Industrial process control by flow injection analysis

The principle of physiochemical and chemical modulation in flow injection analysis is outlined. Advantageous properties of flow injection analyzers for chemical on-line process control are summarized. Three exampls of applications are presented. First, peak-width measurements enable chemical batch processes to be monitored over several orders of magnitude of analyte concentration whereas peak-height measurements are selected in the critical state of the process where very small changes of analyte concentration must be determined precisely. Secondly, exploitation of variable gradient dilution to match sample concentration to the needs of accurate analysis is combined with trapped-zone selective spectrophotometry. Finally, frequency-discriminated chemical analysis is feasible by combining sample gradient formation with reagent injection.     

The fluoride ion-selective electrode in flow injection analysis: Part 1. General Methodology

A simple flow-injection system for determination of traces of fluoride by means of the fluoride-selective electrode is presented. A comparison of several flow-cell arrangements confirmed the advantages of a well-jet design. Systematic investigations of the parameters affecting response times (i.e., polishing procedure, flow rate, carrier composition) established the optimal experimental conditions for measurements down to 1 μg l^?1 fluoride. Calibration plots in the lower μg l^?1 range were neither Nernstein nor linear, but good precision (0.5–5%) was obtained even when the potential differences for concentration steps of one decade were as small as 3 mV.     

Potential of modified reverse flow injection analysis for continuous monitoring and process control

Summary Modified reverse flow injection analysis is introduced as a novel means for monitoring purposes and online process control. The technique is based on the injection of standard solutions into the continuously flowing sample stream. The transient detector response occurring shortly after the injection reflects the deviation between the analyte concentration of the sample stream and the standard injected. Interpolative calibration and the check of nominal values are two interesting execution modes presented. The beneficial features of the novel approach are discussed and exemplarily demonstrated for practical problems. Experimental conditions are given for the photometric determination of chloride in tap water, phosphate in surface water and the potentiometric determination of fluoride. The respective advantages over common monitoring systems are outlined.

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Einsatzmöglichkeiten der modifizierten Umkehrfließinjektions-Analyse zur kontinuierlichen überwachung und Prozeßsteuerung

     

Fluoride ion-selective electrode in flow injection analysis : Part 2. Interference Studies

The influence of the sample composition on the response characteristics of the fluoride ion-selective electrode in flow injection analysis is described. Sample parameters such as ionic strength, viscosity and pH affect the response time of the electrode and cause transient signals when limiting values are exceeded. The respective limiting values depend on the total ionic-strength adjustment buffer (TISAB) used and these interferences can be minimized by proper choice of the TISAB. The complex formation of fluoride by several elements in the presence of TISAB containing CDTA is discussed. Aluminium and magnesium were found to interfere when present at levels above 1 and 100 mg l^?1, respectively. The signal decrease in the presence of iron, calcium and silicon can be attributed to ionic strength effects rather than complexation. Provided that the ionic strength is taken into account and corrected for, no influence occurs even in the presence of 0.5, 2 and 5% of iron, calcium and silicon, respectively.     

The fluoride ion-selective electrode in flow injection analysis : Part 3. Applications

Flow-injection potentiometry with a combination fluoride-selective electrode is used to determine fluoride in tap water, beverages and urine. Excellent sensitivity (down to 1 μg l^?1) and long-term stability are obtained, with a sample throughput of 30–40 h^?1, based on triplicate injections at 120 h^?1. The commonly used buffer TISAB-III is unsuitable for the analysis of undiluted tea and urine samples. The application of a modified citrate-containing TISAB overcomes interferences caused by high natural ionic strength and avoids complexation of fluoride. Recoveries after spiking tap water, tea and urine with fluoride concentration ranging from 0.01 to 1 mg l^?1 are in the range 91–106%. The equipment used provides a flexible system allowing fast changes between different buffers and carrier streams depending on the samples presented.     

Exploitation of gradient techniques in flow injection analysis

Summary Founded on the key role of dispersion, controlled in space and time, the FIA gradient techniques are based on the feasibility of identifying and selecting reproducibly suitable sections of the time/concentration matrix represented by the dispersed sample zone and exploiting the specific concentrations at these points for analytical purposes. The characteristics and common theme of these gradient procedures — comprising gradient dilution and calibration, stopped flow reaction rate measurement, titrations, gradient scanning, and systems associated with simultaneous injection of two zones which completely or partially overlap thereby permitting the execution of selectivity studies and standard addition procedures — are briefly reviewed.

