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.     

Flow injection titrimetric analysis of vitamin C in pharmaceutical products

A flow injection titrimetric method for the assay of vitamin C using cerium(IV) was developed. A 157 mul vitamin C solution was injected into a carrier stream of 7.48 x 10(-3)M cerium(IV) prepared in 0.10M sulfuric acid flowing at a rate of 5.0 ml/min. A 1.3 ml mixing chamber was employed for dispersion of the resultant solution which was propelled through a flow-through cell and detected using the spectrophotometer at a wavelength monitored at 410 nm. The method was applied to the determination of vitamin C in pharmaceutical preparations and a high accuracy was attained when compared with the British Pharmacoepeia (BP) method. Excellent precision indicated by a relative standard deviation of less than 0.9% was always obtained. No interferences were encountered and as many as 36 samples per hour were reached for throughput.     

On Line Determination of Perphenazine Using Flow Injection Analysis

An accurate specific flow injection spectrophotometric method for the determination of perphenazine in the range 50-250 ppm is introduced. In the method, 110 μl of the drug is injected through a stream of 0.2% (w/v) potassium dichromate in 0.25 M sulfuric acid flowing on line as a carrier stream. The drug is oxidized on the flow to the red monocation radical, the peak absorbance of which is monitored at 525 nm. A throughput of up to 300 samples per hour is attained. The mechanism of the reaction is suggested and the method is compared with the United States Pharmacopeia method.     

Flow cytometry-based evaluation and enrichment of multiwalled carbon nanotube dispersions

The uniform aqueous dispersion of carbon nanotubes (CNTs) is a vital but challenging task required for their utilization in most technologies. We propose and demonstrate a technique based on forward- and side-scatter analysis on a flow cytometer to characterize the components in a dispersion of multiwalled CNTs (MWCNTs). The method simultaneously distinguishes various MWCNT components such as short and long CNTs, nanotube bundles, and particulates. It also detects the emergence of new CNT populations as a result of centrifugation. We use this method, together with classical methods such as UV and Raman spectroscopy, to observe and study the multistep MWCNT dispersion process in various surfactants (Pluronic, Triton X-100, sodium dodecyl sulfate, and cetyl trimethylammonium bromide). On the basis of the distinct scatter patterns obtained, we confirm and elaborate the surfactant-assisted unzipping mechanism of MWCNT dispersion. We also show that the ultrasonic energy spent after MWCNT unbundling and unwinding can be minimized and the process optimized for each surfactant by correct end point detection through scatter analysis. The ability to enrich nanotube population in dispersion by using the sorting mode of a flow cytometer is confirmed by electron microscopy and Raman spectroscopy. This method can thus be used for observing and enriching MWCNT components and as a complementary technique to UV spectroscopy for studying and optimizing MWCNT dispersion in surfactants.     

Flow injection calibration methods for atomic absorption spectrometry

The use of an atomic absorption spectrometer as a detector in flow injection analysis is briefly reviewed. A new simplified model is described for the dispersion effects observed with such systems; the model is based on considering the dispersion to be due to a single hypothetical mixing chamber located immediately prior to the measurement stage. The utility of this approach is demonstrated for two methods of calibration commonly used in atomic absorption spectrometry, and it is shown that flow injection sample and standard handling techniques are comparable to manipulation with volumetric apparatus. The flow injection method has a number of advantages for the analogue of the standard addition method. The use of an exponential concentration gradient is proposed as a novel method of calibration using a single concentrated standard. Results are presented for the determination of chromium in standard steels.     

Using flow injection analysis to time-resolve rhythmic and pulsatile signals

Continuous monitoring can be used to detect rhythms, an important aspect of biology. But peaks of concentration are broadened by dispersion so that they overlap their neighbours and obscure high frequency chemoperiodicities. In this study, flow injection was found experimentally to be useful in resolving these. A rhythmically varying pattern of permanganate concentration was measured spectrophotometrically. The rhythm (frequency 0.08 Hz) was observable at a dispersion coefficient of 3.0 but not at 3.9 (when only a single peak was recorded). It was again observable using the same high dispersion manifold but positioned after an injection valve that subsampled the stream at intervals. A design based on this work is proposed for an automated instrument that outputs a time series of concentration measures.     

