Analytical Flow System Research Articles

Electrochemical flow injection approach for routine screening of lipase activity in pancreatic preparations

A state-of-the-art strategy for the determination of lipase activity in pancreatic preparations using flow injection analysis (FIA) with electrochemical detection (FIA-ED) is described. The procedure is based on the enzymatic reaction of a specific substrate (1,3-dilinoleoyl-glycerol) with lipase from porcine pancreas and determination of enzymatically formed linoleic acid (LA) at +0.4 V by applying a cobalt (II) phthalocyanine-multiwalled carbon-nanotubes modified carbon paste electrode (Co(II)PC/MWCNT/CPE). In order to get a high-performance analytical method, sample preparation, flow system, and electrochemical conditions were optimized. Under optimized conditions the lipase activity of porcine pancreatic lipase was calculated to be 0.47 units per mg lipase protein based on the definition that 1 unit hydrolyses 1 microequivalent linoleic acid from 1,3-dilinoleoyl-glycerol per 1 min at pH 9 and 20 °C (kinetic measurement: 0–25 min). Moreover, the developed procedure was shown to be easily adaptable for the fixed-time assay (incubation time 25 min) as well. In this case, linear correlation between flow signal and lipase activity was found in the range from 0.8 to 18 U L−1. LOD and LOQ were determined to be 0.3 U L−1 and 1 U L−1, respectively. The kinetic assay was further preferred for the determination of lipase activity in commercially available pancreatic preparations. The lipase activities of all preparations obtained by the present method were found to be in good correlation with those obtained by the titrimetric method and declared by manufacturers.

Carbon dioxide spectrophotometric determination in whole roasted coffee beans using a total analysis system after super-modified simplex optimization

In this work an automated methodology for the spectrophotometric determination of carbon dioxide in whole roasted coffee beans was developed. The analytical flow system was based on a reaction vessel featuring a lab-made gas diffusion flow cell. The coffee beans, without any pre-treatment, were poured in a vessel into which an acidic solution was added to promote carbon dioxide evolution from the sample. By pervaporation, the gas was thereafter trapped in a gas diffusion flow cell containing an acceptor solution which was afterwards analyzed by spectrophotometry. To enhance the sensitivity of the methodology, an experimental design based on super-modified simplex method was carried out to adjust the chemical composition of the acceptor solution. The developed methodology showed to be suitable to determine carbon dioxide in roasted coffee samples, and present as main advantages a higher determination rate (ca. 7 h−1) and reduced labor comparing with usually followed methodologies. Under the selected experimental conditions, the minimum quantity of carbon dioxide detected was of 4.62 × 10−7 mol. The results obtained by the developed methodology were correlated with those provided by the Hinman methodology.

Compact analytical flow system for the simultaneous determination of l-lactic and l-malic in red wines

During the malolactic fermentation of red wines, l-malic acid is mainly converted to l-lactic acid. Both acids should be precisely measured during the entire process to guarantee the quality of the final wine, thus making real-time monitoring approaches of great importance in the winemaking industry. Traditional analytical methods based on laboratory procedures are currently applied and cannot be deployed on-site. In this work, we report on the design and development of a bi-parametric compact analytical flow system integrating two electrochemical biosensors that could be potentially applied in this scenario. The developed flow-system will allow for the first time the simultaneous measurement of both acids in real scenarios at the real-time and in remote way. Miniaturized thin-film platinum four-electrode chips are fabricated on silicon substrates by standard photolithographic techniques and further implemented in a polymeric fluidic structure. This includes a 15 µL flow cell together with the required fluidic channels for sample and reagent fluid management. The four-electrode chip includes counter and pseudo-reference electrodes together with two working electrodes. These are sequentially modified with electropolymerized polypyrrole membranes that entrap the specific receptors for selectively detecting both target analytes. The analytical performance of both biosensors is studied by chronoamperometry, showing a linear range from 5 × 10−6 to 1 × 10−4 M (LOD of 3.2 ± 0.3 × 10−6 M) and from 1 × 10−7 to 1 × 10−6 M (LOD of 6.7 ± 0.2 × 10−8 M) for the l-lactate and the l-malate, respectively. Both biosensors show long-term stability, retaining more than the 90% of their initial sensitivity after more than 30 days, this being a prerequisite for monitoring the whole process of the malolactic fermentation of the red wines (time between 20 and 40 days). The flow system performance is assessed with several wine samples collected during the malolactic fermentation process of three red wines, showing an excellent agreement with the results obtained with the standard method.

