A novel stopped flow injection-amperometric procedure for the determination of chlorate

A novel stopped flow injection—amperometric (sFI-Amp) procedure for determination of chlorate has been developed. The reaction of chlorate with excess potassium iodide and hydrochloric acid, forming iodine/triiodide that is further electrochemically reduced at a glassy carbon electrode at +200 mV versus Ag/AgCl electrode is employed. In order to increase sensitivity without using of too high acid concentration, promoting of the reaction by increasing reaction time and temperature can be carried out. This can be done without increase of dispersion of the product zone by stopping the flow while the injected zone is being in a mixing coil which is immersed in a water bath of 55 ± 0.5 °C. In a closed system of FIA, a side reaction of oxygen with iodide is also minimized. Under a set of conditions, linear calibration graphs were in the ranges of 1.2 × 10⁻⁶-6.0 × 10⁻⁵ mol l⁻¹and 6.0 × 10⁻⁵-6.0 × 10⁻⁴ mol l⁻¹. A sample throughput of 25 h⁻¹ was accomplished. Relative standard deviation was 2% (n = 21, 1.2 × 10⁻⁴ mol l⁻¹ chlorate). The proposed sFI-Amp procedure was successfully applied to the determination of chlorate in soil samples from longan plantation area.     

A multicommuted flow system for sequential spectrophotometric determination of hydrosoluble vitamins in pharmaceutical preparations

A multicommuted flow system is proposed for spectrophotometric determination of hydrosoluble vitamins (ascorbic acid, thiamine, riboflavine and pyridoxine) in pharmaceutical preparations. The flow manifold was designed with computer-controlled three-way solenoid valves for independent handling of sample and reagent solutions and a multi-channel spectrophotometer was employed for signal measurements. Periodic re-calibration as well as the standard addition method was implemented by using a single reference solution. Linear responses (r=0.999) were obtained for 0.500-10.0 mg l⁻¹ ascorbic acid, 2.00-50.0 mg l⁻¹ thiamine, 5.00-50.0 mg l⁻¹ riboflavine and 0.500-8.00 mg l⁻¹ pyridoxine. Detection limits were estimated as 0.08 mg l⁻¹ (0.5 μmol l⁻¹) ascorbic acid, 0.8 mg l⁻¹ (2 μmol l⁻¹) thiamine, 0.2 mg l⁻¹ (0.5 μmol l⁻¹) riboflavine and 0.1 mg l⁻¹ (0.9 μmol l⁻¹) pyridoxine at 99.7% confidence level. A mean sampling rate of 60 determinations per hour was achieved and coefficients of variation of 1% (n=20) were estimated for all species. The mean reagent consumption was 25-fold lower in relation to flow-based procedures with continuous reagent addition. Average recoveries between 95.6 and 100% were obtained for commercial pharmaceutical preparations. Results agreed with those obtained by reference methods at 95% confidence level. The flow system is suitable for application in quality control processes and in dissolution studies of vitamin tablets.     

Determination of bismuth and zinc in pharmaceuticals by first derivative UV-Visible spectrophotometry

A first order derivative spectrophotometric method has been developed for the simultaneous determination of bismuth and zinc by dithizone without time-consuming extraction step. The reactions of bismuth and zinc with dithizone in a three component solution prepared in water, acetone and n-propanol mixture have been investigated. These cations react with dithizone in this mixture at pH 5.0, forming coloured complex that is stable for at least 2 h. The linear range in D evaluation was between 3.0 × 10⁻⁶ and 1.8 × 10⁻⁵ mol l⁻¹ for Zn and 2.4 × 10⁻⁶ and 1.2 × 10⁻⁵ mol l⁻¹ for Bi. The limits of detection for the analytical procedure were found 0.05 mg l⁻¹ for both cations. The relative standard deviations for the determination of 0.5 mg l⁻¹ bismuth and 0.5 mg l⁻¹ zinc were 1.2 and 1.1%, respectively, for five determinations. The procedure is simple, rapid and reliable. This method was applied to the determination of bismuth and zinc in the pharmaceutical materials successfully. Good agreement was achieved between the results obtained by the proposed and comparative methods.     