Multi-pumping in flow analysis: concepts, instrumentation, potentialities

A novel strategy for the implementation of flow-based analytical procedures using several micropumps is proposed. The pumps are switched individually or in combination, in order to create a pulsed flowing stream through the analytical path, and are the only active devices acting simultaneously as liquid propelling units, sample insertion ports and commuting elements. Configuration and control of the flow system are then greatly simplified. The micropumps produce distinct stroke volumes at distinct pulse frequencies with high reproducibility ensuring the attainment of very stable flow rates. This leads to an enhanced versatility that enables the utilisation of different approaches for sample management including step-wise variable sample volume, binary sampling and merging zones without reconfiguration of the system hardware. In contrast to the typical flow systems, the proposed one is characterised by a pulsed flow ensuring a fast sample/reagent mixing that contributes to improve the reaction development—thus sensitivity—even in situations of limited dispersion. The basic features and the performance of the proposed strategy are evaluated in the spectrophotometric determination of Cr(VI) in natural waters with 1,5-diphenylcarbazide.     

Tunable Bidirectional Electroosmotic Flow for Diffusion-Based Separations

We present a new concept for on-chip separation that leverages bidirectional flow, to tune the dispersion regime of molecules and particles. The system can be configured so that low diffusivity species experience a ballistic transport regime and are advected through the chamber, whereas high diffusivity species experience a diffusion dominated regime with zero average velocity and are retained in the chamber. We detail the means of achieving bidirectional electroosmotic flow using an array of alternating current (AC) field-effect electrodes, experimentally demonstrate the separation of particles and antibodies from dyes, and present a theoretical analysis of the system, providing engineering guidelines for its design and operation.     

Comparisons Between Numerical Model and Experiments for a Direct Current Plasma Flow

The governing equations describing a flowing stream of a hydrogen plasma encountered in applications, such as diamond deposition, and in devices, such as arcjet thrusters, are solved numerically using the linearized implicit (LBI) Method of Briley and McDonald. The results of simulations under the assumption that the plasma can be described by a single temperature are compared with detailed experimental measurements of flow characteristics and species concentrations in a 1 kW arcjet. These comparisons show that by formulating the problem in terms of known experimental operating conditions, such as mass flow rate, power, and current levels, it is possible to predict many of the characteristics of the flowing plasma. As expected, predictions from this one-temperature model show that some deviations from the experimental results occur near the exit plane of the channel, where unequal electron and heavy particle temperatures are encountered because of lower pressures.     

流动注射分析中样品的分散与载流流速关系的研究

根据串联釜模型，流动注射分析响应曲线的峰高与峰面积、１／４峰高处的峰宽两个因素有关，这两个因素与载流流速的关系分别可以用不同的ｅｘｐ（－ｑ）多项式表示（ｑ是无量纲流速），由于分散度与峰高成反比，分散度随流速的变化关系由此能够很好地得到解释。     

Application of a New pH‐Sensitive Electrode as a Detector in Flow Injection Potentiometry

A new pH sensitive detector for flow‐through potentiometry was developed on the basis of a graphite/quinhydrone composite electrode. The detector was constructed of two polymethylmethacrylate plates between which a 0.1 mm thick sensitive layer is situated. After drilling a hole through the plates, a ring of the sensitive layer is exposed to the solution stream. A conventional calomel reference electrode was placed downstream following the flow‐through sensor. The response behavior in different electrolyte solutions was investigated. The detector shows a Nernstian behavior for injections of hydrochloric acid into a KCl background solution as well as for the steady state response in concentration step experiments using hydrochloric acid and buffer solutions. The response time is sufficiently short for FIA applications (T₉₅ between 3 and 5.4 s). The influences of flow rate and injection volume on the detector signal are discussed.     

Tunable Bidirectional Electroosmotic Flow for Diffusion‐Based Separations

We present a new concept for on‐chip separation that leverages bidirectional flow, to tune the dispersion regime of molecules and particles. The system can be configured so that low diffusivity species experience a ballistic transport regime and are advected through the chamber, whereas high diffusivity species experience a diffusion dominated regime with zero average velocity and are retained in the chamber. We detail the means of achieving bidirectional electroosmotic flow using an array of alternating current (AC) field‐effect electrodes, experimentally demonstrate the separation of particles and antibodies from dyes, and present a theoretical analysis of the system, providing engineering guidelines for its design and operation.     

