCE). The electrochemical response to parathion at the MWCNT/Nafion film electrode was investigated by cyclic voltammetry and linear sweep voltammetry. The redox current of parathion at the MWCNT/Nafion film electrode was significantly higher than that at the bare GCE, the MWCNT-modified GCE, and the Nafion-modified GCE. The results indicated that the MWCNT/Nafion film had an efficient electrocatalytic effect on the electrochemical response to parathion. The peak current was proportional to the concentration of parathion in the range 5.0×10^–9–2.0×10^–5 mol L^–1. The detection limit was 1.0×10^–9 mol L^–1 (after 120 s accumulation). In high-performance liquid chromatography with electrochemical detection (HPLC–ED) a stable and sensitive current response was obtained for parathion at the MWCNT/Nafion film electrode. The linear range for parathion was over four orders of magnitude and the detection limit was 6.0×10^–9 mol L^–1. Application of the method for determination of parathion in rice was satisfactory.     

Simple and Fast Determination of Warfarin in Pharmaceutical Samples Using Boron‐doped Diamond Electrode in BIA and FIA Systems with Multiple Pulse Amperometric Detection

This paper proposes the use of the boron‐doped diamond electrode (BDDE) in flow and batch injection analysis (FIA and BIA) systems with multiple‐pulse amperometric (MPA) detection for the determination of warfarin (WA) in pharmaceutical formulations. The electrochemical behavior of WA obtained by cyclic voltammetry (CV) in 0.1 mol L⁻¹ phosphate buffer shows an irreversible oxidation process at +1.0 V (vs Ag/AgCl). The MPA was based on the application of two sequential potential pulses as a function of time on BDDE: (1) for WA detection at +1.2 V/100 ms and; (2) for electrode surface cleaning at −0.2 V/200 ms. Both hydrodynamic systems (FIA‐MPA and BIA‐MPA) used for WA determination achieved high precision (with relative standard deviations around 2 %, n =10), wide linear range (2.0−400.0 μmol L⁻¹), low limits of detection (0.5 μmol L⁻¹) and good analytical frequency (94 h⁻¹ for FIA and 130 h⁻¹ for BIA). The WA determination made by the proposed methods was compared to the official spectrophotometric method. The FIA‐MPA and BIA‐MPA methods are simple and fast, being an attractive option for WA routine analysis in pharmaceutical industries.     

Voltammetric study of glibenclamide at carbon paste and Sephadex-modified carbon paste electrodes

The oxidative behaviour of the antidiabetic agent glibenclamide on a bare carbon paste electrode (CPE) and a Sephadex-modified carbon paste electrode (SMCPE) was explored by cyclic and differential pulse voltammetry (DPV). The analysis procedure consisted of an open circuit accumulation step in stirred sample solution of Britton-Robinson buffer (0.04 mol L^–1, pH 2.0). This was followed by medium exchange to a clean solution of Britton-Robinson buffer (0.04 mol L^–1, pH 5.0), and subsequently an anodic potential scan was effected to obtain the voltammetric peak. The glibenclamide oxidation peak current obtained by DPV was proportional to the concentration of the glibenclamide in the range of 1.0×10^–9 mol L^–1 to 5.0×10^–8 mol L^–1 for 180 s accumulation time, with a detection limit of 4.0×10^–10 mol L^–1. A method was developed for the determination of glibenclamide in formulation and spiked human serum. Moreover, the proposed procedure was used to estimate the serum concentrations after oral administration of a 5 mg tablet of glibenclamide to three diabetic subjects.     

Determination of trace europium by use of the new fluorescence system europium–sparfloxacin–1,10-phenanthroline–sodium dodecyl sulfate

A new lanthanide-sensitized luminescence system: europium–sparfloxacin–1,10-phenanthroline–sodium dodecyl sulfate has been discovered. The spectrofluorimetric properties of the system were studied. The effect of experimental conditions on the fluorescence intensity was defined. Under the optimum conditions, the fluorescence intensity of the system is a linear function of the concentration of europium in the range 5.0×10^–9–1.0×10^–6 mol L^–1 and the detection limit is 1.0×10^–10 mol L^–1. The system was used for the determination of trace amounts of europium in rare earth samples with satisfactory results.     

