Laminar dispersion in flow-injection analysis

Simple expressions are given for the dispersion and the travel times of samples in simple flow-injection analysis systems. The sum of these two quantities is the total residence time of the sample in the system. The expressions are based on numerical solutions of the diffusion-convection equation. Preliminary experiments are in agreement with the derived simple expressions, as are peak curve shapes. Diffusion coefficients can be obtained in a straightforward manner.     

Experimental studies on the effect of temperature on dispersion in a flow-injection system

The effect of temperature on dispersion in a flow-injection system is described. Systems both with and without chemical reaction were investigated. The reaction studied was that of p-nitrophenol with sodium hydroxide. The results indicate that temperature does indeed have a significant and predictable effect on the dispersion of a sample plug. Findings agree with the random walk model for flow-injection systems.     

Chemical kinetics with reagent dispersion in single-line flow-injection systems

Simultaneous chemical kinetics and dispersion kinetics in single-line flow-injection manifold can be modeled by using total rate laws for dispersion and chemical reaction. The model combines a (non-equilibrium) mass-transfer rate coefficient with residence-time theory based on extended tanks-in-series. Reagent dispersion is found to play a key role in defining a zone of mixing. Double product peaks and narrow analyte peaks observed in the literature are predicted by the model. When applied to a slow chemical reaction under conditions of constant length of mixing, predicted and experimentally observed response curves agree well.     

Integrated reaction and photometric detection with enzymes immobilized in the flow cell of a flow-injection system

A biosensor based on the integration of reaction and detection with a conventional photometric detector is proposed. An enzyme is immobilized on the walls of the flow cell and reaction-rate measurements of the enzymatic reaction are made by halting the flow when the sample-coenzyme plug is in the flow cell and monitoring the increase in the reduced form of the coenzyme (λ_max = 340 nm).     

Determination of mercury in saliva with a flow-injection system

A simple, fast and reliable method, with a low detection limit, has been developed for the determination of total mercury in saliva samples. The method uses a brominating reagent, followed by on-line addition of KMnO₄ at room temperature to convert organically bound mercury to inorganic mercury ions, and determines mercury by flow-injection cold-vapour atomic absorption spectrometry. Using the method described, complete recoveries of five mercury compounds from saliva were attained. Results obtained on real samples using the new method were comparable to that obtained using the established method with batch system. The detection limit of this method, based on three standard deviations of the blank, is 0.05 μg l⁻¹ Hg in a saliva sample of 500 μl. A sample throughput of 80 measurements per hour is possible with the method. The calibration curves are linear up to 20 μg l⁻¹ and the dynamic range extends to 40 μg l⁻¹ Hg. At a concentration of 1μg l⁻¹ mercury in saliva, the relative standard deviation is about 2% for 11 replicates; a total volume of 0.5 ml saliva is required for three replicates.     

Smallest chemical reaction system with Hopf bifurcation

The smallest at most bimolecular chemical reaction system with Hopf bifurcation is presented. First the notion smallest reaction system is explained. Since the lowest number of intermediates has the highest priority in this characterization and since it has already been shown that three-component systems can have a Hopf bifurcation [1], the smallest reaction system must contain three intermediates. On the basis of a sufficient condition for a Hopf bifurcation in three-dimensional systems it is possible to find one reaction system which is according to the given characterization, the smallest one. In the first part of this paper it is shortly pictured and in the second part a more extensive proof that this system is really the searched smallest one is given.     

Two trends of sample dispersion variation with carrier flow rate in a single flow-injection manifold

Two trends of sample dispersion variation with carrier flow rate in a single flow-injection manifold are completely revealed for the first time and an inflection point in the dispersion coefficient (D) vs. flow rate (q) curve is discovered. With the increase of the flow rate, the value of D increases before the inflection point but decreases after the inflection point. The value of the carrier flow rate at the inflection point (q_m) is independent of the sample injection volume, the tube length, the tube coil radius and the tube inner diameter. It is only affected by the substance diffusion coefficient (D_m) of the analysis. The value of q_m decreases as D_m increases. Therefore, the value of D_m for a sample can be estimated according to the D_m vs. q_m curve.     

