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.     

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.     

Design and performance of a new continuous-flow sample-introduction system for flame infrared-emission spectrometry: Applications in process analysis, flow injection analysis, and ion-exchange high-performance liquid chromatography

A new sample introduction system for the analysis of continuously flowing liquid streams by flame infrared-emission (FIRE) spectrometry has been developed. The system uses a specially designed purge cell to strip dissolved CO(2) from solution into a hydrogen gas stream that serves as the fuel for a hydrogen/air flame. Vibrationally excited CO(2) molecules present in the flame are monitored with a simple infrared filter (4.4 mum) photometer. The new system can be used to introduce analytes as a continuous liquid stream (process analysis mode) or on a discrete basis by sample injection (flow injection analysis mode). The key to the success of the method is the new purge-cell design. The small internal volume of the cell minimizes problems associated with purge-cell clean-out and produces sharp, reproducible signals. Spent analytical solution is continuously drained from the cell, making cell disconnection and cleaning between samples unnecessary. Under the conditions employed in this study, samples could be analyzed at a maximum rate of approximately 60/h. The new sample introduction system was successfully tested in both a process analysis- and a flow injection analysis mode for the determination of total inorganic carbon in Waco tap water. For the first time, flame infrared-emission spectrometry was successfully extended to non-volatile organic compounds by using chemical pretreatment with peroxydisulfate in the presence of silver ion to convert the analytes into dissolved carbon dioxide, prior to purging and detection by the FIRE radiometer. A test of the peroxydisulfate/Ag(+) reaction using six organic acids and five sugars indicated that all 11 compounds were oxidized to nearly the same extent. Finally, the new sample introduction system was used in conjunction with a simple filter FIRE radiometer as a detection system in ion-exchange high-performance liquid chromatography. Ion-exchange chromatograms are shown for two aqueous mixtures, one containing six organic acids and the second containing six mono-, di-, and trisaccharides.     

Fast and simple sample introduction for capillary electrophoresis microsystems

A newly designed capillary electrophoresis (CE) microchip with a simple and efficient sample introduction interface is described. The sample introduction is carried out directly on the separation channel through a sharp inlet tip placed in the sample vial, without an injection cross, complex microchannel layouts or hardware modification. Alternate placement of the inlet tip in vials containing the sample and buffer solutions permits a volume defined electrokinetic sample introduction. Such fast and simple sample introduction leads to highly reproducible signals with no observable carry over between different analyte concentrations. The performance of the system was demonstrated in flow-injection and CE measurements of nitroaromatic explosives and for on-chip enzymatic assays of glucose in the presence of ascorbic acid. Employing an 8 cm long separation channel and a separation voltage of 4000 V it offers high-throughput flow-injection assays of 100 samples h(-1) with a relative standard deviation of 3.7% for TNT (n= 100). Factors influencing the analytical performance of the new microchip have been characterized and optimized. Such ability to continuously introduce discrete samples into micrometer channels indicates great promise for high-speed microchip analysis.     

Integration of continuous-flow sampling with microchip electrophoresis using poly(dimethylsiloxane)-based valves in a reversibly sealed device

Here we describe a reversibly sealed microchip device that incorporates poly(dimethylsiloxane) (PDMS)-based valves for the rapid injection of analytes from a continuously flowing stream into a channel network for analysis with microchip electrophoresis. The microchip was reversibly sealed to a PDMS-coated glass substrate and microbore tubing was used for the introduction of gas and fluids to the microchip device. Two pneumatic valves were incorporated into the design and actuated on the order of hundreds of milliseconds, allowing analyte from a continuously flowing sampling stream to be injected into an electrophoresis separation channel. The device was characterized in terms of the valve actuation time and pushback voltage. It was also found that the addition of sodium dodecyl sulfate (SDS) to the buffer system greatly increased the reproducibility of the injection scheme and enabled the analysis of amino acids derivatized with naphthalene-2,3-dicarboxaldehyde/cyanide. Results from continuous injections of a 0.39 nL fluorescein plug into the optimized system showed that the injection process was reproducible (RSD of 0.7%, n = 10). Studies also showed that the device was capable of monitoring off-chip changes in concentration with a device lag time of 90 s. Finally, the ability of the device to rapidly monitor on-chip concentration changes was demonstrated by continually sampling from an analyte plug that was derivatized upstream from the electrophoresis/continuous flow interface. A reversibly sealed device of this type will be useful for the continuous monitoring and analysis of processes that occur either off-chip (such as microdialysis sampling) or on-chip from other integrated functions.     

