Single-line Flow-injection Systems Research Articles

Flow injection simultaneous determination of synthetic colorants in food using multiple pulse amperometric detection with a boron-doped diamond electrode

A single-line flow injection system and multiple pulse amperometric detection using a boron-doped diamond electrode were employed to develop and optimize a simple, low-cost, and rapid method for the simultaneous determination of two pairs of food colorants: tartrazine and sunset yellow (TT–SY) or brilliant blue and SY (BB–SY). A dual-potential waveform was used: Edet.1=−150mV (400ms duration) and Edet.2=−450mV (100ms duration) vs. Ag/AgCl (3.0molL–1 KCl). Polarization at Edet.1 or Edet.2 causes reduction of SY or the respective pair of colorants, TT–SY or BB–SY; hence, with proper current correction, both colorants in each pair can be determined. The obtained linear response ranges (detection limits) were 5.0–60.0 (2.5) and 1.0–50.0 (0.80)μmolL−1, for TT and SY, or 5.0–60.0 (3.5) and 1.0–50.0 (0.85)μmolL−1, for BB and SY, respectively. Investigation of possible interferents (other food colorants or additives) showed no significant interference with the methods here proposed, which were then used to simultaneously determine the pairs of colorants in industrialized food samples, with results that showed good agreement with those obtained using a comparative HPLC method.

Cobinamide-based cyanide analysis by multiwavelength spectrometry in a liquid core waveguide.

A novel cyanide analyzer based on sensitive cobinamide chemistry relies on simultaneous reagent and sample injection and detection in a 50 cm liquid core waveguide (LCW) flow cell illuminated by a white light emitting diode. The transmitted light is read by a fiber-optic charge coupled device (CCD) spectrometer. Alkaline cobinamide (orange, lambda(max) = 510 nm) changes to violet (lambda(max) = 583 nm) upon reaction with cyanide. Multiwavelength detection permits built-in correction for artifact responses intrinsic to a single-line flow injection system and corrects for drift. With optimum choice of the reaction medium, flow rate, and mixing coil length, the limit of detection (LOD, S/N = 3) is 30 nM and the linear dynamic range extends to 10 microM. The response base width for 1% carryover is <95 s, permitting a throughput of 38 samples/h. The relative standard deviations (rsd) for repetitive determinations at 0.15, 0.5, and 1 microM were 7.6% (n = 5), 3.2% (n = 7), and 1.7% (n = 6), respectively. Common ions at 250-80,000x concentrations do not interfere except for sulfide. For the determination of 2 microM CN(-), the presence of 2, 5, 10, 20, 100, and 1000 microM HS(-) results in 22, 27, 48, 58, 88, and 154% overestimation of cyanide. The sulfide product actually has a different characteristic absorption, and in those samples where significant presence is likely, this can be corrected for. We demonstrate applicability by analyzing the hydrolytic cyanide extract of apple and pear seeds with orange seeds as control and also measure HCN in breath air samples. Spike recoveries in these sample extracts ranged from 91 to 108%.

Electrochemical evidence of self-substrate inhibition as functions regulation for cellobiose dehydrogenase from Phanerochaete chrysosporium

The reaction mechanism of cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium, adsorbed on graphite electrodes, was investigated by following its catalytic reaction with cellobiose registered in both direct and mediated electron transfer modes between the enzyme and the electrode. A wall-jet flow through amperometric cell housing the CDH-modified graphite electrode was connected to a single line flow injection system. In the present study, it is proven that cellobiose, at concentrations higher than 200 μM, competes for the reduced state of the FAD cofactor and it slows down the transfer of electrons to any 2e −/H + acceptors or further to the heme cofactor, via the internal electron transfer pathway. Based on and proven by electrochemical results, a kinetic model of substrate inhibition is proposed and supported by the agreement between simulation of plots and experimental data. The implications of this kinetic model, called pseudo-ping-pong mechanism, on the possible functions CDH are also discussed. The enzyme exhibits catalytic activity also for lactose, but in contrast to cellobiose, this sugar does not inhibit the enzyme. This suggests that even if some other substrates are coincidentally oxidized by CDH, however, they do not trigger all the possible natural functions of the enzyme. In this respect, cellobiose is regarded as the natural substrate of CDH.