平头pH电极-流动注射联用技术快速滴定食醋中醋酸的方法研究

采用一种具有平头结构的pH电极作为流动注射分析(FIA)的检测器,构建了流动注射自动化酸度滴定系统.优化了样品进样量、流速、载液浓度和反应管长度等参数.用NaOH溶液作为载液,在4.639×10-4～0.212 mol·L-1范围内醋酸浓度的对数与FIA峰的峰面积成正比,该方法的相对标准偏差(RSD)小于0.5%.采用...     

Flow injection potentiometric detection of trimethylamine in seafood using tungsten oxide electrode

A simple flow injection gas/diffusion method for the determination of trimethylamine (TMA) in seafood with potentiometric detection using tungsten oxide electrode has been developed. The method is based on the diffusion of TMA through a PTFE membrane from a sodium hydroxide donor stream to a phosphate buffer acceptor stream. The TMA in the acceptor stream passes through an electrochemical flow cell containing a tungsten oxide wire and a silver/silver chloride electrode, where TMA was sensitively detected. The parameters affecting the sensitivity of the electrode such as sodium hydroxide concentration, buffer concentration, pH, flow rate and injected volume were studied in details. The electrode response was linear in the concentration range from 1 to 10 μg ml⁻¹ TMA with a correlation coefficient (R²) of 0.991 and a detection limit of 0.05 μg ml⁻¹ TMA. The intra- and inter-days precision (R.S.D.) was found to be, respectively, 1.20 and 1.6% (n=6). The method was applied to the determination of TMA in fish tissue and recoveries of 99-100% were obtained for fish extracts. Results were in close agreement with those obtained by the existing classical official method. Common interference from those species that can diffuse through the membrane were removed by the addition of formaldehyde to the seafood extract. The method is simple, feasible with satisfactory accuracy and precision and thus, could be used for monitoring seafood quality with a sampling rate of 20±2 sample h⁻¹.     

Development of an on-chip injector for microchip-based flow analyses using laminar flow

A new on-chip injector for microchip-based flow analyses has been designed and characterized. The microchip design utilizes separate laminar flow streams of buffer and sample that are brought into parallel contact for a distance of 300 microm. The buffer flow stream is first routed through a conventional 6-port injection valve fitted with a 5 microm i.d. sample loop. When the 6-port valve is actuated from load to inject for a given time, the on-chip buffer flow stream is constricted and the sample flow stream is pressurized into the buffer flow channel. Once the valve returns to the load state the separate laminar flow streams resume. Fluorescence detection was used to characterize the injector and it was found that 50 injections of a 100 microM fluorescein sample led to an average peak height of 174.32 +/- 2.05 AFU (RSD 1.18%) and average peak skew of 1.37 +/- 0.06. The injector was also interfaced with amperometric detection. Injections of catechol solutions ranging in concentration from 500 nM to 100 microM resulted in a linear response (sensitivity = 2.49 pA microM(-1), r(2) = 0.998) and a limit of detection of 155 nM (S/N = 3). Compared to an off-chip injection scheme, plug dilution, band broadening, and peak asymmetry are much reduced. Finally, the injection and subsequent lysis of an erythrocyte sample was demonstrated, with an injected plug of erythrocytes being lysed 5.72 +/- 0.15 s after injection into a flow stream containing sodium dodecyl sulfate (n = 10). The new injection scheme does not require complex valving mechanisms or high pressures and enables reproducible injections from a continuous sample flow stream in a manner where changes in analyte concentration can be monitored with high temporal resolution.     

The dynamic behavior of a stratified column bed packed with porous adsorbent particles having partially fractal structures and a nonuniform ligand density distribution

