Kinetic determination of alcohols and their binary mixtures with a stopped-flow spectrophotometric technique

The kinetic method described for the determination of a single alcohol (? 5 X 10^-4 M), or of a binary mixture of alcohols in aqueous solution is based on fast oxidation by silver(II), monitored spectrophotometrically by using a stopped-flow technique. Mixtures of alcohols having rate constants differing by a factor of ?3 can be resolved. The lower limit of determination is ca. 5 × 10^-4 M.     

Kinetic determination of adrenaline,l-dopa and their mixtures with a stopped-flow spectrophotometric technique

Catecholamines (adrenaline and L-Dopa) can be determined by a stopped-flow spectrophotometric technique. For individual determinations, catecholamines are oxidized to the corresponding o-benzoquinones by hexachloroiridate(IV). Concentrations in the range 2·10^-4–2·10^-3 M can be determined with errors of about 2 %. For evaluation of mixtures, aminochromes are formed. The method allows a catecholamine concentration of about 5·10^-6 M to be determined in the presence of a ten-fold amount of another catecholamine, with a maximum error of about 10 %.     

Kinetic enzymatic determination of ethanol/methanol mixtures by the stopped-flow technique

The analytical use of the enzymatic oxidation of alcohols by alcohol oxidase was assessed applying the stopped-flow technique for mixing sample and reagents, and the DTNB (5,5'-dithiobis-(2-nitrobenzoic acid)/L-cysteine system as a new chromogenic reagent. The oxidation reaction was monitored by measuring the rate of absorbance decrease at 412 nm, the wavelength of maximum absorption of the reduced form of DTNB. The calibration graphs for the individual determination of the alcohols were linear over the range 1.0 x 10(-6)-1.0 x 10(-5)M, and the precision ranged between 2.1 and 4.8%. A differential rate principle was applied to the determination of ethanol/methanol mixtures involving mutual kinetic effects which allowed mixtures in molar ratios between 25:1.0 and 1.0:1.0 to be accurately resolved with good precision (r.s.d. less than 9 and 5%, respectively). Compared to the flow-injection analysis method, the proposed approach offers higher sensitivity and sample throughput, as well as the wider concentration ratio range for mixture resolution.     

Kinetic determination of ascorbic acid by the 2,6-dichlorophenolindophenol reaction with a stopped-flow technique.

The reaction of ascorbic acid with 2.6-dichlorophenolindophenol is applied for the kinetic determination of ascorbic acid in 0.05M oxalic acid. Stopped-flow techniques are used; the concentrations of the reactants of the second-order reaction can be adjusted so that the transmittance signal remains nearly invariant for a wide range of voltage. Theoretical and experimental results are in very good agreement. Analytical working curves are presented for the determination of ascorbic acid in the ranges 5.0·10^-4–1.0·10^-2M and 5.0·10^-5–1.0-·10^-3M with errors of 1.0% and 2.2%. respectively. The method is simple, fast and sensitive.     

Kinetic fluorimetric determination of carbamazepine in serum by the stopped-flow technique

Summary A simple, fast method for the determination of carbamazepine in serum is reported for the first time. It is based on the fluorescent reaction of this drug with cerium(IV) in an acid medium. A stopped-flow mixing module coupled to a conventional spectrofluorimeter is used for this purpose. The linear range of the proposed method is 0.04–140 g ml^–1 of carbamazepine and the detection limit is 0.01 g ml^–1. The within- and between-assay precision data and selectivity results show the method to be adequate for the determination of carbamazepine in serum by including a preliminary extraction step with dichloromethane. Analytical recoveries from different serum samples are reported.     

Kinetic-photometric determination of spermine, spermidine and their mixtures by the stopped-flow technique

Summary Kinetic photometric stopped-flow methods are described for the individual and simultaneous determination of two biogenic amines: spermine and spermidine. They are based on the coloured reaction of these amines with sodium 1,2-naphthoquinone-4-sulfonate. The parameters used for the quantitative determinations are the initial rate and the increase in absorbance of the kinetic curves. Each amine can be rapidly determined in the range of 0.5–150 g ml^–1 with a precision (%RSD) of less than 1.8. The simultaneous determination of mixtures of spermine and spermidine is carried out by the proportional equations method. Spermine/spermidine mixtures in ratios between 120 and 201 are successfully resolved.

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Kinetisch-photometrische Bestimmung von Spermin, Spermidin und deren Gemischen mit dem Stopped-Flow-Verfahren

     

Kinetic determination of phenols by a fixed-time method

The fixed-time kinetic method is used for the determination of 15 phenols in methanol and acetic acid by measurement of the yellow products from the oxidation of the phenols with sodium metaperiodate. An increase in acid and metaperiodate concentrations enhances the rate of reaction, whereas the use of 2-propanol as the solvent decreases it.     