Carbon Ceramic Electrodes Modified with Laccase from Trametes hirsuta: Fabrication, Characterization and Their Use for Phenolic Compounds Detection

AbstractFungal laccase (Lc) from the basidiomycete Trametes hirsuta was immobilized on top of a carbon ceramic electrode using physical absorption. Direct, unmediated heterogeneous electron transfer between Lc and the carbon ceramic electrode (CCE) under aerobic conditions was shown. The bioelectrocatalytic reduction of oxygen on Lc‐CCE started at about 430 mV vs. Ag|AgCl|KClsat at pH 3.5 and moved with about 57 mV in the cathodic region per pH unit. The Lc‐modified CCE was then used as a biosensing detection element in a single line flow injection system for the amperometric determination of a variety of phenolic substrates of the enzyme. The experimental conditions were studied and optimized for catechol serving as a model compound. Statistical aspects were applied and the sensor characteristics and Michaelis‐Menten constants of the investigated phenolic compounds were calculated and compared with those obtained for solid graphite electrodes modified with Trametes hirsuta laccase. The results showed that the CCE based biosensor in comparison with the solid graphite based biosensor offers a lower detection limit, a wider linear dynamic range, and excellent operational stability with no sensor passivation, indicating that the sol–gel lattice improves the electrochemical behavior of the biosensor.

Dispersion-convolution model for simulating peaks in a flow injection system

A dispersion-convolution model is proposed for simulating peak shapes in a single-line flow injection system. It is based on the assumption that an injected sample plug is expanded due to a “bulk” dispersion mechanism along the length coordinate, and that after traveling over a distance or a period of time, the sample zone will develop into a Gaussian-like distribution. This spatial pattern is further transformed to a temporal coordinate by a convolution process, and finally a temporal peak image is generated. The feasibility of the proposed model has been examined by experiments with various coil lengths, sample sizes and pumping rates. An empirical dispersion coefficient ( D*) can be estimated by using the observed peak position, height and area ( t p * , h* and A t * ) from a recorder. An empirical temporal shift ( Φ*) can be further approximated by Φ* = D* /u 2, which becomes an important parameter in the restoration of experimental peaks. Also, the dispersion coefficient can be expressed as a second-order polynomial function of the pumping rate Q, for which D*( Q) = δ 0 + δ 1 Q + δ 2 Q 2. The optimal dispersion occurs at a pumping rate of Q opt = δ 0 / δ 2 . This explains the interesting “Nike-swoosh” relationship between the peak height and pumping rate. The excellent coherence of theoretical and experimental peak shapes confirms that the temporal distortion effect is the dominating reason to explain the peak asymmetry in flow injection analysis.

Sequential fluorometric quantification of γ-aminobutyrate and l-glutamate using a single line flow-injection system with immobilized-enzyme reactors

A single line flow-injection system with immobilized-enzyme reactors is proposed for the sequential quantification of γ-aminobutyrate (GABA) and l-glutamate. A co-immobilized l-glutamate oxidase and catalase reactor and an immobilized GABase reactor were introduced into the flow line in series. Sample and reagent were injected into the flow line using an open sandwich method. GABA was selectively detected by GABase when α-ketoglutarate at a high concentration and NADP + were injected as the reagents with a sample. When GABA at a high concentration and NADP + were injected as the reagents with a sample, l-glutamate only was determined by the series of enzymatic reactions. NADPH produced by the immobilized-enzyme reactors was monitored fluorometrically at 455 nm (excitation at 340 nm). Linear relationships between the responses and concentrations of GABA or l-glutamate were observed in the ranges of 5.0 × 10 −6–5.0 × 10 −4 M and 1.0 × 10 −5–5.0 × 10 −4 M, respectively. The relative standard deviations for ten successive injections were less than 2% at the 0.5 mM level. This analytical method was applied to the sequential quantification of GABA and l-glutamate that were produced and consumed, respectively, by lactic acid bacteria, and the results showed good agreement with those obtained using liquid chromatography.

Sequential fluorometric quantification of malic acid enantiomers by a single line flow-injection system using immobilized-enzyme reactors

A method for the sequential enantiomeric quantification of d-malate and l-malate by a single line flow-injection analysis was developed using immobilized-enzyme reactors and fluorescence detection. An immobilized d-malate dehydrogenase ( d-MDH) reactor and an immobilized l-malate dehydrogenase ( l-MDH) reactor were introduced into the flow line in series. Sample and coenzyme (NAD + or NADP +) were injected into the flow line by an open sandwich method. d-Malate was selectively oxidized by d-MDH when NAD + was injected with a sample. When NADP + was injected with a sample, l-malate was oxidized only by l-MDH. NADH or NADPH produced by the immobilized-enzyme reactors was monitored fluorometrically at 455 nm (excitation at 340 nm). Linear relationships between the responses and concentrations of d-malate and l-malate were observed in the ranges of 1 × 10 −6–1 × 10 −4 M and 1 × 10 −6–2 × 10 −4 M, respectively. The relative standard deviations for ten successive injections were less than 2% at the 0.1 mM level. This analytical method was applied to the sequential quantification of d-malate and l-malate in fruit juices and soft drinks, and the results showed good agreement with those obtained using conventional method (F-kit method).