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Ausnutzung von Gradientenverfahren bei der Fließinjektionsanalyse

     

Flow injection analysis : Part IX. A New Approach to Continuous Flow Titrations

Studies of dispersion patterns in nonsegmented streams, flowing through narrow open tubes, show that it is possible to obtain highly reproducible concentration gradients within a sample zone injected into the moving stream. By varying the geometry of the flow path, low, medium and high dispersion patterns can be achieved; the high dispersion pattern forms the basis for a new approach to continuous flow titrimetry. In this type of titration, discrete samples are passed through a gradient device and are then mixed with a continuously flowing stream of titrant of fixed concentration. The new technique has been tested for potentiometric as well as spectrophotometric end-point indication. A simple one-channel system allows titrations to be performed automatically in less than 1 min.     

The Application of strongly oxidizing agents in flow injection analysis : Part 1. Silver(II)

The application of silver(II) as a powerful oxidizing reagent in flow injection analysis is described in detail. Under the experimental conditions, the half-life of the very unstable silver(II) was about 120 s. Nevertheless, various organic and inorganic substances could be determined. Spectrophotometric detection was at 390 nm where silver(II) in nitric acid solutions absorbs strongly. As neither iron(III) nor copper(II) reacts with silver(II), oxidizable compounds can be determined in the presence of large amounts of these species. Special attention is given to manganese(II), which can be determined selectivity by this method in the range 10^?5–10^?4 mol l^?1.     

Flow injection analysis in on-line process control

Flow injection systems are serious candidates for a new generation of chemical on-line analyzers because there is a growing interest in instruments that combine versatility with the possibility of attaining high sampling frequencies. For real on-line applications the instrument and its component parts have to meet the highest standards with respect to reliability and maintenance. These aspects are considered in some detail, and some industrial applications are briefly discussed.     

Sandwich techniques in flow-injection analysis : Part 2. Simultaneous Determination of Iron(II) and Total Iron

Samples (1500 μl) are inserted between zones of water and ascorbic acid solution with subsequent addition of 1,10-phenanthroline at pH 5.0. The signal provides a plateau region corresponding to Fe(Il) followed by a peak corresponding to total iron. The proposed system allows up to 90 injections per hour. Linear working ranges are 0.1–9 and 0.3–12 mg l^?1 Fe(II) and total iron, with relative standard deviations of < 0.6 and 1.2%, respectively. Results obtained for various ground waters agree well with those obtained by a standard method.     

Application of flow injection techniques for the analysis of inorganic anions

Summary A survey of applications of flow injection analysis for the analysis of inorganic anions is given in form of a table. The turbidimetric determination of sulphate is improved by the intermittent addition of a wash solution via a separate pump. A comparison of results for the determination of sulphate in surface waters using the barium-methylthymol blue method and ion-chromatography shows good agreement. For the determination of sulphite a gas-diffusion technique is used. The method allows 90 injections/h and shows good reproducibility, 1% r.s.d. for consecutive injections of standards at the 10 mg/l SO₂-level. As the sample line is physically separated from the detection line, the method is insensitive to matrix effects from coloured or turbid samples. Results for the determination of free and total sulphite in wine, beer and fruit juices are given.

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Anwendung der Fließ-Injektionsanalyse in der anorganischen Anionenanalyse

Zusammenfassung In tabellarischer Form wird ein Überblick über die Anwendung der Fließ-Injektionsanalyse in der anorganischen Anionenanalyse gegeben. Die Zusammenstellung enthält neben Literaturhinweisen einen kurzen Kommentar zur verwendeten Methode. Für die turbidimetrische Sulfatbestimmung wird ein Verfahren mit intermittierendem Zusatz einer Waschlösung über eine separate Pumpe vorgeschlagen. Ein Vergleich von Ergebnissen der Sulfatbestimmung mit Barium-Methylthymolblau als Reagens in Oberflächengewässern mit Ergebnissen der Ionen-Chromatographie zeigt gute Übereinstimmung. Zur Bestimmung des Sulfits wird ein Verfahren mit Hilfe der Gasdiffusion vorgestellt. Die Reproduzierbarkeit von Mehrfachinjektionen liegt, bei einer Probenfrequenz von 90 Injektionen/h, bei 1%. Das Verfahren zeichnet sich durch Unempfindlichkeit gegenüber Probeneigenfärbungen und -trübungen aus. Ergebnissen für die Bestimmung von gesamtem und freiem Sulfit in Wein, Bier und Fruchtsäften werden gegeben.