Chemiluminescence determination of citrate and pyruvate and their mixtures by the stopped-flow mixing technique

A novel kinetic chemiluminescent method using the stopped-flow mixing technique has been investigated for the rapid and sensitive determination of citrate and pyruvate. The method is based on a tris(2,2′-bipyridiyl)ruthenium(III) (Ru(bpy)₃³⁺) chemiluminescence (CL) reaction. Ru(bpy)₃³⁺ was generated in the mixing chamber by oxidising tris(2,2′-bipyridyl)ruthenium(II) with cerium(IV). After selecting the best operating parameters, calibration graphs were obtained over the concentration ranges 0.38-38 μg ml⁻¹ and 8.7-1300 ng ml⁻¹ for citrate and pyruvate, respectively. The limits of detection were 0.1 μg ml⁻¹ for citrate and 0.3 ng ml⁻¹ for pyruvate. Based on the differential rate of the chemiluminescent reaction corresponding to citrate and pyruvate, a very simple kinetic procedure was developed for the simultaneous determination of both compounds. Mixtures of citrate and pyruvate in ratios between 15:1 and 1.5:1 were satisfactorily resolved. The proposed method was successfully applied to the determination of citrate in pharmaceutical formulations, pyruvate in animal blood serum and both compounds in human urine.     

Kinetic determination of urea in serum by stopped-flow spectrophotometry

The determination of urea based on its reaction with biacetyl in the presence of thiosemicarbazide and iron(III) has been studied kinetically by using a modular stopped-flow system. This study gave some new data about this coloured reaction and led to the development of a simple, fast method which overcomes some of the disadvantages (e.g., long analysis times, high temperatures and acid concentrations) of the conventional equilibrium method for the determination of urea, which hinder its application to routine analyses. The linear range (0.5–15 μg ml^?1), detection limit, precision and selectivity of the proposed kinetic stopped-flow method are reported. The method has been satisfactorily applied to the analysis of serum samples without pretreatment. The results correlate well with those obtained by the urease method.     

Kinetic spectrophotometric method for determination of perphenazine based on monitoring the oxidation intermediate by applying a stopped-flow technique

A novel and high-sensitive stopped-flow kinetic spectrophotometic method for the determination of perphenazine based on monitoring the variation of absorbance of the intermediate within a few seconds has been developed. The optimum conditions for various parameters on which the forming of the intermediate depends, were investigated. It was found that the initial reaction rate increased linearly with an increase in the perphenazine concentration in the range from 1.0 x 10(-5) to 1.6 x 10(-4)M. The detection limit was calculated to be 5.3 x 10(-6)M. The kinetics of the reaction was established by the aid of single-mixing or double-mixing stopped-flow techniques, a successive reaction model was proposed to analyze and simulate the reaction. The influence of both ascorbic acid and NADH on the time courses was also investigated.     

Kinetic determination of magnesium and calcium by stopped-flow injection analysis

A kinetic method for the simultaneous determination of magnesium and calcium ions in solution is described. The method is based on the dissociation reactions of cryptand (2.2.2) complexes; a stopped-flow injection technique is used with spectrophotometric monitoring of the phthalein complexone complexes of the released metal ions. Analyses were done at a rate of 80 h^?1, with injected sample volumes of 80 μl. A microcomputer system for data acquisition and control of the system is described.     

Fluorimetric determination of total ascorbic acid by a stopped-flow mixing technique

A simple, rapid and automatic fluorimetric method for the determination of total ascorbic acid is described. The method makes use of the stopped-flow mixing technique in order to achieve the rapid oxidation of ascorbic acid by dissolved oxygen to dehydroascorbic acid, which then reacts with o-phenylenediamine to form a fluorescent quinoxaline. The initial rate and fluorescence signal of this system are directly proportional to the ascorbic acid concentration. The calibration graph was linear over the range 0.1-30 microg ml(-1) (kinetic method) and 0.25-34 microg ml(-1) (equilibrium method). The precision (% RSD) was close to 0.5%. The method has been used for the determination of ascorbic acid in pharmaceutical formulations, fruit juices, soft drinks and blood serum.     

Kinetic determination of creatinine in biological fluids by stopped-flow injection analysis

Summary A kinetic photometric method for the determination of creatinine in biological fluids based on the Jaffé reaction and the use of a flow injection manifold where the reacting plug is halted prior to detection is proposed. The wide linear range of the calibration curve obtained (between 3 and 150 g ml^–1) allows the determination of the analyte in serum and urine. The kinetic measurements require no sample pretreatment (urine samples need dilution). The proposed method compares favourably with the standard recommended procedure.     