Chemiluminometric photo-induced determination of Strychnine in a Multicommutation flow assembly

This paper deals on the determination of Strychnine, a potent and dangerous pesticide and the analytical procedure is based on the photo-induced chemiluminescence of the pesticide by means of the Multicommutation continuous-flow methodology. Small segments of the pesticide solution were sequentially alternated with segments of the solution for adjusting the suitable medium for the photodegradation. The required time of UV irradiation was obtained by stopped-flow during 150 s; then, the resulting solution formed alternated segments with the oxidizing solution containing 5 × 10⁻³ mol l⁻¹ Ce(IV) in 0.6 mol l⁻¹ nitric acid. The calibration range, from 2 μg l⁻¹ to 50 mg l⁻¹, resulted in a linear behaviour over the range 25 μg l⁻¹ to 20 mg l⁻¹ and fitting the equation: I = 4706x + 624 with a correlation coefficient of 0.9955. The limit of detection was 2 μg l⁻¹ and the sample throughput 15 h⁻¹. After testing the influence of a large series of potential interferents, the method was applied to different kinds of samples.     

p-Dimethylaminobenzaldehyde thiosemicarbazone: a simple novel selective and sensitive fluorescent sensor for mercury(II) in aqueous solution

A novel and simple fluorophore, p-dimethylaminobenzaldehyde thiosemicarbazone (DMABTS), was prepared in order to find available fluorescent chemosensor for mercuric ion in aquesous solution. DMABTS emitted fluorescence at 448 nm in aqueous solution and its fluorescence intensity was completely quenched upon interaction with Hg²⁺ ions, which should be attributed to the 1:1 complex formation between DMABTS and Hg²⁺. The binding constant of the complex was determined as 7.48 × 10⁶ mol l⁻¹. The linear range of quantitative detection of 0 to 5.77 × 10⁻⁶ mol l⁻¹ and the detection limit of 7.7 × 10⁻⁷ mol l⁻¹ for Hg²⁺ in the 6.3 × 10⁻⁶ mol l⁻¹ DMABTS aqueous solution were obtained from a calibration curve. The coexistence of several transition metal ions and anions did interfere the fluorometric titration of Hg²⁺ ion by less than 4% in the emission change.     

Analytical strategy photodegradation/chemiluminescence/continuous-flow multicommutation methodology for the determination of the herbicide Propanil

The present paper is dealing with an analytical strategy based on coupling photodegradation, chemiluminescence and multicommutation continuous-flow methodology for the determination of the pesticide Propanil, a common herbicide. The pesticide solution is inserted as small segments sequentially alternated with segments of the solution for adjusting the suitable medium for the photodegradation. Both flow-rates (sample and medium) are adjusted to required time for photodegradation, 2.0 min; and then, the resulting solution is also sequentially inserted as segments alternated with segments of the oxidizing solutions system, 1.00 × 10⁻⁴ mol l⁻¹ potassium permanganate in 2.00 mol l⁻¹ sulphuric acid medium. The calibration range, from 10 μg l⁻¹ to 25 mg l⁻¹, resulted in a linear behaviour over the range 10 μg l⁻¹-5 mg l⁻¹ and fitting the linear equation: I = 780.30C + 95.28; correlation coefficient 0.9999. The limit of detection was 8 μg l⁻¹ and the sample throughput 20 h⁻¹. After testing the influence of a large series of potential interferents the method is applied to water samples obtained from different places and to one formulation. The method is valid for the determination of other pesticides from the same chemical family, namely: alachlor, flumetsulam, furalaxyl and ofurace. Calibration graphs, limits of detection, repeatability and determination in water samples are obtained for each reported pesticide.     