On-line coupling of gas diffusion to capillary electrophoresis

On-line gas diffusion has been coupled to a capillary electrophoresis system (CE) via a specially designed interface. The sample is merged with a modifying solution, e.g., a strong acid, in a flow system to transform the analytes of interest into their respective gaseous forms. These transformed, gaseous analytes permeate through a PTFE membrane into an acceptor stream comprising of a tris-buffer. The continuously flowing acceptor stream is led into an injector forming an integrated part of a flow injection analysis (FIA) system. The sample receiving carrier stream in the FIA system, a chromate buffer, brings the sample, 50 mul, to the FIA-CE interface into which one end of a separation capillary has been inserted. A small portion of the injected sample enters the capillary (electrokinetic injection) and separation takes place. A UV detector is placed at the other capillary end and a run potential of 25 kV is applied to two platinum electrodes positioned in the flow system. Multiple sample injections can be performed in one uninterrupted electrophoretic run. A typical sampling frequency is 15 h(-1); each run may result in quantitation of at least five anions. The overall repeatability is in the range 1.8-3.6% (RSD). The technique has been applied to the analysis of real samples such as soft drinks, vinegar and wine. Selective discrimination of anions which are unable to form volatile species is accomplished. No off-line sample pre-treatment is needed.     

Ion mobility spectrometry as flow-injection detector and continuous flow monitor for aniline in hexane and water

Ion mobility spectrometry (IMS) was used as a flow-injection detector to quantitatively examine the ionization chemistry of aniline in hexane. A 5-microl sample was vaporized at 15-90-sec intervals in a flowing air stream and analyzed with an IMS equipped with acetone reactant ion chemistry, ambient temperature drift tube and membrane-based inlet. Precision was 3-11% relative standard deviation for 1-100 ppm aniline in hexane with 90-sec injection intervals and detection limits were ca. 0.5 ppm with 5-microl injections. Matrix effects with amine and organic solvent mixtures were observed and corrected for low and medium proton affinity interferences with standard addition methods. Pronounced fouling of the IMS occurred when a continuous water flow was introduced for aqueous flow injection-IMS. Continuous water monitoring without degraded IMS performance was possible by sampling air flow through a Silastic tube immersed in an aqueous sample.     

Flow injection analysis — a survey of its potential for continuous monitoring of industrial processes

Flow-injection methods provide a number of approaches to monitoring. Those discussed include sample injection into a flowing reagent stream, continuous pumping of sample and merging with a reagent stream, and injection of reagent into a sample stream. Peak-height measurements are normally used in these systems, but peak-width measurements can have advantages. Means of achieving multidimensional flow-injection analysis are discussed briefly.     

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

     

Continuous sample monitoring by flow reversal methodology

A flow reversal methodology based on a single change of the flow direction was used for the continuous monitoring of samples injected into an unsegmented flow-system. The photometric detector was located in the loop of the injection valve and the flow was reversed once each whole plug emerged. Sample dispersion in the carrier stream was continuously monitored in systems with and without chemical reaction. The influence of various physico-chemical variables was studied and some analytical possibilities are discussed.     

Spectrophotometric simultaneous determination of creatinine and creatine by flow injection with reagent injection

The reactions of creatinine with picric acid and of creatine with 1-naphthol and biacetyl, both in an alkaline medium, have been used to develop a flow injection method for the simultaneous determination of creatinine and creatine, respectively. The sample containing both analytes is continuously merged with a picrate stream and mixed through the reactors; the coloured stream passes through a flow cell in a spectrophotometer set at 520 nm, recording a continuous signal proportional to the creatinine concentration. The mixture of the reagents 1-naphthol and biacetyl is inserted into the stream by use of the injection valve, which results in a peak (superimposed on the continuous signal), proportional to the creatine concentration. Linear calibration graphs for both analytes were obtained up to 30 mg l^–1 with relative standard deviations <2%, and a sampling rate of 42 measurements h^–1. The method was applied to the determination of creatinine and creatine in broth cube samples.     