Rapid determination of telmisartan in pharmaceuticals and serum by the parallel catalytic hydrogen wave method

The polarographic characteristics of telmisartan have been investigated in 0.8 mol L^–1 NH₃.H₂O–NH₄Cl (pH 8.9)–0.01 mol L^–1 H₂O₂ as supporting electrolyte. The results demonstrate that the polarographic reduction wave at ca. –1.30 V in the absence of H₂O₂ is a catalytic hydrogen wave, and the reduction wave enhanced by H₂O₂ is a so-called parallel catalytic hydrogen wave. The analytical sensitivity of the parallel catalytic hydrogen wave is ca. 60 times higher than that of the corresponding catalytic hydrogen wave. Based on the parallel catalytic hydrogen wave a novel method has been developed for determination of telmisartan by linear sweep polarography. The calibration curve is linear in the range 2.0×10^–8–2.0×10^–6 mol L^–1 and the detection limit is 1.0×10^–8 mol L^–1. The precision is excellent with relative standard deviations of 2.6% at a concentration of 1.0×10^–7 mol L^–1 telmisartan. The proposed method has been applied to the direct determination of the telmisartan in capsule forms and biological samples. The proposed method has been proved to be advantageous over existing CZE and MEKC methods in simplicity, rapidity, and reproducibility.     

Spectrophotometric determination of manganese with derivatives of 1,3,3-trimethyl-2-[3-(1,3,3-trimethyl-1,3- H-indol-2-ylidene)propenyl]-3 H-indolium

A new, simple, rapid, and sensitive spectrophotometric method has been developed for the determination of manganese in sewage. The method is based on the reaction of manganese with derivatives of 1,3,3-trimethyl-2-[3-(1,3,3-trimethyl-1,3-H-indol-2-ylidene)propenyl]-3H-indolium to form a colored ion associate with a sensitive absorption maximum at 560 nm. The appropriate reaction conditions have been established: pH 8.5–10.0, 1.25–2.3×10^–3 mol L^–1 1-nitroso-2-naphthol, and 1.6–2.4×10^–4 mol L^–1 dye reagent. Beer's law is obeyed for manganese concentrations up to 4.2 mg L^–1. The limit of detection is 0.01 mg L^–1 Mn²⁺; the molar absorptivity of the ion associate was 7.5×10⁴ L mol^–1 cm^–1. The effect of various foreign ions was examined. A reaction mechanism is suggested. The developed procedure was tested for determination of manganese in sewage with satisfactory precision and accuracy.     

Flow‐Injection Pulsed‐Amperometric Determination of Free Glycerol in Biodiesel at a Gold Electrode

This work reports the highly‐sensitive amperometric determination of free glycerol in biodiesel at a gold electrode adapted in a flow‐injection analysis (FIA) cell. The amperometric method involved the continuous application of three sequential pulses to the working electrode (+250 mV, +700 mV, and ?200 mV, for 100 ms each). This sequence of potential pulses eliminated electrode passivation and dramatically increased the analytical signal. The proposed FIA‐amperometric method presented low relative standard deviation between injections (1.5 %, n=15), high analytical frequency (85 h^?1), satisfactory recovery values (93–118 %) for spiked samples, wide linear range (from 1 to 300 µmol L^?1), and low detection limit (0.5 µmol L^?1).     

Batch‐injection versus Flow‐injection Analysis Using Screen‐printed Electrodes: Determination of Ciprofloxacin in Pharmaceutical Formulations

This article highlights the potential use of multi‐walled carbon‐nanotube modified screen‐printed electrodes (SPEs) for the amperometric sensing of ciprofloxacin and compares the association of batch‐injection analysis (BIA) and flow‐injection analysis (FIA) with amperometric detection. Both analytical systems provided precise (RSD<5 %) and sensitive determination of ciprofloxacin (LOD<0.1 μmol L^?1) within wide linear range (up to 200 μmol L^?1). Accuracy of both methods was attested by recovery values (93–107 %) and comparison with capillary electrophoresis. The BIA system is completely portable (especially due to association with SPEs) and provided faster analyses (130 h^?1) and more sensitive detection than the FIA system due to the higher flow rates of injection.     