New potentiometric sensors for creatinine

Three main types of creatinine potentiometric membrane sensors are described. They are based on the use of dibenzo-30-crown-10 (DB30C10) with potassium tetrakis(p-chlorophenyl)borate type (I), dibenzo-30-crown-10 alone type (II), and potassium tetrakis(p-chlorophenyl)borate alone type (III), incorporating in poly(vinyl chloride) matrix membrane plasticized with either o-nitrophenyl octyl ether or dioctylphthalate. The sensors are used for quantification of creatinine after soaking the membranes in 0.1 M creatinine solution for 2 days. The sensors show almost the same potentiometric response characteristics. Sensor type (I) exhibits Nernstian responses over a concentration range of 5.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 59.5 ± 0.1 and 60 ± 0.2 mV decade⁻¹ and detection limits of 1.1 × 10⁻⁵ mol l⁻¹ and 8 × 10⁻⁶ mol l⁻¹ creatinine, over the pH range of 3.5-6.5 and 3.5-7.0, for o-NPOE and DOP solvent mediators, respectively. Sensor type (II) displays Nernstian responses over a concentration range of 6.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 60.0 ± 0.1 and 65.0 ± 0.2 mV decade⁻¹ and detection limits of 1.5 × 10⁻⁵ mol l⁻¹ and 1.4 × 10⁻⁵ mol l⁻¹ creatinine over the pH range of 2.6-6.2 and 2.5-6.0, for o-NPOE and DOP solvent mediators, respectively. Sensor type (III) shows Nernstian responses over a concentration range of 7.0 × 10⁻⁵ mol l⁻¹-1.0 × 10⁻² mol l⁻¹ creatinine with cationic slopes of 60 ± 0.1 and 62.0 ± 0.2 mV decade⁻¹ and detection limits of 2.7 × 10⁻⁵ mol l⁻¹ and 2.0 × 10⁻⁵ mol l⁻¹ creatinine over the pH range of 2.5-6.0, for o-NPOE and DOP solvent mediators, respectively. The response times of the sensors for 10⁻³ mol l⁻¹ creatinine solution are instantaneous (4-10 s). The sensors show long-term stability with life span of ∼6 months. The sensors are used for determination of serum creatinine of rats (Rattus Norvigicus) with mean R.S.D. of 2.62%, and the results agreed well with the Jaffe kinetic method.     

Bia-amperometric quantification of salbutamol in pharmaceutical products

This paper presents a simple, rapid and reproducible method of analysis of salbutamol in pharmaceutical products, utilizing batch injection analysis (BIA) associated with amperometric detection. A study of salbutamol oxidation demonstrated a strong dependence between electrode fouling and pH. All determinations were done utilizing a glassy carbon electrode in presence of 3.0 mol l⁻¹ NaOH. A large linear dynamic range from 8×10⁻⁷ to 2×10⁻⁴ mol l⁻¹ was obtained by using an injected volume of 100 μl with a detection limit of 2.5×10⁻⁷ mol l⁻¹. R.S.D. of 0.92% for 50 successive injections of 4×10⁻⁶ mol l⁻¹ of salbutamol and a sample throughput of 60 samples per hour were achieved. The method was applied for salbutamol quantification in syrups.     

Flow-injection turbidimetric determination of homatropine methylbromide in pharmaceutical formulations using silicotungstic acid as precipitant reagent

A flow-injection turbidimetric procedure exploiting merging zones is proposed for determining homatropine methylbromide (HMB) in pharmaceutical preparations. The determination is based on the precipitation reaction of homatropine methylbromide with silicotungstic acid in acidic medium to form a precipitate, which was measured at 410 nm. The analytical curve was linear in the HMB concentration range from 8.1 × 10⁻⁵ to 2.2 × 10⁻⁴ mol l⁻¹, with a detection limit of 5.0 × 10⁻⁶ mol l⁻¹. The recoveries ranged from 96 to 103%, the sampling frequency was 70 determinations per hour and relative standard deviations were less than 1.5% (n = 10). The results obtained for commercial formulations using the FIA procedure were in good agreement with those obtained by using a comparative method.     