The dynamic behavior of adsorption in a single column and in stratified column beds packed with porous adsorbent particles having partially fractal structures is studied when all columns have the same total length and the spatial ligand density distribution in the porous microspheres from which the porous adsorbent particles are made, is either uniform or nonuniform and such that the concentration of the immobilized ligands (active sites) increases monotonically from the center of the microspheres to their outer surface. The total number of immobilized ligands in the porous adsorbent particles has the same value whether the spatial ligand density distribution is uniform or nonuniform. The results in this study clearly show that for a given value of the superficial velocity of the flowing fluid stream in the column (for a given value of throughput) the breakthrough time is significantly increased when the radius of the microspheres is decreased, the total number of sections of the stratified column bed is increased, and the spatial ligand density distribution employed in the microspheres is nonuniform. Furthermore, when the superficial velocity of the flowing fluid stream in the column is increased (throughput is increased) the effect that (i) the reduction in the radius of the microspheres and (ii) the increase in the number of sections of the stratified column bed have on providing robust and effective dynamic adsorptive capacity and smaller reductions on the breakthrough time is substantially larger than that realized through the use of the nonuniform ligand density distribution. Similar trends are also observed in the dynamic behavior of adsorption in the systems studied here when the value of the concentration of the adsorbate in the flowing fluid stream entering the column (inlet concentration) has such a high magnitude that the value of the equilibrium concentration of the adsorbate in the adsorbed phase determined from the equilibrium Langmuir isotherm that would correspond to the inlet concentration of the adsorbate in the flowing fluid stream is, for all practical purposes, at its saturation limit.     

A detailed study of sample injection into flowing streams with potentiometric detection

A new method developed by combining manual or coulometric injection into flowing streams and potentiometric detection is described. The potential-time curves obtained in a flow-through detector as a result of single injections are handled theoretically and examined practically. The effects of experimental parameters, concentrations and flow rates on the signal are discussed. A systematic survey of the potential use of the technique in analysis is given. Species can be determined easily either in the flowing stream or in the injected sample. Some applications in pharmaceutical and other types of analysis are listed, and the main advantages of the technique are summarized.     

Flow analysis deconvolution for kinetic information reconstruction

In this work it is proposed a methodology for the kinetic information reconstruction based on the definition of a macrotransport transfer function and a numerical regularisation method. In continuous flowing systems there could be a discrepancy, for fast enough processes, between the read measure in the detector and what actually happens in the chemical reactor. This difference is a consequence of the solute dispersion along the tube. To solve this problem we define a macrotransport transfer function from the Aris–Taylor dispersion theory which enables us a direct interpretation of the experimental data (convolution) or signal reconstruction (deconvolution). The methodology proposed consists in data processing using Tikhonov regularisation method in combination with a specific experimental procedure which allows to characterize the dispersion of the solute along the flowing system. Preliminary results for the determination of the specific area of a gas–liquid reactor are shown analysing the reaction data between the ozone and the Blue Indigotrisulfonate.     

Continuous monitoring with microfabricated capillary electrophoresis chip devices

We report on the direct coupling of hydrodynamically flowing stream to a microchip capillary electrophoresis (CE) for continuous assays of liquid samples. The new interface relies on mounting the sample tubing onto a sharp inlet tip and allows rapid, convenient and reproducible electrokinetic loading from a continuously flowing stream directly into the narrow separation microchannel. The sharp inlet interface is characterized by its efficiency, stability and simplicity. The effect of the sample flow rate, applied voltages and other relevant variables, is described. It was found that the peak intensity is independent of the flow rate. The performance of the new interface is illustrated for on-line CE-electrochemical monitoring of phenolic and explosive compounds. Conditions simulating continuous long-term monitoring, led to a highly stable response for a 15 ppm 1,3,5-trinitrobenzene solution (RSD = 3.7%, n= 40). Such ability to continuously introduce flowing samples into micrometer channels makes 'lab-on-a-chip' devices highly compatible with real-life monitoring applications.     

Confluent streams in flow injection analysis

The addition of a confluent stream increases the mean length of the sample zone and simultaneously decreases the involved concentrations. Simple equations describing these effects under ideal mixing conditions are proposed. The effects of the confluent stream on the overall sample dispersion may be more or less compensated, depending on the decrease in the post-confluence dispersion. When this compensation is almost total, a paradoxical situation occurs, in which the recorded peak height becomes practically unaffected by variations in the flow rate of the merging stream. In this situation, the peak width approaches a limiting value which is independent of the injected volume and of the flow rate of the confluent stream. Limiting values for the peak width and for the mean length of the sample zone are postulated. When the post-confluence dispersion is negligible, the effects of the confluence appear in the recorded peak. The confluent stream addition reduces the overlap between successive sample zones. The site of the confluence may be an important dispersion factor. A saturation index is proposed and practical implications are discussed.