Methods for Multivariate Calibrations for Processing of the Dynamic Response of a Flow-Injection Multiple-Sensor System

Application of chemometric methods to processing of the dynamic response of a flow-injection multiple-sensor system of the “electronic tongue” type is described. A through flow-injection multiple-sensor system of a new type is developed. The data processing is carried out using the method of projection to latent structures and the n-variate method of projection to latent structures (in processing of three-dimensional data). The results obtained in determining ions of lead and zinc in the case of their simultaneous presence in solution and comparative characteristics of various calibration models are presented.__________Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 1, 2005, pp. 90–96.Original Russian Text Copyright © 2005 by A. Legin, Rudnitskaya, K. Legin, Ipatov, Vlasov.     

Flow injection analysis as a diagnostic technique for development and testing of chemical sensors

An immobilized urease sensor is developed for continuous, on-line analysis. The sensor consists of the enzyme urease, cross-linked with bovine serum albumin into a cellulose pad, with an acid-base indicator dye covalently bound to the surface of the cellulose. The sensor is placed within a flow-injection optosensing system to monitor the change in pH, and subjected to a through evaluation, using the flow-injection technique; sensor stability (both dye and enzyme stability), speed of sensor response, sensor sensitivity, sensor-to-sensor reproducibility, response to a typical interferent, and sensor lifetime data are obtained. Sensor poisoning upon exposure to low levels of mercury, and subsequent regeneration of the immobilized enzyme pad, is investigated for use as an on-line mercury sensor. The urea sensor is also evaluated for use as a continuous monitor for urea in kidney dialysate. Enzyme Michaelis-Menten constants are determined for the immobilized urease, under given assay conditions, using a stopped-flow flow-injection technique.     

Photo-cured polymers in ion-selective electrode membranes : Part 3. A Potassium Electrode for Flow Injection Analysis

The preparation of a potassium-selective electrode based on a photo-cured polymer membrane containing valinomycin is reported. This electrode has similar response characteristics to one based on PVC with the same neutral carrier. It is suitable for use in flow-injection systems because of its fast initial response and the hardness and mechanical strength of the membrane; nearly Nernstian response is obtained in the range 10^?1?10^?4 M potassium ion.     

Simultaneous determination of multiple components in flow-injection systems by square-wave amperometry

Two or more components can be determined in a single sample by using a flow-injection system with an electrochmical detector, a microprocessor-based potentiostat, and a microcomputer. The computer generates a repeating staircase potential program with a superimposed square wave. Square-wave amperometric measurements at each potential step are used to construct the current/time response at the potential. By appropriate selection of step potentials, the reconstructed response from each step corresponds to the flow-injection response of a particular component. The approach is evaluated by applying it to the simultaneous determination of copper, lead, cadmium and zinc. Limits of detection range from 8 to 18 μg 1^?1. Sample throughput is 80 h^?1.     

Flow-injection approach for the determination of the dynamic response characteristics of ion-selective electrodes. Part 1. Theoretical considerations

A single-line flow-injection system with a straight tube reactor is proposed for investigating the dynamic response behaviour of ion-selective electrodes. The principle of the method is based on the fact that the concentration—time curve at the electrode surface can be described theoretically in the flow-injection system under certain practically realizable conditions. The response of the ion-selective electrode to that input signal can be measured experimentally. Thus, knowing the input and the output signal of an ion-selective electrode, an appropriate model describing its dynamic behaviour can be selected among the relevant models existing in the literature. Theoretical expressions for predicting the transient response of ion-selective electrodes in the flow system when the rate-determining step is an ion-transport process through a diffusion layer or a kinetic process were elaborated.     

Automated exploration and exploitation of flow-injection response surfaces

Three-dimensional plots of instrumental responses vs. chemical concentrations or flow parameters have been 1 obtained in an automated manner on a computer-controlled flow-injection methods development system. Consideration of several alternative responses for flow-injection systems is shown to help characterize a chemistry more thoroughly and reveal the best experimental conditions. One may see the effects of individual experimental variables (reagent concentrations, pH, flow-rates, etc.), the interactions of these variables, instrumental factors and limitations of the surface exploration procedure employed. Chemical systems studied were the photometric determination of phosphate, palladium(II), iron(II) and persulfate. The propriety of automated response surface mapping is demonstrated and the efficacies of simplex and grid search approaches to response surface exploration are contrasted. Responses obtained include absorbance at peak maximum, relative standard deviation of maximum absorbance, time from injection to peak maximum and wavelength of maximum absorbance. Higher dimensional response surface representations of peak shape and absorbance spectra are also presented. The results show that the response chosen governs the general shape of the surface and the height at any point. This approach to automated characterization of chemical reactions in flow analysis is critically assessed.     