Voltammetry determination of trace manganese with pretreatment glassy carbon electrode by linear sweep voltammetry

This paper described the determination of trace manganese using linear sweep voltammetry at a pretreatment glassy carbon electrode. The glassy carbon electrode pretreated by electrochemical method in the 0.1 mol l⁻¹ NaOH solution greatly improved the electrode responsibility in the determination of manganese(II). The barrier to the detection of low manganese concentration was overcome by means of autocatalytic effect of manganese oxide deposited on the electrode in advance. Under the optimum experiments condition (0.04 mol l⁻¹ NH₃-NH₄Cl buffer solution, pH 9.0), the linear range was 4×10⁻⁸ to 1×l0⁻⁶ mol l⁻¹ Mn(II) for linear sweep voltammetry and 1×10⁻⁹ to 4×10⁻⁸ mol l⁻¹ Mn(II) for convolution voltammetry. The relative standard deviation for 2×10⁻⁸ mol l⁻¹ Mn(II) is 3.4%. The proposed method is simple, rapid, sensitive and selective. It had been applied to the determination of trace manganese in samples with satisfactory results.     

Flow-injection chemiluminescence determination of fluoroquinolones by enhancement of weak chemiluminescence from peroxynitrous acid

A flow-injection chemiluminescence (CL) method is described for the determination of fluoroquinolones including ciprofloxacin, norfloxacin and ofloxacin. The method is based on the enhancement by these compounds of the weak CL from peroxynitrous acid. The linear ranges are 1.0×10⁻⁷ to 1.0×10⁻⁵ mol l⁻¹ for ciprofloxacin and norfloxacin, and 3.0×10⁻⁷ to 3.0×10⁻⁵ mol l⁻¹ for ofloxacin, respectively. The detection limits (S/N=3) are 4.5×10⁻⁸ mol l⁻¹ ciprofloxacin, 5.9×10⁻⁸ mol l⁻¹ norfloxacin and 1.1×10⁻⁷ mol l⁻¹ ofloxacin, respectively. The proposed method was applied to the determination of fluoroquinolones in pharmaceutical preparations.     

Determination of noradrenaline and dopamine in pharmaceutical injection samples by inhibition flow injection electrochemiluminescence of ruthenium complexes

Two maximal potential-resolved flow injection-electrochemiluminescent (FI-ECL) peaks were observed for Ru(bpy)₃²⁺/TPrA system at 0.90 and 1.05 V, and for Ru(phen)₃²⁺/TPrA at 1.01 and 1.25 V (vs. Ag/AgCl) in pH 8.0 phosphate buffer solutions. Sensitive ECL inhibition effects were observed in the presence of noradrenaline and dopamine for both of these systems. Therefore, an FI-ECL inhibition method for determination of noradrenaline and dopamine has been developed. Under optimal conditions, linear responses between logarithm of ECL intensity changes and logarithm of sample concentration were found for noradrenaline in the linear range (LR) of 4×10⁻⁸-1×10⁻⁵ mol l⁻¹ with theoretical detection limit (DL) of 2.5×10⁻⁸ mol l⁻¹ for Ru(bpy)₃²⁺/TPrA system, and in LR of 2×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 7.1×10⁻⁹ mol l⁻¹ for Ru(phen)₃²⁺/TPrA system; and for dopamine in LR of 8×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 5.2×10⁻⁸ mol l⁻¹ for Ru(bpy)₃²⁺/TPrA system, in LR of 4×10⁻⁸-2×10⁻⁵ mol l⁻¹ with DL of 1.5×10⁻⁸ mol l⁻¹ for Ru(phen)₃²⁺/TPrA system. It was applied for determination of commercial pharmaceutical injection samples with satisfied results. The mechanism of the inhibition effects was proposed in the preliminary way.     

Determination of benorilate in pharmaceutical formulations and its metabolite in urine at carbon paste electrode modified by silver nanoparticles

Benorilate was determined by the differential pulse voltammetry (DPV) using a carbon paste electrode modified by silver nanoparticles in 1.25 × 10⁻³ mol l⁻¹ KH₂PO₄ and Na₂HPO₄ buffer solution (pH = 6.88, 25 °C) .The anodic peak potential was +0.970 V (versus SCE). A good linear relationship was realized between the anodic peak currents and benorilate concentrations in the range of 1.0 × 10⁻⁷ to 2.5 × 10⁻⁴ mol l⁻¹ with the detection limit of 1.0 × 10⁻⁸ mol l⁻¹. The recovery was 95.2-103.6% with the relative standard deviation of 3.6% (n = 9). The pharmaceutical preparations, benorilate tablets samples and its metabolite (salicylic acid) in urine were determined with the desirable results.     