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Lecture given at the Symposium for Inorganic Anion Analysis, Regensburg, 19.–21. 9. 1984     

Hydrodynamically limited precision of gradient techniques in flow injection analysis

Based on the law of error propagation, a general expression is derived to study theoretically the hydrodynamically limited precision associated with each single element of fluid in a concentration profile in flow injection analysis. Convolution of the injection and residence time distribution functions is used to obtain response functions for a homogeneously stirred mixing chamber and for a straight capillary tube. The effects of stochastic variations on overall precision by sample introduction, pumping and timing are elucidated and compared with experimental findings. Resulting practical implications for the use of these two dispersing elements are outlined.     

Continuous flow techniques in organic synthesis

As part of the dramatic changes associated with the need for preparing compound libraries in pharmaceutical and agrochemical research laboratories, the search for new technologies that allow automation of synthetic processes has become one of the main topics. Despite this strong trend for automation high-throughput chemistry is still carried out in batches, whereas flow-through processes are rather restricted to production processes. This is far from understandable because the main advantages of that approach are facile automation, reproducibility, safety, and process reliability, because constant reaction parameters can be assured. Indeed, methods and technologies are missing that allow rapid transfer from the research level to process development without time-consuming adaptation and optimization of methods from the laboratory scale to production plant scale. Continuous-flow processes are considered as a universal lever to overcome these restrictions and, only recently, joint efforts between synthetic and polymer chemists and chemical engineers have resulted in the first continuous-flow devices and microreactors; these allow rapid preparation of compounds with minimum workup. Many of these approaches use immobilized reagents and catalysts, which are embedded in a structured flow-through reactor. It is generally accepted, that for achieving best reaction and kinetic parameters for convective-flow processes monolithic materials are ideally suited as solid phases or polymer supports. In addition, immobilization techniques have to be developed that allow facile regeneration of the active species in the reactor.     

Flow injection analyses: Part I. A new concept of fast continuous flow analysis

The concept of a new continuous flow analyser system is described. Based on instant discrete sampling by injection into a carrier stream, the system allows continuous flow analysis to be performed in a fast, much simplified way. As the continuous flowing stream is characterized by a turbulent rather than a laminar flow, the discrete instant sampling creates geometrically well-defined segments of sample solution within the flowing stream. Because of the absence of lag phase, an unprecedented sampling rate for continuous flow analysis of well over 200 samples per hour can be achieved; and even manual injection of the samples allows a very high degree of accuracy and precision to be obtained ( ? ± 1%). Uses of the system in various analytical procedures are described and discussed. A potentiometric sensor (the air-gap electrode used in a flow-through unit) and a spectrophotometric arrangement with a flow-through cell have been used as detector units.     

Single-point titrations: Part 4. Determination of acids and bases with flow injection analysis

A single-point titrimetric system for acids and bases based on the flow injection principle is reported. The sample (30 μl) is introduced into a water stream with a pneumatic injector; this stream reacts with a linear acidic or basic buffer solution in a merging stream, and the peak height is recorded potentiometrically with a glass electrode in a flowthrough cell. The peak maxima are a linear function of the acid or base concentration in the range 0.01–0.1 M. At a sampling rate of 180 samples per hour, the relative standard deviation is less than 1%. The method can be used at sampling rates as high as 720 samples per hour.     

A microprocessor control system for automated multiple flow injection analysis

A microprocessor control system is reported for automated multiple flow injection analysis. The control system consists of an IMSAI-8048 microprocessor, some associated electronic interfacing and a control computer command language. The system can be programmed to control any of three versions of automated multiple flow injection analysers. This control system is relatively inexpensive and is suitable for use by inexperienced personnel.     

Development of solid-phase chemiluminescence immunoassays for digoxin comparing flow injection and sequential injection techniques.

The development of a competitive solid-phase immunoassay for digoxin making use of the acridinium chemiluminescence system is described. Two different instrumental approaches are compared. One is based on a continuous flow system using a peristaltic flow injection analysis pump; the other uses a new sequential injection technique. In both systems a flow cell, consisting of transparent PTFE tubing packed with immobilized antibodies, acts as an immunoreactor. The entire assay, including both the immunoreaction and the chemiluminescence reaction, takes place in this immunoreactor cell. Compared with the flow injection technique, the sequential injection mode showed higher precision, ranging from 2.16 to 5.5% RSD depending on concentration. The total assay time, including regeneration, is less than 8 min with the sequential injection technique. The detection limit for both techniques is in the low femtomole range.