Simultaneous determination of choline and acetylcholine based on a trienzyme chemiluminometric biosensor in a single line flow injection system.

A detector for the simultaneous determination of choline (Ch) and acetylcholine (ACh) based on a sensitive trienzyme chemiluminometric biosensor in a single line flow injection (FI) system is described. Immobilized choline oxidase (ChOx), immobilized peroxidase (POx), immobilized acetylcholinesterase, and coimmobilized ChOx/POx were packed, in turn, in a transparent ETFE tube (1 mm i.d., 75 cm) and the tube was placed in front of a photomultipier tube as a flow cell. Two-peak response was obtained by one injection of the sample solution. The first and second peaks were dependent on the concentrations of Ch and ACh, respectively. The influence of some experimental parameters such as flow rate, amounts of immobilized enzymes on the behavior of the sensor was studied in order to optimize the sensitivity, sample throughput and resolution. Calibration curves were linear at 1 - 1000 nM for Ch and 3 - 3000 nM for ACh. The sample throughput was 25/h without carryover. The FI system was applied to the simultaneous determination of Ch and ACh in rabbit brain tissue homogenates.

A nanoinjector for microanalysis.

We describe a simple miniature injection device that can be used for introduction of nanoliter sample volumes in microfluidic systems. The hybrid microstructure consists of two hydraulically connected parts, a pulse micropump, and a multilevel cross-flow injector. Sample injection is accomplished by creating a transient pressure pulse in the sample line by means of the solenoid-based micropump. The sample line is aligned at right angles to the main carrier flow line. The two flow channels are located in two different parallel planes. The cross section of the two channels is defined by a self-sealing aperture in an elastomer. During the pressure pulse, the sample is introduced through this aperture directly into the main flow stream. Fast impulse-based injection causes rapid mixing of the injected sample with the main flow stream. This permits simple single-line manifold micro flow injection (MFI) systems. The deformation/relaxation of the elastomer is fast and repeatable; as such, rapid serial actuations essentially result in a larger injected sample volume without significantly affecting the peak shape. In the present form, 2-40-nL samples are easily injected by single injection, and the injected volume can be chosen by system parameters. The injection repeatability as observed by a photometric detector is better than 1.2% (n = 100).

Characterization of graphite electrodes modified with laccase from Trametes versicolor and their use for bioelectrochemical monitoring of phenolic compounds in flow injection analysis

Spectrographic graphite electrodes were modified through adsorption with laccase from Trametes versicolor. The laccase-modified graphite electrode was used as the working electrode in an amperometric flow-through cell for monitoring phenolic compounds in a single line flow injection system. The experimental conditions for bioelectrochemical determination of catechol were studied and optimized. The relative standard deviation of the biosensor for catechol (10 μM, n=12) was 1.0% and the reproducibility for six laccase-modified graphite electrodes, prepared and used different days was about 11%. The optimal conditions for the biosensor operation were: 0.1 M citrate buffer solution ( at pH 5.0), flow rate of 0.51 ml min −1 and a working potential of −50 mV versus Ag|AgCl. At these conditions the responses of the biosensor for various phenolic compounds were recorded and the sensor characteristics were calculated and compared with those known for biosensors based on laccase from Coriolus hirsutus, cellobiose dehydrogenase (CDH) from Phanerochaete chrysosporium and horseradish peroxidase (HRP).

Mass distribution in a dynamic sample zone inside a flow injection manifold: modelling integrated conductimetric profiles

A mathematical model for fitting the experimental ICM (integrated conductimetric method) curves developed by the authors in a previous work, is presented for the first time in this study. The proposed model fits the experimental curves with great precision and allows to predict physical dispersion for single-line flow injection system. The correlation of the model’s parameters with typical reactionless FIA peak parameters is also assessed. The IDQ coefficient—a novel dispersion estimator previously reported by the authors—can also be predicted when operational FIA variables are changed. Experimental and modelled profiles are compared as a function of the system’s variables, showing an excellent agreement.