Determination of levodopa methyl ester and its metabolites in rat serum by CZE with amperometric detection

A reliable and reproducible method, capillary zone electrophoresis with amperometric detection (CZE–AD), has been developed for separation and quantification of levodopa methyl ester (LDME) and its biotransformation products levodopa (L-DOPA) and dopamine (DA) in rat serum. A carbon-disk electrode was used as working electrode. The optimum conditions for CZE detection were 50 mmol L^–1 phosphate solution at pH 7.0 as running buffer, 17 kV as separation voltage, 1.0 V (vs Ag/AgCl, 3.0 mol L^–1) as detection potential, and sample injection for 8 s at 17 kV. The linear ranges were from 2.4×10^–2 to 2.2 g mL^–1 for LDME, 2.9×10^–1 to 49.5 g mL^–1 for L-DOPA, and 1.4×10^–2 to 1.5 g mL^–1 for DA with correlation coefficients of 0.9997, 0.9994, and 0.9999, respectively. The detection limits for LDME, L-DOPA, and DA were 14.6, 98.0, and 9.7 ng mL^–1, respectively. Recoveries were 80.3% for LDME, 93.5% for L-DOPA, and 86.5% for DA. This method was applied to serum samples after intravenous injection of LDME and L-DOPA to rats.     

Determination of four kinds of carbamate pesticides by capillary zone electrophoresis with amperometric detection at a polyamide-modified carbon paste electrode

In this paper, a polyamide-modified carbon paste electrode in capillary zone electrophoresis with amperometric detection (CZE-AD) was firstly applied to the determination of four carbamate pesticides: fenobucarb, isoprocarb, metolcarb and carbaryl. The four carbamates were hydrolyzed in alkalescent aqueous solutions, resulting in the formation of 2-sec-butylphenol, 2-isopropylphenol, m-cresol and α-naphthol, which could be determined by amperometry after capillary electrophoretic separation. Under the selected optimum conditions, the four analytes could be perfectly separated within 23 min. The linear ranges of 2-sec-butylphenol, 2-isopropylphenol and m-cresol were from 1.0 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹ and that of α-naphthol was from 2.0 × 10⁻⁷ to 2.0 × 10⁻⁵ mol L⁻¹ and their detection limits were 3.0 × 10⁻⁸, 3.0 × 10⁻⁸, 3.0 × 10⁻⁸ and 6.0 × 10⁻⁸ mol L⁻¹, respectively (S/N = 3). Fenobucarb, isoprocarb, metolcarb and carbaryl can be indirectly determined by this CZE-AD method with recovery of 105, 104, 110 and 98% and R.S.D. of 4, 3, 4 and 3%, respectively. Above results demonstrated that this method was of high sensitivity, good repeatability and could be used in the rapid determination of the pesticide residues.     

Enzyme sensor for L-lactate with a chitosan-mercury film electrode

Summary An amperometric enzyme sensor composed of a mercury film electrode and an enzyme-immobilized chitosan membrane is developed. This biosensor is based on both a mercury film electrode detecting the consumption of dissolved dioxygen following enzymatic reaction, and a chitosan membrane. The latter provides an excellent permselectivity and excludes electroactive interferents. The detection range of this biosensor was 1.0×10^–5–3.0×10^–4 mol/l and the relative standard deviation, R.S.D. at 5.0×10^–5 mol/l was 1.4% (n=3). This biosensor was applied to the direct determination of L-lactate in human serum without pretreatment.     