Characterization of a condensed-phase membrane introduction mass spectrometry (CP-MIMS) interface using a methanol acceptor phase coupled with electrospray ionization for the continuous on-line quantitation of polar, low-volatility analytes at trace levels in complex aqueous samples

We report the development and application of a capillary hollow fibre membrane interface using methanol as an acceptor phase to deliver target analytes to an electrospray ionization source and a triple quadrupole mass spectrometer. Superior fluid handling systems lead to greater signal stability and membrane integrity for the continuous on‐line monitoring of polar and charged analytes in complex aqueous samples with detection limits in the parts‐per‐trillion to parts‐per‐billion range. The system can be operated in either a continuous flow or a stopped acceptor flow mode – the latter giving rise to greater sensitivity. We report detection limits, enrichment factors and signal response times for selected analytes with polydimethylsiloxane and Nafion^® polymer membrane interfaces. In addition, we demonstrate the use of this interface to detect pharmaceuticals and other contaminants in natural water and artificial urine. The improved sensitivity and analytical response times of our CP‐MIMS system make it possible to continuously monitor dynamic chemical systems with temporal resolutions on the order of minutes. Presented is a comparison of the performance of CP‐MIMS versus direct infusion electrospray ionization, demonstrating the potential advantages over direct infusion for trace analyte measurements in complex, high ionic strength samples. Furthermore, by continuously flowing a reaction mixture in a closed loop over the interface, we demonstrate the use of the system as an in situ reaction‐monitoring platform for the chlorination of a model organic compound in aqueous solution. Copyright © 2011 John Wiley & Sons, Ltd.     

“Stat” methods in continuous flow analysis : Determination of Copper,Iron, Iodide and Hydrochloric Acid

A continuous flow “stat” method is described in which a certain arbitrarily imposed state in the flowing stream is automatically maintained by regulating the rate of flow of one of the components. The electronic system is regulated by measuring a physical phenomenon in the flowing solution. The method is illustrated by the examples of a continuous flow absorptiostat [Fe(III)/S₂O₃^2-/Cu(II)]for determinations of copper(II) (1–10 μg ml^-1), iron(III) (25–250 or 12.5–125 μg ml^-1), as well as for determination of iodide (12.8–128 μg ml^-1). A continuous flow conductostat [HCl/NaOH] for determination of 1–2.5 × 10^-4 M HCl is also described. This analytical technique is intended for automatic continuous monitoring of sample streams.     

热解吸调制,顶空—毛细管气相色谱法测定二元液系的活度系数

本文采用热解吸调制器作毛细管气相色谱直接顶空进样测定二元液体混合物苯－甲苯和丙酮－氯仿的活度系数。热解吸调制器是在毛细管柱前端的一段短的加热段。调制器段外部镀以一薄层导电膜。所体样品连续不断地流经调制器和毛细管柱。     

Multiple injection techniques for microfluidic sample handling

This paper presents an experimental and numerical investigation into electrokinetic focusing flow injection for bioanalytical applications on 1 x N (i.e., 1 sample inlet port and N outlet ports) and M x N (i.e., M sample inlet ports and N outlet ports) microfluidic chips. A novel device is presented which integrates two important microfluidic phenomena, namely electrokinetic focusing and valveless flow switching within multiported microchannels. The study proposes a voltage control model which achieves electrokinetic focusing in a prefocusing sample injection system and which allows the volume of the sample to be controlled. Using the developed methods, the study shows how the sample may be prefocused electrokinetically into a narrow stream prior to being injected continuously into specified outlet ports. The microfluidic chips presented within this paper possess an exciting potential for use in a variety of techniques, including high-throughput chemical analysis, cell fusion, fraction collection, fast sample mixing, and many other applications within the micrototalanalysis systems field.     

Increased size‐sorting performance in gravitational SPLITT by using a pinched sample inlet design

Split‐flow thin fractionation is a continuous, flow‐assisted separation technique for sorting macromolecules and particulate matter on a preparative scale. On reducing the thickness of the sample inlet conduit of a gravitational split‐flow thin fractionation channel, size‐sorting performance is found to increase since particles that are continuously fed into the channel can be more rapidly compressed toward the upper wall of the channel. Experiments are carried out by measuring the number percentage of particles eluted at each outlet as a function of different thickness values of the sample inlet conduit. The effects that the total thickness of the gravitational split‐flow thin fractionation channel and the sample feed concentration have on the size‐fractionation performance are examined with the goal of determining the best pinched sample inlet, gravitational split‐flow thin fractionation channel design.     