SIMPLE FLOW INJECTION TITRATION METHOD BASED ON VARIATION OF THE SAMPLE VOLUME

ABSTRACT A simple single-line flow injection system incorporating the fully rotary valve (FRV) has been developed for titration purposes. The FRV enables to inject the titrand solution to the titrant stream in four different volumes producing a set of four adequate analytical signals. Due to this fact the titration procedure can be performed with the use of only two standard solutions which allow four parallel calibration lines to be constructed. The method was tested on the example of spectrophotometric acid–base titration. Under optimised flow conditions (flow rate of 8.3 mL/min, the volumes injected of 30, 100, 300, and 800 µL, titrant containing 0.1 mmol/L NaOH and 4 × 10−4% bromothymol blue) the analytical results of precision less than 0.6% (RSD) and of accuracy less than 0.5% (relative error) were obtained for the HCl samples of pH ranged from 1 to 2.

Potentiometric flow injection determination of glycerol in distilled spirits.

A single-line flow injection system including a tubular periodate-selective electrode without inner reference solution is proposed for glycerol determination in distilled spirits, based on oxidation of this polyol by periodate. Interferences due to 5.0 mg L(-1) Cu, 5000 mg L(-1) sucrose, and 3000 mg L(-1) fructose plus glucose were investigated. The procedure is characterized by a linear response for 20-500 mg L(-1) glycerol (r > 0.9999, n = 7), a relative standard deviation of results of <0.03, and an analytical throughput of 30 determinations per hour. Accuracy was assessed by applying the procedure to distilled spirits of sugarcane and grape already analyzed by HPLC; in addition, recoveries within 96 and 120% were obtained.

Na+-selective nasicon electrode used as potentiometric and amperometric sensor in a wall-jet FI system

A Na+-selective NASICON-membrane electrode (Na+-SNME) was tested in a single-line flow-injection (FI) system, both in potentiometric and amperometric operational modes. The Na+-SNME was homemade, using a NASICON pellet and an Ag/AgCl, 0.02 M NaCl internal reference electrode, and it was inserted into a wall-jet electrochemical cell. For a 28 μL sample loop and an injector-cell connecting tube (i.d. 0.5 mm) as short as possible, the optimum experimental conditions (flow rate, jet-nozzle membrane distance, and applied potential value) were established. The FI results obtained in the steady-state potentiometric operational mode confirmed those previously reported for batch measurements: lower limit of the linear range 4 × 10−4 M; slope 58.6 mV/pNa; detection limit 2 × 10−4 M; selectivity coefficients KNaK = 2 × 10−2, KNaLi = 1.6 × 10−2, KNaH = 3.8. For FI amperometric operational mode, the analytical parameters were better than those corresponding to the potentiometric one: detection limit 10−5 M; selectivity coefficients KNaK = 7.1 × 10−3, KNaLi = 7.6 × 10−3, KNaH = 3.1; a linear dependence between the current intensity and the Na+-concentration in the low concentration domain (20–100 μM). The FI amperometric sensitivity was quite high, leading to a good repeatability: the relative standard deviations for 10 repeated injections were 2.3% and 0.7% for pNa = 4 and pNa = 3, respectively. © 1998 John Wiley & Sons, Inc. Lab Robotics and Automation 10: 339–345, 1998

Sequential flow-injection spectrofluorimetric determination of coumarins using a double-injection single-line system

A simple and rapid flow-injection (FI) technique with spectrofluorimetric detection is presented for the sequential determination of two model compounds: 6-methoxy-7-hydroxycoumarin (scopoletine) and 7-hydroxycoumarin (umbelliferone) without prior separation. A double-injection valve with a single-line flow-injection system is used for the simultaneous injection of a sample and a phosphate buffer solution in aqueous 50% ( v v ) ethanol in order to measure the fluorescence signal of the analytes successively at pH ∗ 6.0 and 11.0. The optimum excitation and emission wavelengths are 350 nm and ≥ 418 nm, respectively. Using a calculation method based on differential fluorimetry it is possible to determine ≥ 1 μmol l −1 of scopoletine in the presence of ≥ 9-fold molar excess of umbelliferone or 1 μmol l −1 of umbelliferone in the presence of ≥ 23-fold molar excess of scopoletine. This technique can be used for sequential determination of various other fluorescent compounds (exhibiting pH-dependent excitation spectra) in two-component mixtures; it allows for a quick interchange of reagents within a single-line FIA system and the reagent consumption is decreased substantially.