Evaluation of free hydroxyl radical scavenging activities of some Chinese herbs by capillary zone electrophoresis with amperometric detection

Due to the severe damage caused by free hydroxyl radicals (OH·) to cells and tissues, there is much interest in finding and studying effective and non-toxic OH· scavengers, including traditional Chinese herbs. In this paper, the simple and highly-sensitive technique of capillary zone electrophoresis with amperometric detection (CZE-AD) was used to study the OH· scavenging activities of aqueous extracts from some traditional Chinese herbs. Salicylic acid (SAL) was used as an OH· trap, and the content of OH· could be determined by assaying their products, 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA). The optimum conditions for CZE-AD for the determination of 2,3-DHBA and 2,5-DHBA were explored. The linearity ranges of 2,3-DHBA and 2,5-DHBA were 1.0 ×10^–7~1.0 ×10^–4 mol L^–1, and their detection limits were as low as 2×10^–8 mol L^–1, which were much better than the CE-UV method often used. The traditional Chinese herbs studied included Radix angelicae sinensis, Rhizoma coptidis, Ligustrum lucidum, Ligusticum wallichii, Radices glycyrrhizae and Semen plantaginis. The experiments showed that the aqueous extracts from all of the above traditional Chinese herds had free OH· scavenging activities, although to different degrees.     

Voltammetric behaviour and determination of moxifloxacin in pharmaceutical products and human plasma

The oxidative behaviour of moxifloxacin was studied at a glassy carbon electrode in different buffer systems using cyclic, differential pulse, and Osteryoung square-wave voltammetry. The oxidation process was shown to be irreversible over the entire pH range studied (2.0–10.0) and was diffusion-controlled. The methods were performed in Britton–Robinson buffer and the corresponding calibration graphs were constructed and statistical data were evaluated. When the proposed methods were applied at pH 6.0 linearity was achieved from 4.4×10^–7 to 1.0×10^–5 mol L^–1. Applicability to tablets and human plasma analysis was illustrated. Furthermore, a high-performance liquid chromatographic method with diode-array detection was developed. A calibration graph was established from 4.0×10^–6 to 5.0×10^–5 mol L^–1 moxifloxacin. The described methods were successfully employed with high precision and accuracy for estimation of the total drug content of human plasma and for pharmaceutical dosage forms of moxifloxacin.     

Assay for uric acid level in rat striatum by a reagentless biosensor based on functionalized multi-wall carbon nanotubes with tin oxide

A novel reagentless amperometric uric acid biosensor based on functionalized multi-wall carbon nanotubes (MWCNTs) with tin oxide (SnO₂) nanoparticles has been developed. This was successfully applied to assay uric acid levels from an in vivo microdialysis sampling. Compared with unfunctionalized or traditional carboxylic acid (–COOH)-functionalized MWCNTs, the MWCNTs–SnO₂ electrode exhibited higher electrocatalytic oxidation to uric acid. Here, MWCNTs–SnO₂ may act as an efficient promoter, and the system exhibited a linear dependence on the uric acid concentration over the range from 1.0 × 10^–7 to 5.0 × 10^–4 mol L^–1. In addition, there was little ascorbic acid interference. The high sensitivity of the MWCNTs–SnO₂ modified enzyme electrode enabled the monitoring of trace levels of uric acid in dialysate samples in rat striatum.     

Determination of N-Acetyl-l-cysteine Ethyl Ester (NACET) by Flow Injection Analysis and Spectrophotometric Detection Using Different Thiol-Sensitive Ligands

A new flow injection spectrophotometric method for the determination of N-acetyl-l-cysteine ethyl ester (NACET) was developed and validated. The method is based on the reduction of Cu(II)-ligand complexes to chromophoric Cu(I)-ligand complexes with the analyte. The studied ligands were neocuproine (NCN), bicinchoninic acid (BCA) and bathocuproine disulfonic acid (BCS). The absorbance of the Cu(I)-ligand complex was measured at 458, 562 and 483 nm for the reactions of NACET with NCN, BCA and BCS, respectively. The method was validated in terms of linear dynamic range, limit of detection and quantitation, accuracy, selectivity, and precision. Experimental conditions were optimized by a univariate method, yielding linear calibration curves in a concentration range from 2.0 × 10⁻⁶ mol L⁻¹ to 2.0 × 10⁻⁴ mol L⁻¹ using NCN; 2.0 × 10⁻⁶ mol L⁻¹ to 1.0 × 10⁻⁴ mol L⁻¹ using BCA and 6.0 × 10⁻⁷ mol L⁻¹ to 1.2 × 10⁻⁴ mol L⁻¹ using BCS. The achieved analytical frequency was 90 h⁻¹ for all three ligands. The method was successfully employed for NACET determination in pharmaceutical preparations, indicating that this FIA method fulfilled all the essential demands for the determination of NACET in quality control laboratories, as it combined low instrument and reagent costs with a high sampling rate.     