Automated on-line microdialysis sampling coupled with high-performance liquid chromatography for simultaneous determination of malondialdehyde and ofloxacin in whole blood

We have developed a system that couples an on-line microdialysis (MD) system with flow injection high-performance liquid chromatography (HPLC)-fluorescence detection for simultaneous measurement of the concentrations of malondialdehyde (MDA) and ofloxacin (OFL) in whole blood samples. The sample matrix was first cleaned with an MD system using an MD probe. A continuously flowing dialysate stream was derivatized on-line and auto-injected into a separation column. MDA and OFL were separated through a reverse-phase C18 column (250 mm × 4.6 mm) at a flow rate of 0.8 mL min⁻¹ and then detected using a fluorescence detector (excitation: 532 nm; emission: 553 nm); the system's components were connected on-line using a valve control. Validation experiments demonstrated good linearity, precision, accuracy, and recovery. The precisions for the determinations of MDA and OFL, measured in terms of relative standard deviations, were 6.5% and 4.6%, respectively, for intra-day assays and 7.5% and 8.7%, respectively, for inter-day assays. The average recoveries of MDA and OFL spiked in plasma were each close to 100%. The use of this on-line MD-HPLC system permitted continuous monitoring of MDA and OFL in OFL-treated whole blood subjected to UV-A irradiation. Based on our results, the UV-A irradiation markedly increased the level of MDA in the OFL-treated whole blood.     

A Novel Interface for Optimized Coupling of Gas Chromatography and Supersonic Jet Spectroscopy

The laser-based methods Laser Induced Fluorescence (LIF) and Resonance-Enhanced Multi-Photon Ionization (REMPI) can be used as highly selective detection modes for gas chromatography (GC). One major prerequisite for successful application of these detection methods is the availability of appropriate and reliable interfaces between the GC effluent and the molecular beam inlet. When a pulsed supersonic molecular beam (jet) source is used, the analyte molecules are efficiently cooled, allowing maximum selectivity of the laser spectroscopic detection methods. However, several technical problems have to be solved for practical realization of a GC-supersonic jet valve hyphenation. The pulsed jet interface should not interfere with the GC properties and the supersonic molecular beam properties. Further a good working cycle for the conversion from the continuously flowing GC current to the pulsed jet gas flow should be attained. This paper presents a novel setup of a GC-pulsed jet interface. The construction allows temporal and spatial compression of the analyte molecules in jet gas pulse and thus an increase of the detection sensitivity. Moreover, the GC effluent comes into contact only with glass surfaces and not with valve parts like plungers and seals. This reduces memory effects and sample decomposition. The valve setup is tested with a REMPI-TOFMS instrument.     

Automated sampling with capillaries

A system is described for the analysis of serum contained in capillaries. The capillaries, filled with samples, are placed directly into a moving stream of diluent which flushes the capillaries, carrying the samples into a continuous flow or discrete system of analysis. The capillaries are inserted into holes in a plastic block which is pushed forward sequentially by a drive mechanism. As each capillary comes into line with an entrance tube and exit tube, reagent is pumped through these tubes and through the capillary. As an alternative, a dispenser is attached to the inlet tube, and as each capillary comes into position, a measured amount of liquid is dispensed through the capillary and into a container. The system is applied to continuous flow analysis of phosphate, alkaline phosphatase, uric acid, and creatinine. The construction of an efficient and reliable peristaltic pump is described for the continuous flow system.     

A reverse-flow injection analysis method for the determination of dissolved oxygen in fresh and marine waters

A reverse flow injection method (rFIA) based on the Winkler titration chemistry, is reported for the determination of dissolved oxygen (DO) in natural waters. Manganese(II) sulfate is injected into a continuously flowing stream of sample and subsequently merges with a reagent stream of sodium hydroxide and sodium iodide. Manganese(II) hydroxide that is formed reacts with DO in the sample to form an oxidized manganese hydroxyoxide floc. Addition of 10% sulfuric acid dissolves this floc, and under acidic conditions, the triiodide ion formed is detected by photometry in a flow through cell at a wavelength of 440 nm. The method is rapid (48 measurements per h), repeatable (R.S.D. ca. 3%, n=3), and has a calculated detection limit of 0.25 mg l⁻¹ (P=0.001). No interference from nitrite or ferrous ions was observed at concentrations typically found in natural waters. The method has been successfully applied to on-line measurement of DO in sediment respiration reactors.