Flow-injection analysis of phenols at a graphite electrode modified with co-immobilised laccase and tyrosinase

The kinetic parameters of the oxidation reaction of phenolic compounds by molecular oxygen catalysed by fungal laccase have been studied. An amperometric biosensor for detection of phenols in environmental analysis is proposed. The enzymes laccase and tyrosinase were co-immobilised by adsorption onto a spectrographic graphite electrode. The bienzyme electrode was used as a sensor in a single-line flow-injection system. The analysis is based on the amperometric detection of the enzymatic reaction products at a potential of -0.05 V vs. Ag AgCl . The experimental parameters were optimised. The joint use of laccase and tyrosinase in the analytical procedure allows the detection of a large group of phenolic compounds.

Mathematical modelling of flow-injection systems

Analysis of the great variety of flow-injection (FI) manifolds used in analytical practice nowadays has shown that most of them can be decomposed into two basic flow configurations, i.e., the single-line and the conjugated two-line system. The former system has one influent and one effluent stream through which it can contact with the environment. The conduit walls are totally impermeable. The most distinctive characteristic of a conjugated two-line system is the existence of a flow-through section with two separate streams (e.g., donor and acceptor) which exchange matter continuously along a common semipermeable interface (e.g., membrane). It can be concluded that two of the cornerstones in the modelling of FI manifolds are the successful mathematical description of the two basic flow systems mentioned above. Numerous mathematical models of FI systems employing ideas from different scientific areas (e.g., statistics, chemical engineering, artificial intelligence, chromatography) have been developed so far. It should be pointed out that the majority of them describe only single-line FI systems. A classification of all these models based on the main principles on which they are built, is proposed. The models have been compared with respect to their predictive power, the complexity of their mathematical treatment, and the requirements for computation time when applied to single-line and conjugated two-line FI systems. It is concluded that the axially dispersed plug flow model deserves special attention because it offers an acceptable compromise between the conflicting requirements for maximal possible mathematical simplicity and maximal possible precision. It can be used as the basis for an unified approach to the modelling of FI systems.

Determination of chloride ions by reaction with mercury thiocyanate in the absence of iron(III) using a UV-photometric, flow injection method

The absorbance at 254 nm of a previously undocumented complex, which forms when chloride and mercury thiocyanate are mixed in aqueous solution, provides the basis for a new technique for the determination of chloride ions in water. Performance of this new technique is compared to that of the widely applied method described by Utsumi, in which FeIII is included as a reagent and the absorbance of the absorbing species, Fe(SCN)2+, is measured at 450 nm. A single-line flow injection (FI) system was employed for studies of the Cl–Hg(SCN)2 system at 254 nm. A logarithmic response was obtained for chloride concentrations in the range of 0–2000 µg ml–1 without the need for sample dilution. The detection limit of the system was determined to be 0.16 µg ml–1. Sulfate, magnesium, and calcium ions do not interfere with chloride ion determination. Bromide and iodide ions interfere significantly in the determination of Cl– over the concentration ranges of 10–100 µg ml–1 of interferent.

Tubular detectors for flow-injection potentiometric determination of tetrafluoroborate in electroplating baths

PVC tetrafluoroborate tubular electrodes without an inner reference solution, based on tetraoctylammonium as the ion exchanger and 2-nitrophenyloctyl ether as the mediator solvent, were constructed. The general operating characteristics of the tubular detectors were evaluated by a single line flow-injection system with low sample dispersion and compared to those of corresponding conventionally-shaped electrodes, determined in batch conditions. In flow-injection analysis (FIA), the detector response was linear in the concentration range between 4 × 10 −6 and 10 −1 M, with a mean slope of 59.2 ± 0.9 mV/dec in the pH range between of 2.7 and 11.5 The reproducibility of the detector potentials was approximately 0.2 mV during a day of continuous operation. The membrane used showed a high degree of selectivity. The tubular detectors remained operational over a long period of time (approximately 36 months) and had a response time that enabled a sampling rate of 360 h −1. The electrodes were later used in the determination of tetrafluoroborate in concentrated electrolytic baths for which a high dispersion FIA system was developed. The analyses of seven baths with different compositions presented results with mean recovery values of 98.8% with a coefficient variation of 1.9%.

Spectrofluorometric determination of acrinol by a single line flow injection system.

Spectrofluorometric determination of acrinol by a single line flow injection system.