An environmentally friendly multicommutated alternative to the reference method for anionic surfactant determination in water

It has been developed a fully mechanized procedure for the spectrophotometric determination of anionic surfactants in water expressed in terms of SDS concentration. The reference method, based on the reaction of SDS with methylene blue (MB) followed by extraction in chloroform, was mechanized in order to reduce the consumption of organic solvents. The system was based on the multicommutation approach and provided a 35 times reduction of the waste production without sacrificing the figures of merit of the method in terms of sensitivity and repeatability, for a dynamic linear range from 0.2 to 1.7 mg l⁻¹. Results obtained for washing water samples were comparable with those obtained using the reference method and no significant differences, at 95% confidence level, were observed. Other useful characteristics are a solvent consumption of 0.7 ml per determination, a sampling throughput of 40 determinations per hour, a relative standard deviation of 5.9% (n = 10) for a sample containing 2 × 10⁻⁶ mol l⁻¹ (576 μg l⁻¹) surfactant and a limit of detection of 6.1 × 10⁻⁹ mol l⁻¹ (1.7 μg l⁻¹).     

Study of the voltammetric behaviour of maleic hydrazide and its determination at a hanging mercury drop electrode

Voltammetric behaviour of maleic hydrazide pesticide dissolved in a Britton-Robertson buffer was investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). It was found that the process at the Hg electrode was diffusion controlled; the reaction was irreversible and involved a change of one proton and a transfer of one electron. A quantitative differential pulse voltammetric method for determination of maleic hydrazide was developed on the basis of these studies involving the reduction of the compound at a hanging mercury drop electrode. A linear calibration was obtained in the range of 0.5-5.5 mg l⁻¹, and the developed DPV methodology was then applied for the determination of maleic hydrazide in spiked vegetable samples by the standard addition method. Satisfactory percentage R.S.D. (∼2%), percentage recovery values (∼85%) and LOD (0.215 mg l⁻¹) were obtained. These compared well with the results from the alternative spectrophotometric method.     

Electrochemical determination of maltol in beverages with glassy carbon electrode and its silica sol-gel modified electrode

The electrochemical behavior of maltol on a glassy carbon (GC) electrode was investigated. The results were applied to differential pulse voltammetric determination of maltol in beverages pretreated by ultrafiltration. Under the optimum experimental conditions, the linear range is 1×10⁻⁵ to 6×10⁻⁴ mol l⁻¹ maltol and the relative standard deviation for 0.4 mmol l⁻¹ maltol is 0.6% (n=9). The detection limit was 5 μmol l⁻¹. Furthermore, silica sol-gel film on GC electrode could be used as suitable selective membrane, which integrated selective membrane on the electrode and substituted for the pretreatment of ultrafiltration. Under the above conditions, maltol was determined by semi-differential linear sweep voltammetry at a silica sol-gel modified GC electrode in the concentration range of 5×10⁻⁶ to 5×10⁻⁴ mol l⁻¹. The detection limit was 2 μmol l⁻¹ and the relative standard deviation for 0.1 mmol l⁻¹ maltol was 0.7% (n=7). The proposed method is of sensitivity, simplicity, rapidness and no contamination. It had been applied to the direct determination of maltol in beverages such as grape wines, drinks and beers without any pretreatment. The results obtained with the present method were satisfactory with those obtained by spectrophotometry. It could be used as a simple and practical method for the determination of the flavor enhancer maltol in beverages.     

Affinity biosensors based on disposable screen-printed electrodes modified with DNA

Simple and sensitive DNA sensors have been developed on a base on graphite screen-printed electrodes modified with DNA and enzymes. Cholinesterase and peroxidase immobilized by treatment with glutaraldehyde were used for the detection of human DNA antibodies of systemic lupus erythematosus and bronchial asthma patients. The amperometric signal was measured at +680 mV versus Ag/AgCl for DNA-cholinesterase sensor and −150 mV for DNA-peroxidase sensor 5 min after the injection of acethylthiocholine and hydroquinone, respectively. The addition of serum samples results in the sharp decrease of the signal due to the formation of DNA-antibody adducts followed by the suppression of the access of substrate to the enzyme active site. Sulfonamide medicines suppress the DNA-antibody interaction due to the competitive binding along DNA minor grooves. DNA sensor labeled with peroxidase showed the linear calibration range of 5×10⁻⁹ to 7×10⁻⁵ mol l⁻¹ of sulfamethoxazole and of 5×10⁻⁸ to 1×10⁻⁴ mol l⁻¹ of sulfathiazole.     