Determination of thiol compounds in rat striatum microdialysate by HPLC with a nanosized CoHCF-modified electrode

A cobalt hexacyanoferrate (CoHCF) nanoparticle (size ca. 60 nm) chemically modified electrode (CME) was fabricated and the electrochemical behavior of thiols at this nanosized CoHCF CME was studied. In comparison with a bare glassy carbon (GC) electrode and with a general CoHCF CME which was electrodeposited in the traditional manner, the present nanosized CoHCF CME efficiently performed electrocatalytic oxidation for glutathione (GSH) and L-Cysteine (L-Cys) with relatively high sensitivity, outstanding stability, and long-life. Combined with high-performance liquid chromatography (HPLC), the nanosized CoHCF CME was used for electrochemical determination (ECD) of GSH and L-Cys. The peak currents were a linear function of concentrations in the range 2.0×10^–7 to 2.0×10^–4 mol L^–1 for both GSH and L-Cys, with detection limits of 1.2×10^–7 and 1.0×10^–7 mol L^–1, respectively. Coupled with microdialysis sampling, the HPLC–ECD system has been successfully used to assess the GSH and L-Cys content of rat striatum.     

Determination of subnanomolar concentrations of cobalt by adsorptive stripping voltammetry at a bismuth film electrode

Procedures for trace cobalt determinations by adsorptive stripping voltammetry at in situ and ex situ plated bismuth film electrodes are presented. These exploit the enhancement of the cobalt peak obtained by using the Co(II)–dimethylglyoxime–cetyltrimethylammonium bromide–piperazine-N,N-bis(2-ethanesulfonic acid) system. The calibration graph for an accumulation time of 120 s was linear from 2 × 10^–10 to 2 × 10^–8 mol L^–1. The relative standard deviation from five determinations of cobalt at a concentration of 5 × 10^–9 mol L^–1 was 5.2%. The detection limit for an accumulation time of 300 s was 1.8 × 10^–11 mol L^–1. The proposed procedure was applied to cobalt determination in certified reference materials and in tap and river water samples.     

Perchlorate-selective membrane electrode based on a new complex of uranil

A potentiometric ion-selective electrode based on new compound, as a carrier, has been successfully developed for detection of perchlorate anion in aqueous solution. Within the perchlorate ion concentration range 1.0×10^–6 to 1.0 mol L^–1 the electrode had a linear response with a Nernstian slope of 60.6±1.0 mV per decade . The limit of detection as determined from the intersection of the extrapolated linear segments of the calibration plot was 8.0×10^–7 mol L^–1. The proposed electrode has fairly a good discriminating ability towards ClO₄^– ion in comparison to other anions. The sensor has a response time of 10 s and can be used for at least 2 months without substantial divergence in potential. It was successfully applied to direct determination of perchlorate in urine and water.     

Micro-pumping flow system for spectrophotometric determination of anionic surfactants in water

A novel procedure has been developed for spectrophotometric determination of anionic surfactants in water using a solenoid micro-pump as fluid-propulsion device. The proposed method is based on substitution of methyl orange (MO) by anionic surfactants in the formation of an ion-pair with the cetyl pyridine ion (CPC⁺) at pH 5.0. The flow network comprised four solenoid micro-pumps which, under microcomputer control, enabled sample and reagent introduction, and homogenisation in the reaction zone. The system is flexible and simple to operate and control, and sensitive and precise. The analytical plot for the anionic surfactant was linear between 1.43×10^–6 and 1.43×10^–5 mol L^–1 (0.5 to 5.0 mg L^–1; R=0.997, n=5). The relative standard deviation was 0.8% (n=11) for a sample containing 5.74×10^–6 mol L^–1 (2 mg L^–1) surfactant. The limit of detection was 9.76×10^–8 mol L^–1 (0.034 mg L^–1) and the sampling throughput was 60 determinations per hour. The results obtained for washing-water samples were comparable with those obtained by use of the reference method, and no significant differences at the 95% confidence level were observed.