Extraction-spectrophotometric determination of anionic surfactants with a flow-injection system

Anionic surfactants are preconcentrated from 5-ml samples by extraction as the ion-pair with ethyl violet into toluene. The absorbance of aliquots of the toluene phase is measured at 610 nm in a flow-injection system. A phase converter is located prior to the injection valve to convert a water stream to, pumped with ordinary pump tubing, to a toluene stream. The working range was 0.01–1.0 mg l^?1 and the reproductibility (r.s.d, n = 10) was 2% for 0.4 mg l^?1 sodium dodecyl sulphate. The non-aqueous flow-injection system serves to miniaturize the extraction from separatory funnel (200 ml) to test tube (10 ml) scale without loss of precision or validity.     

Amperometry of thiols in a flow-injection system with a nickel oxide electrode

The electrochemical oxidation of a series of thils on the surface of a nickel oxide electrode is investigated in a flow-injection system. The background electrolyte is 0.1 M NaOH containing 5 × 10^?65 M NiSO₄ and the injected sample volume is 25 μl. The net peak current is observed at +0.46 V vs. SCE for thiols dissolved in pH 3.0 phthalate buffer. The upper limit of linearity extends to 10^?3 M. The relative standard deviation for ten replicate measurements on 10^?4 M ethanethiol is 1.6%. The lower limits of detection are between 3 × 10^?5 and 2 × 10^?4 M for a series of thiols. Peak shapes indicate that thiolate oxidation mediated by a higher oxide layer on the electrode rather than mass transport or adsorption / rearrangement is the rate-determining step.     

Calculation of the mass transfer coefficient with a chemical reaction in a gas-liquid system

A solution of the problem of mass transfer in a turbulent boundary layer has been obtained with a first-order chemical reaction occurring in the liquid phase. The dependence of the enhancement factor for absorption and of the mass transfer coefficient on the model parameters can be used for building up a hierarchic model of the gas-liquid reactor.

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Indirect determination of iron in a flow-injection system with amperometric detection

The change in peak current resulting from the reaction of Fe(II) with nitroso-R salt in a flow-injection system is used to quantify Fe(II) with either single- or dual-electrode amperometric detectors. The current change varies linearly with Fe(II) concentration from 0 to 200 mg 1^?1. The relative standard deviation was about 5% with the single-electrode detector and about 10% with the dual-electrode detector. The method is evaluated for the determination of iron in dietary supplements.     

Entropy production in a mesoscopic chemical reaction system with oscillatory and excitable dynamics

Stochastic thermodynamics of chemical reaction systems has recently gained much attention. In the present paper, we consider such an issue for a system with both oscillatory and excitable dynamics, using catalytic oxidation of carbon monoxide on the surface of platinum crystal as an example. Starting from the chemical Langevin equations, we are able to calculate the stochastic entropy production P along a random trajectory in the concentration state space. Particular attention is paid to the dependence of the time-averaged entropy production P on the system size N in a parameter region close to the deterministic Hopf bifurcation (HB). In the large system size (weak noise) limit, we find that P ～ N(β) with β = 0 or 1, when the system is below or above the HB, respectively. In the small system size (strong noise) limit, P always increases linearly with N regardless of the bifurcation parameter. More interestingly, P could even reach a maximum for some intermediate system size in a parameter region where the corresponding deterministic system shows steady state or small amplitude oscillation. The maximum value of P decreases as the system parameter approaches the so-called CANARD point where the maximum disappears. This phenomenon could be qualitatively understood by partitioning the total entropy production into the contributions of spikes and of small amplitude oscillations.     

Electrochemiluminescent determination of L-cysteine with a flow-injection analysis system.

A flow-injection electrochemiluminescent method for L-cysteine determination has been developed based on its enhancement of the electrochemiluminecence of luminol at a glassy carbon electrode. This method is simple and sensitive for cysteine determination. Under the selected experimental parameters, the linear range for cysteine concentration was 1.0 x 10(-6) - 5.0 x 10(-5) mol/l, and the detection limit was 0.67 micromol/l (S/N = 3). The relative standard deviation for 11 measurements of 1.0 x 10(-5) mol/l cysteine was 4.5%. The proposed method has been applied to the detection of cysteine in pharmaceutical injections with satisfactory results.     