Simultaneous determination of metal ion concentrations in binary mixtures with a multi-LED photometer

 A compact photometer is based on up to seven different light-emitting diodes whose light is guided into a photometric cell by means of a fibre-optic coupler. The wavelength is selected by switching on the appropriate light source. The application of this multi-wavelength photometer to the determination of binary metal ion samples is demonstrated using model mixtures. Solutions of Fe, Co, Cu, Ni, Cr and Mn were used in combination with several photometric reagents. It was found that the instrument yielded concentration measurements which were usually within a few percent of the correct concentration. These results are comparable to reported data obtained with conventional instruments for similar measurements. The application of the instrument for simultaneous determination in flow-injection analysis employing rapid computer controlled switching of the light-emitting diodes is also demonstrated. A limitation in the use of light-emitting diodes for the simultaneous determination was found to arise from the fact that in some instances non-linear calibration curves are obtained when employing light-emitting diodes as light sources. This occurs when the absorptivity of the measured compound changes over the emission band of the diodes. Multiple regression analysis is then not easily possible. Received: 16 September 1996/Revised: 14 October 1996/Accepted: 16 October 1996     

Simultaneous determination of metal ion concentrations in binary mixtures with a multi-LED photometer

 A compact photometer is based on up to seven different light-emitting diodes whose light is guided into a photometric cell by means of a fibre-optic coupler. The wavelength is selected by switching on the appropriate light source. The application of this multi-wavelength photometer to the determination of binary metal ion samples is demonstrated using model mixtures. Solutions of Fe, Co, Cu, Ni, Cr and Mn were used in combination with several photometric reagents. It was found that the instrument yielded concentration measurements which were usually within a few percent of the correct concentration. These results are comparable to reported data obtained with conventional instruments for similar measurements. The application of the instrument for simultaneous determination in flow-injection analysis employing rapid computer controlled switching of the light-emitting diodes is also demonstrated. A limitation in the use of light-emitting diodes for the simultaneous determination was found to arise from the fact that in some instances non-linear calibration curves are obtained when employing light-emitting diodes as light sources. This occurs when the absorptivity of the measured compound changes over the emission band of the diodes. Multiple regression analysis is then not easily possible. Received: 16 September 1996/Revised: 14 October 1996/Accepted: 16 October 1996     

Kinetic determination of atrazine in foods based on stopped-flow fluorescence polarization immunoassay

A very simple and fast method for the direct determination of atrazine in food samples based on the use of stopped-flow fluorescence polarization immunoassay is described. Unlike other immunoassay methods where the analytical signal is obtained when the immunochemical reaction has reached or is close to the equilibrium, this method uses the initial rate of this reaction as the analytical parameter, which is measured in only 4-5 s. This approach minimizes the static signal from the sample matrix, allows the direct analysis of the samples and can be easily adapted to the routine automatic determination of atrazine. The dynamic range of the calibration graph was 0.7-100 ng ml(-1) and the detection limit was 0.2 ng ml(-1). The precision and selectivity of the method were also studied. The analytical recoveries obtained by applying the method to white and red wine, orange juice and tea samples ranged from 80 to 104%.     

Kinetic spectrophotometric determination of Ga(III)-Al(III) mixtures by stopped-flow injection analysis using principal component regression

A multivariate calibration method based on principal component regression analysis is applied to the resolution of mixtures by kinetic methodology. Gallium(III) and Al(III) were determined in mixtures on the basis of their different rate of reaction with 4-(2-pyridylazo)resorcinol in a slightly basic medium by using a stopped-flow injection procedure. Mixtures containing 1-5 mg/l. Ga(III) and 20-100 mg/l. Al(III) were successfully resolved with errors less than 10%.     

Sensitive determination of diquat by a kinetic method using the stopped-flow mixing technique

A simple, sensitive, selective, fast and inexpensive assay for the determination of diquat is proposed. The method is based on the reduction of the herbicide to a strongly fluorescent monocation radical with sodium dithionite. The initial rate of this reaction is directly proportional to the diquat concentration. The stopped-flow mixing technique was used because the kinetic data can be obtained in only 7 s, meaning that the method can be automated. The calibration graph is linear over the range 5-500 ng ml-1 and the precision (RSD) is close to 1.2%. The applicability of the method was demonstrated by determining the herbicide in different kinds of samples.     

Kinetic spectrophotometric determination of indium/gallium mixtures

A kinetic method is presented for the determination of 0.5–5 μg ml^?1 gallium based on its activating effect on the copper(II)-catalyzed oxidation of 4,4′-dihydroxybenzophenone thiosemicarbazone by hydrogen peroxide. The reaction is monitored spectrophotometrically at 415 nm. Two sets of reaction conditions are established; one for the direct determination of gallium, and another, in which indium affects the gallium response, for determination of indium. Mixtures of these cations can be determined at μg ml^?1 levels and in gallium/indium ratios from 7.5:1 to 1:1.6, with an accuracy and precision of ca. 4.5%.