Determination of diclofenac in pharmaceutical preparations by diffuse reflectance photometry

A quantitative analytical method for the determination of diclofenac in pharmaceutical preparations by diffuse reflectance in the visible region of the spectrum is presented. The color reaction is done directly in the measuring cell immediately after mixing, using small volumes of the analyte solution, of the reagent and of the buffer solutions. All reflectance measurements were carried out in a home made reflectometer equipped with a red LED as light source and a LDR as detector. The calibration curves were constructed from 1.0 to 18 mg mL⁻¹ (about 3.0 × 10⁻³ to 5.5 × 10⁻² mol L⁻¹) of sodium diclofenac or of potassium diclofenac in the analytical solution, with typical correlation coefficients equal to 0.999. The detection limit was estimated to be about 0.7 mg mL⁻¹ (2 × 10⁻³ mol L⁻¹). The method was applied to determine diclofenac in solid and liquid pharmaceutical preparations. The R.S.D. varied from 2% to 4% depending of the sample. The results were compared with those obtained with the HPLC procedure recommended by the United States Pharmacopoeia using the statistical Student's t-test procedure.     

Inhibition biosensor for determination of nicotine

An amperometric nicotine inhibition biosensor has been substantially simplified and used for determination of nicotine in tobacco sample. Besides the use of single enzyme choline oxidase to replace bienzyme, the use of 1,4-benzoquinone as an electron mediator makes it possible to avoid the use of oxygen or hydrogen peroxide sensor as the internal transducer. Choline oxidase was immobilized on the carbon paste electrode through cross-linking with bovine serum albumin (BSA) by glutaraldehyde. In the presence of choline oxidase and its endogenous cofactor flavin-ademine dinneleotide (FAD), choline was oxidized into betaine while FAD was reduced to FADH₂ which subsequently reduced 1,4-benzoquinone into hydroquinone. The later was finally oxidized at a relatively low potential of +450 mV versus saturated calomel electrode (SCE). Nicotine inhibits the activity of enzyme with an effect of decreasing of oxidation current. The experimental conditions were optimized. The electrode has a linear response to choline within 1.25×10⁻⁴ to 1.25×10⁻³ mol l⁻¹. The nicotine measurements were carried out in 0.067 mol l⁻¹phosphate buffer of pH 7.4 at an applied potential of 450 mV versus SCE. The electrode provided a linear response to nicotine over a concentration range of 2.0×10⁻⁵ to 9.2×10⁻⁴ mol l⁻¹ with a detection limit of 1.0×10⁻⁵ mol l⁻¹. The system was applied to the determination of nicotine in tobacco samples.     

Flow injection spectrophotometric method for chloride determination in natural waters using Hg(SCN)(2) immobilized in epoxy resin

A flow injection (FI) spectrophotometric method was proposed for the determination of chloride ion in natural waters. The determination of chloride was carried out by reaction with Hg(SCN)₂ immobilized in an epoxy resin bead in a solid-phase reactor (SPR) and the thiocyanate ions released were determined spectrophotometrically at 480 nm after complexing reaction with Fe(III). The analytical curve for chloride was linear in the concentration range from 5.6 × 10⁻⁵ to 2.2 × 10⁻⁴ mol l⁻¹ with a detection limit of 1.4 × 10⁻⁵ mol l⁻¹. The relative standard deviation (R.S.D.) was 2.2% for a solution containing 2.2 × 10⁻⁴ mol l⁻¹ (n = 10). The simple manifold allows a routine analytical frequency of 100 determinations per hour. The main advantage of the developed method is the 400% reduction of the Hg waste solution generated when compared to conventional methods for chloride determination based on the same spectrophotometric reaction.