The bioluminescent determination of adenosine triphosphate with a flow-injection system

A procedure is described, for the bioluminescent determination of adenosine- 5′-triphosphate (ATP), based on firefly luciferin and luciferase and a purpose-built flow-through detector. The limit of detection is 10^?14 mol and the sample throughput is 200 h^?1. The useful response range for a 30-μl sample was 10^?6–10^?4 M ATP and the correlation coefficient (r) for a log-log plot over this range was 0.9991.     

Polarographic determination of penicilloic acid in penicillin preparations with a flow-injection system

A flow-injection method for the determination of the penicilloic acid content of benzylpenicillin, phenoxymethylpenicillin, ampicillin, cloxacillin and carbenicillin is described. The penicilloate is detected polarographically at a dropping mercury flow-through detector mounted on a conventional polarographic capillary. A constant potential of + 0.04 V vs. SCE is applied and the current is measured in the sampled direct current mode. A pH 9.2 borate buffer containing 0.05% Triton X-100 is used. Triton displaces the reduction wave of oxygen about 400 mV towards negative potentials so that deaeration is not necessary. The penicilloate gives linear calibration graphs for 2 × 10^-6–1 × 10^-4 M solutions. The relative standard deviation for 10 injections of a 5 × 10^-5 M solution is 0.2%. Results for the determination of penicilloate in the presence of large amounts of penicillin (the latter in 10–1000-fold excess) are compared with results obtained by two titrimetric procedures.     

Determination of l-alanine in a flow-injection system with an immobilized enzyme reactor

A flow-injection system for the determination of l-alanine is described. Alanine dehydrogenase is immobilized on poly(vinyl alcohol) beads and used in a packed-bed enzyme reactor. The system responds linearly to injected samples (50 μl) in the concentration range 0.5–500 μM. The maximum throughput was 40 samples per hour. The immobilized enzyme reactor was stable for at least 6 weeks. Its usefulness for assay of l-alanine in serum and beverages is described.     

Determination of clenbuterol in pharmaceutical preparations by reaction with o-phthalaldehyde using a flow-injection fluorimetric procedure

A new procedure for the determination of clenbuterol is proposed using flow-injection and fluorimetric detection. The method is based on the derivatization reaction of the primary amine group with o-phthalaldehyde in the presence of 2-mercaptoethanol. The calibration graph based on peak area was linear in the range 0.2-5 mug ml(-1) and the detection limit was 0.06 mug ml(-1). The method was validated using a reference spectrophotometric procedure and was applied to the determination of the drug in commercial pharmaceutical preparations.     

Stability of solutions to a reaction diffusion system based upon chemical reaction kinetics

In this paper, we deal with the stability problem to some mathematical models that describe chemical reaction kinetics. One is a set of ordinary differential equations induced by one reversible chemical reaction mechanism containing three chemical species. The other is a set of reaction diffusion equations based on the same chemical reaction. We show that all solutions of the model are asymptotically stable by applying the Liapunov method. We thus find that the concentration of each species has certain limits as time proceeds.      

Rapid determination of microorganisms using a flow-injection system

An automated method for the rapid determination of microorganisms using a flow-injection system is presented. Electrochemical measurement of a mediator reduced by microbial metabolism allowed the determination of fungi and bacteria in a few minutes. The lowest detection limit was 5 × 10⁶ colony-forming units (cfu) ml^?1 for Escherichia coli. Correlation between the flow-injection method and standard microbiological methods was excellent (r = 0.997, n = 4 for Beauveria bassiana; r = 0.997, n = 7 for E. coli). The flow-injection system was applied to the on-line control of an E. coli cultivation.     

Differential pulse voltammetry with fast pulse repetition times in a flow-injection system with a copper-amalgam electrode

The reductive determination of cadmium and zinc is used to illustrate the effects of pulse repetition times in differential pulse voltammetry at solid electrodes. With 100-ms repetition times, signal-to-background ratio is shown to improve in flow-injection response studies at a copper-amalgam working electrode for both voltage scanning and amperometric operation, allowing scan rates up to 100 mV s^?1, and detection limits of 0.07–0.15 ng for cadmium and zinc after deaeration of sample solutions. Peak widths at baseline in the range 12–30 s, depending on flow rate, are obtained in the flow-injection system for cadmium and zinc at working potentials of ?0.77 and ?1.27 V, respectively. Interferences occur when cadmium is in large excess over zinc, although the voltammetric peaks do not overlap.