Spontaneous Charge Generation in Flowing Albumin Solutions at 35 °C and 38 °C.

The time dependence of a charge accumulation in a 10−15 M albumin solution, flowing through a measuring cell of an analytical flow system injector, had a nonlinear character under certain conditions, within a human physiological temperature range. Sharp charge increases depended on albumin concentration. This effect must be taken into consideration when generating models that describe electrokinetic phenomena in flowing protein solutions and when developing analytical flow systems for the registration of biomolecules in low concentration ranges.

Development of a chromatographic low pressure flow injection system using amperometric detection: Application to the analysis of niacin in coffee

In this work, an analytical flow system able to perform low pressure chromatography with amperometric detection is presented. As case study, the determination of niacin (vitamin B3) in coffee brewed samples was selected. The manifold comprised a 1.0cm length monolithic column coated with didecyldimethylammonium bromide, a laboratory-made boron doped diamond electrode, and featured in-line ionic strength adjustment of the mobile phase. The figures of merit concerning the selected case study namely, detection limit, 7.90×10−7M, determination rate, ca. 10 samplesh−1, mobile phase and ISA solution consumption, ca. 2.6mL per analysis, and CV, below 5% for retention time and peak height, showed the competitiveness of this analytical strategy comparing to the described HPLC methods for niacin determination. The strategy displays a simple configuration, low cost, fast and easy assembling, foreseeing its use to general purpose applications.

Multipumping flow systems devoid of computer control for process and environmental monitoring

In this paper we describe for the first time the use of astable multivibrator circuits for computer-less control of solenoid micropumps and application for analytical flow techniques. Triggering and powering were accomplished using the NE555 integrated circuit. The activation and deactivation time intervals were adjusted using 10-turn trimmer potentiometers. The potential and characteristics of the instrumentation were studied on a two-channel flow system injecting an indigo carmine indicator solution. Subsequently, a three-channel flow system was assembled and successfully applied to the determination of nitrite in well waters. One circuit was used to control the activation time intervals of three further circuits used for the control of the flow rates or pulsation of solenoid micropumps. These were used for driving carrier, reagent, and sample in an analytical flow system. In the present work, the utility of the circuits for the construction of simple multipumping flow systems was demonstrated. A main feature to be highlighted was the simplicity and very low costs of the controlling circuits, favouring economic and miniaturised flow analysers. Second, no expenses or knowledge are required for the usual software control of the solenoid micropumps. This allows working with every existing detector without considering any problems of software and control compatibility. Third, owing to these features, the proposed assembly is especially suited for simple monitoring analysers, sample provision from an environmental or industrial process, or chemical education.

Chemiluminometric determination of captopril in a multi-pumping flow system

In this work, a simple, versatile and fully automated analytical methodology for the chemiluminometric determination of captopril – an angiotensin II-converting-enzyme (ACE) inhibitor – in pharmaceutical formulations, is proposed. The developed methodology was based on the enhancement by captopril of the chemiluminescence emission of tris(2,2′-bipyridyl)ruthenium(II). In sulphuric acid medium tris(2,2′-bipyridyl)ruthenium(II) was oxidized by cerium(IV) and converted into a reactive oxidant specie [Ru(bpy)3]3+, which was subsequently reduced with captopril in order to yield a significant enhancement of the original chemiluminescence emission that was directly related to captopril concentration. The analytical process was implemented by resorting to an automated multi-pumping flow system (MPFS) that enabled the establishment of multiple reaction interfaces, which, in combination with the created pulsed flowing stream assured a fast and reproducible sample/reagent mixing and reaction development essential to guarantee the generation and subsequent measurement of the short-lived species involved in the chemiluminescent process. The developed system employed three solenoid micro-pumps as the only flow manifold active components. These assured the insertion, propelling and commuting of all solutions. The automatic actuation of the solenoid micro-pumps provided an easily programmed, operated and controlled analytical flow system, exhibiting high versatility, efficiency and compactness at a low cost.Under the optimized experimental conditions, the proposed method allowed the determination of captopril for concentrations between 2×10−3 and 1.5×10−1mmolL−1 (r=0.9996, n=6) and a sampling frequency of about 58 determinations per hour, producing 620μL of waste per determination. The results obtained for pharmaceutical formulations were statistically comparable to those provided by the reference procedure with a relative deviation between −2.32 and 1.39%. The possible mechanism of the chemiluminescence reaction was also discussed in this work.