Flow-through bulk optode for the fluorimetric determination of perchlorate

A flow-through fluorescence bulk optode for the flow-injection determination of perchlorate is described. As the active constituents the optode incorporates the lipophilized pH indicator fluorescein octadecyl ester and methyl tridodecyl ammonium chloride, dissolved in a plasticized poly (vinyl) chloride membrane entrapped in a cellulose support. The optode is applied in conjunction with the flow injection technique for perchlorate determination at pH 4.5 (acetic-acetate). The sensor is readily regenerated with the pH 10.4 (TRIS) carrier solution. The analytical characteristics of this optode with respect to perchlorate response time, dynamic measurement range, reproducibility and selectivity are discussed. The proposed FI method is applied to the determination of perchlorate in waters from different sources.     

Enzymatic flow-injection determination of urea in blood serum using potentiometric gas sensor with internal nonactin based ISE

Ion-selective electrode with cellulose triacetate membrane containing nonactin is employed for the potentiometric detection of ammonia produced in biocatalytic reaction in flow-injection system with enzyme reactor. The elimination of interferences occurring in the presence of alkali metal ions was achieved by covering a nonactin membrane with outer hydrophobic gas permeable membrane. The obtained flow-injection response to ammonia indicates a possibility of ammonia determination down to 10 microM ammonia. In the flow-injection system for urea determination 200 microl of 10-fold diluted blood serum sample was injected into carrier stream of distilled water merged with TRIS buffer, passed through the urease flow-through reactor and then after merging with NaOH stream delivered to the detector. It was found in several series of natural blood serum samples, that the correction for endogenous ammonia in such a determination is not indispensable.     

Determination of penicillin V in fermentation samples by flow injection analysis

Penicillin V in fermentation broths is determined, with linear response over the range 100–1000 U ml^?1, by enzymatic hydrolysis to penicilloic acid and subsequent formation of a molybdenum blue. The Auto-Analyzer and flow-injection procedures are compared.     

Potentiometric flow-injection determination of copper-complexing organic ligands with a copper-wire indicating electrode

The potentiometric response of a copper-wire indicator electrode in a flow-injection system with a phosphate-buffered carrier stream can be used to determine copper-complexing ligands; glycine, histidine, L-cysteine, EDTA, ethylenediamine, triethylenetetramine, dopamine and imidazole are discussed. The electrode response is shown to give peak potentials with a Nernstian relationship to total injected ligand concentration over limited ranges, depending on the stoichiometry, stability, and oxidation state of the copper complexes formed. Galvanostatic measurements showed that complex formation with Cu²⁺ or Cu⁺ or mixed species can be responsible for the response characteristics. The effects of adding 0.1 M sodium chloride to the carrier stream are generally beneficial, particularly in obtaining sharper responses. Detection limits can be improved to about 10^?5 M by adding about 10^?5 M Cu²⁺ to the carrier stream, but the linear range of Nernstian response is then narrow.     

Flow-Injection Analysis of Pharmaceuticals

The current state and outlooks of the development of the flow-injection analysis of medicinal substances in pharmaceuticals and biological fluids are considered. The role of chemical, photochemical and enzymatic reactions of derivatization in the flow-injection determination of pharmaceuticals is outlined. The role of detection methods in improving the selectivity and sensitivity of the flow-injection analysis of pharmaceuticals and expanding its possibilities in pharmaceutical analysis is considered     

Universal response model for a corona charged aerosol detector

The universality of the response of the Corona Charged Aerosol Detector (CoronaCAD) has been investigated under flow-injection and gradient HPLC elution conditions. A three-dimensional model was developed which relates the CoronaCAD response to analyte concentration and the mobile phase composition used. The model was developed using the response of four probe analytes which displayed non-volatile behavior in the CoronaCAD and were soluble over a broad range of mobile phase compositions. The analyte concentrations ranged from 1μg/mL to 1mg/mL, and injection volumes corresponded to on-column amounts of 25ng to 25μg. Mobile phases used in the model were composed of 0-80% acetonitrile, mixed with complementary proportions of aqueous formic acid (0.1%, pH 2.6). An analyte set of 23 compounds possessing a wide range of physicochemical properties was selected for the purpose of evaluating the model. The predicted response was compared to the actual analyte response displayed by the detector and the efficacy of the model under flow-injection and gradient HPLC elution conditions was determined. The average error of the four analytes used to develop the model was 9.2% (n=176), while the errors under flow-injection and gradient HPLC elution conditions for the evaluation set of analytes were found to be 12.5% and 12.8%, respectively. Some analytes were excluded from the evaluation set due to considerations of volatility (boiling point <400°C), charge and excessive retention on the column leading to elution outside the eluent range covered by the model. The two-part response model can be used to describe the relationship between response and analyte concentration and also to offer a correction for the non-linear detector response obtained with gradient HPLC for analytes which conform to the model, to provide insight into the factors affecting the CoronaCAD response for different analytes, and also as a means for accurately determining the concentration of unknown compounds when individual standards are not available for calibration.     

Formation of two reaction zones in flow-injection systems for kinetic determinations of cobalt and nickel

The injection of a large sample volume (ca. 1 ml) into a single-channel flow-injection system was studied with a dye (to examine physical dispersion) and with chemical systems having easily-controlled reaction rates (to examine chemical kinetics). With the dye, the response curve has a central plateau caused by non-mixing of carrier and sample. When a chemical reaction takes place, two peaks are obtained with a central minimum corresponding to little or no mixing and reaction. Comparison of these two types of response provides relationships of analytical interest between response parameters and variables in te flow-injection system. The configuration is used for individual kinetic determinations of cobalt and nickel (2.5–30 μg ml^?1 based on the rate of their complex formation with 2-hydroxybenzaldehyde thiosemicarbazone. Sample injection rates were 15 h^?1 for cobalt and 40 h^?1 for nickel. Differential kinetic determinations of cobalt and nickel in mixtures are based on the increment in peak height (or area) between the two peaks obtained for each injection; sample throughput is 7 h^?.     

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.     

Sensitive and selective determination of gallic acid in green tea samples based on an electrochemical platform of poly(melamine) film

In this work, an electrochemical sensor coupled with an effective flow-injection amperometry (FIA) system is developed, targeting the determination of gallic acid (GA) in a mild neutral condition, in contrast to the existing electrochemical methods. The sensor is based on a thin electroactive poly(melamine) film immobilized on a pre-anodized screen-printed carbon electrode (SPCE*/PME). The characteristics of the sensing surface are well-characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and surface water contact angle experiments. The proposed assay exhibits a wide linear response to GA in both pH 3 and pH 7.0 phosphate buffer solutions (PBS) under the optimized flow-injection amperometry. The detection limit (S/N = 3) is 0.076 μM and 0.21 μM in the pH 3 and pH 7 solutions, respectively. A relative standard deviation (RSD) of 3.9% is obtained for 57 successive measurements of 50 μM GA in pH 7 solutions. Interference studies indicate that some inorganic salts, catechol, caffeine and ascorbic acid do not interfere with the GA assay. The interference effects from some orthodiphenolic compounds are also investigated. The proposed method and a conventional Folin–Ciocalteu method are applied to detect GA in green tea samples using the standard addition method, and satisfactory spiked recoveries are obtained.     

Amperometric detection of cationic neurotransmitters at nafion-coated glassy carbon electrodes in flow streams

Substantial improvements in amperometric monitoring of flowing streams are obtained by using Nafion-coated working electrodes. The charged coating tends to exclude anionic and neutral interferences, thus adding a new dimension of selectivity to electrochemical detection for flow-injection and liquid-chromatographic systems. A highly selective response is observed for cationic neurotransmitters in the presence of otherwise interfering substances (e.g., ascorbic acid, uric acid, bilirubin or chlorpromazine). The permselectivity and transport characteristics are evaluated with respect to solution pH, solvent, flow rate, film thickness, and other variables. The reduced flow-rate dependence results in low noise levels and detection limits of 0.04 and 0.10 ng of norepinephrine and epinephrine, respectively. A bilayer electrode coating, with a cellulose acetate film over the Nafion layer, offers a bifunctional (selectivity, protection) capability. Applicability to urine samples is demonstrated.     

Anodic response to oxygen of an electrochemical detector in high-performance liquid chromatography and flow-injection analysis

A strong anodic response associated with the presence of oxygen in liquid chromatography and flow-injection solvent streams is found to occur at the porous-graphite electrodes of a commonly available detector cell. Removal of oxygen lowers the residual current and improves the detection limits considerably in oxidatively-detected analyses.     

Flow-injection system with ion-exchange separation and potentiometric detection for the determination of three halides

The use of ion exchangers in flow-injection systems is reviewed briefly. In the method described, halides are separated on a short column of a strongly basic ion-exchange resin (Dowex 1-X8) placed in the flow-injection conduit, with a laboratory-made tubular silver/silver halide ion-selective electrode as potentiometric sensor. The response capabilities of the different halide-selective electrodes to a wide concentration range (20-5000 mg dm^?3) of single and mixed halide solutions with and without the incorporated ion-exchange column are compared. By careful selection of suitable concentrations of the potassium nitrate carrier/eluent stream to satisfy the requirements of both the ion-exchange column and the halide-selective electrode, it is possible to separate and determine chloride, bromide and iodide in mixed halide solutions with a detection limit of 5 mg dm^?3. The bromide-selective electrode is the most satisfactory detector.     

Limitation of linear response in flow-injection systems with ion-selective electrodes

The lower limit of linear response in flow-injection systems employing membrane and second-kind ion-selective electrodes as detectors depends on the dispersion in the system and is usually much worse than in batch measurements. Below a certain value of the peak height, linearity of the electrode response is not maintained. The limiting value depends on the process causing the loss of linear response (solubility of the electrode material, contamination, or adsorption at the electrode surface), and varies from 15 to 70 mV. Chloride- iodide-, fluoride- and copper(II)-selective electrodes are discussed.