A Flow System for Video Demonstrations: Comparative Evaluation of the Stability of Some Cuprate(II) Complexes

An analytical flow system for class demonstrations is proposed. A digital camera monitors the flow-through cuvette, allowing images of the flowing sample to be real-time displayed on a large screen. The demonstration involves in-line formation and monitoring of different coloured cuprate(II) complexes. By taking into account the typical coloration of the gathered images, it is possible to classify the complexes according to their stability constants. Concepts of analytical selectivity and sensitivity are discussed. The flow system is rugged and portable, requiring minute amounts of solutions.

A critical comparison of analytical flow systems exploiting streamlined and pulsed flows

Multipumping (MPFS) and multicommuted (MCFS) flow systems relying on pulsed and laminar flows were critically compared. The mixing conditions and dispersion associated with both systems were evaluated by simulating the sample with bromocresol green. The molybdenum blue method for phosphate determination in soil extracts was also implemented in both flow systems. Furthermore, laser-induced fluorescence (LIF) was applied to visualize the dispersing sample; rhodamine B was used as the fluorescent species. The pulsed flow enhanced the mixing of the solutions involved, thus reducing reagent consumption (48 and 96 microl for MPFS and MCFS), and improving sampling rate (67 and 144 h(-1) for MCFS and MPFS). For phosphate determination, results obtained with both systems were precise (r.s.d. < 0.5%; n = 10) and accurate. Analyses of the absorbance vs time/space LIF plots revealed that exploitation of pulsed flow led to a pronounced radial dispersion and to a limited axial dispersion, typical aspects of turbulent flows.

Optimization of the geometry of a double-path spray chamber for inductively coupled plasma-atomic emission spectrometry by computer simulation and an evolutionary strategy

Computational fluid dynamics (CFD) is used to investigate the analytical performance of variously sized inductively coupled plasma (ICP)-double-path spray chambers (Scott-type) and for the optimization of the geometry of the spray chamber using an evolutionary strategy. It can be seen from the simulation of the aerosol transport in chambers of various lengths that shorter chambers have better characteristics. This result was confirmed experimentally. A higher mass flow rate at the outlet of the chamber (aerosol yield) is obtained by reducing the diameter of the inner tube and keeping the other dimensions constant. An evolutionary strategy in combination with CFD was applied in order to find the optimum geometry for this type of chamber. For the evaluation of the virtual chambers the development of a quantitative objective function was necessary. The search for the best chamber geometry was then performed by searching for the global maximum of this function. As result, a modified ICP-double-path chamber with a much better analytical performance than that of the initial one was obtained. Replacing the original chamber by the optimized one, in the same inductively coupled plasma-atomic emission spectrometry instrument, resulted in the detection limits being lowered by a factor of 2–6 depending on the element. The combination of CFD and evolutionary algorithms can be used as a new and powerful tool for the optimization of various analytical flow systems.

A novel analytical low-cost flow system based on a 0.6 MPa (84 psi) diaphragm pump applied to on-line trace pre-concentration in flame AAS and ICP-OES.

A novel inexpensive 0.6 MPa (84 psi) flow system based on a low-cost diaphragm pump has been developed. The unfavourable strong pulsation of the pump has been overcome by using highly flexible silicone tubing as a pulse suppression coil. This results in a smooth pulse-free continuous flow of 100 mL min(-1) in circulation. This flow rate is much too high for a flow-injection system; however, with a restrictor capillary the flow rate required can be tapped off down to a range of 0.1-50 mL min(-1). By employing diaphragm pumps in an analytical flow system the pressure gap between HPLC pumps (2-40 MPa) and peristaltic pumps (<0.2 MPa), mainly used in FIA systems, can be filled. Due to the higher pressure delivered by diaphragm pumps relative to peristaltic pumps, the new flow system can be applied to on-line sample pre-concentration and matrix separation in flame AAS and ICP-OES by using standard HPLC pre-columns or small ion-exchange columns, respectively. In this way, very low detection limits in flame AAS have been reached (Cd 0.07 micro g L(-1), Cu 0.05 micro g L(-1), Co 0.9 micro g L(-1), Ni 0.8 micro g L(-1), Mn 0.7 micro g L(-1), Pb 0.8 micro g L(-1) and Tl 0.2 micro g L(-1)).

Approaches to in-line removal of dissolved oxygen in flow systems for process analysis

Two methods for the removal of oxygen in analytical flow systems are described. Efficient deoxygenation can be realized by introducing N 2 into the stream to form a two-phase flow combined with a membrane gas-liquid separator. A similar effect can be obtained by passing the flow through a length of Poreflon membrane tubing placed in a water bath at 100 °C. These methods are simple and rapid and remove more than 98% of the oxygen originally present. In order to evaluate the efficiencies of oxygen removal by these methods, we developed a reversed (reagent injection)-FIA system utilizing pyrogallol for spectrophotometric oxygen determination.

On-line organic-phase enzyme detector

The utility of flow carbon-fiber enzyme electrodes for on-line biomonitoring of organic streams is illustrated. The enzymes peroxidase and tyrosinase are immobilized by simple adsorption onto an electrochemically pretreated carbon-fiber surface. The resulting detectors respond very rapidly to dynamic changes in the concentration of organic peroxides and phenolic compounds in flowing chloroform and acetonitrile solutions. The flow-injection operation is characterized with high sample throughputs (60 h −1, good precision (1.1–1.7% R.S.D.) and detection limits of 2–4 × 10 −5 M. The organic-phase enzyme electrode could be combined with various types of analytical flow systems and should allow on-line monitoring of previously inaccessible organic matrices.

Cold-vapor mercury atomic absorption spectrometry III. Signal reporting options

Methodology is presented for the normalization of integrated absorbance signals (absorbance·time) to time-independent absorbance values in analytical flow systems. This measure, absorbance·cm 2, represents the absorbance if all the analyte atoms were present simultaneously in the vapor phase in a normalized optical analysis volume. The measure, for which the name “normalized absorbance” is suggested, is one which is totally independent of optical analysis cell dimensions and carrier gas flow rates. Application of this method to cold-vapor mercury atomic absorption (CVMAA) is reported.

New sample introduction devices for analytical atomic spectrometry

New devices for sample introduction into instruments for analytical atomic spectrometry are beginning to see the light of day. These devices facilitate the coupling of the instruments to analytical flow systems, e.g., flow injection analysis and high-performance liquid chromatography. The use of such devices gives markedly enhanced limits of detection compared to conventional nebulizer systems.

フローインジェクション分析法を応用した冷却水水質管理システム

For an open cooling water system with a zinc-polyphosphate inhibitor, a water quality monitoring system adopted Flow Injection Analysis was introduced. Concentrations of phosphate (inhibitor) and Fe(II) ions were measured automatically by the analytical flow system. Analytical curves for phosphat and Fe(II) ions have good linearity, and their correlation coefficients were 0.9995 and 0.9999, respectively.Reproducibility of measured value for Fe(II) ions was very good and that for phosphate ions was not so good. But these results were satisfactory for practical purposes. When the analytical flow system was applied to on-line monitoring, measured value for phosphate ions have a little amount of scatter. The reason may be explained as follows: 1) the conversion rate of phosphorus forms, during water sample is running through the heated zone, has slightly different at each measurment, and 2) the capability of ascorbic acid as a reducing agent for antimony-phospho-molybdate complex becomes weak with lapse of time. However, this water quality monitoring system has been developed to the point of being usable in practical applications.

A flow detector based on square-wave polarography at the dropping mercury electrode

Square-wave polarography (s.w.p.) at the DME has been adapted for detection in various analytical flow systems. The combination of high sensitivity with rapid potential scan rates results in detector characteristics significantly superior to those previously reported for techniques involving constant applied potential. In an automated flow system for the analysis of discrete samples, s.w.p. allows sensitive and reproducible multi-component sample analysis, at a sampling rate of 22.5 samples per hour (at a 1:1 sample/wash ratio and with relatively low sample volumes). The electrochemical selectivity of the detector may be exploited for monitoring Chromatographic column effluents, in cases where the chromatographic separation is incomplete: species eluted simultaneously but having different reduction potentials can be determined with satisfactory sensitivity. The in-situ monitoring system based on s.w.p. allows rapid simultaneous determinations (~ 300 per hour) of a number of contaminants present at the sub-ppm level; because of its long-term stability and reproducibility, it seems well suited for continuous contaminant control.

An analytical flow system based on reversible immobilization of enzymes and whole cells utilizing specific lectin—glucoprotein interactions

An analytical flow system based on reversible immobilization of enzymes and whole cells utilizing specific lectin—glucoprotein interactions