Mário A. Feres

A downsized flow set up based on multicommutation for the sequential photometric determination of iron(II)/iron(III) and nitrite/nitrate in surface water

In this work, a downsized flow set up designed based on multicommutation concept for photometric determination of iron(II)/iron(III) and nitrite/nitrate is surface water is described. The flow system network comprised a set of three-way solenoid valves, reaction coil and a double-channel flow cell, which were nested in order to obtain a compact and small-size instrument. To accomplish the downsizing requirement light source (LED) and radiation detection (phototransistor) were coupled to the flow cell. In order to demonstrated the effectiveness of the system, the photometer methods based on Griess reaction and 1-10-phenantroline for nitrite and iron(II) determination, respectively, were selected. Under computer control the set up provided facilities to handle four reagent solutions employing a single pumping channel, thus permitting also the determination of nitrate and iron(III) after its reduction to nitrite and to iron(II), respectively. The overall system performance was demonstrated working several days running standard solution, no significant variation of base line, linear response range and slop (less than 1%) were observed. The usefulness of the downsized system was ascertained by analyzing a set of surface water. Aiming to access the accuracy sample were also analyzed employing reference procedures and no significant difference at 95% confidence level were observed for the four analytes. Other profitable features such as analytical throughput of 40 determination per hour; relative standard deviation of 1%; linear response range between 50 and 300mugl(-1) for nitrite and nitrate, 0.5-6.0mgl(-1) iron(II) and iron(III); low reagent consumption 75mug for nitrate/nitrite and 0.6mg for iron(II)/iron(III) per determination; and 2.4ml waste generation per determination were also achieved.

Multi-commutation in flow analysis: Recent developments and applications

The concept of multi-commutation in flow analysis is revisited, and emphasis is given to recent methodological and applicative achievements. Multi-commutation is compatible with different flow patterns (unsegmented, segmented, pulsed, tandem) and amenable to concentration-oriented feedback mechanisms. Its exploitation has led to significant attainments mainly in relation to versatility of the flow system. Characteristics and potentialities of the multi-commuted flow systems are discussed, and guidelines for assisting methodological implementation are given. The number of applications has experienced remarkable increase during last years; therefore, the applicative part of this review is focused on the recent noteworthy applications, mainly in relation to environmental, agronomical, pharmaceutical, biological, food and industrial samples.

Downscaling a multicommuted flow injection analysis system for the photometric determination of iodate in table salt.

In this work a downscaled multicommuted flow injection analysis setup for photometric determination is described. The setup consists of a flow system module and a LED based photometer, with a total internal volume of about 170 microL. The system was tested by developing an analytical procedure for the photometric determination of iodate in table salt using N,N-diethyl-henylenediamine (DPD) as the chromogenic reagent. Accuracy was accessed by applying the paired t-test between results obtained using the proposed procedure and a reference method, and no significant difference at the 95% confidence level was observed. Other profitable features, such as a low reagent consumption of 7.3 microg DPD per determination; a linear response ranging from 0.1 up to 3.0 m IO(3)(-), a relative standard deviation of 0.9% (n=11) for samples containing 0.5 m IO(3)(-), a detection limit of 17 microg L(-1) IO(3)(-), a sampling throughput of 117 determination per hour, and a waste generation 600 microL per determination, were also achieved.

Evidences of turbulent mixing in multi-pumping flow systems

Multi-pumping flow systems exploit pulsed flows delivered by solenoid pumps. Their improved performance rely on the enhanced radial mass transport inherent to the pulsed flow, which is a consequence of the establishment of vortices thus a tendency towards turbulent mixing. This paper presents several evidences of turbulent mixing in relation to pulsed flows, such as recorded peak shape, establishment of fluidized beds, exploitation of flow reversal, implementation of relatively slow chemical reactions and/or heating of the reaction medium. In addition, Reynolds number associated with the GO period of a pulsed flow is estimated and photographic images of dispersing samples flowing under laminar regime and pulsed flow conditions are presented.

Multi‐commutation in Flow Analysis: A Versatile Tool for the Development of the Automatic Analytical Procedure Focused on the Reduction of Reagent Consumption

Abstract In this work, a general view of multi‐commutation in flow analysis process is presented. The manuscript comprises a comprehensive description of the basic manifolds designed to demonstrate the multi‐commutation concept including the operational modes and the inherent features. A multi‐function flow system with the ability to carry out three‐operation modes without any hardware modification is also described. Intending to demonstrate the versatility of the flow system based on multi‐commutation process, a set of experiments was designed comprising an in‐line dilution and stopped flow approach. Standard solutions of iron and of orthophosphate were selected as models to demonstrate the analytical potential of the proposed system. The results obtained are presented and discussed. The authors were invited to contribute this paper to a special issue of the journal entitled “Spectroscopy and Automation”. This special issue was organized by Miguel de la Guardia, Professor of Analytical Chemistry at Valencia University, Spain.

Total and inorganic mercury determination in fish tissue by flow injection cold vapour atomic fluorescence spectrometry.

A simple and reliable method for Hg determination in fish samples has been developed. Lyophilised fish tissue samples were extracted in a 25% (w/v) tetramethylammonium hydroxide (TMAH) solution; the extracts were then analysed by FI-CVAFS. This method can be used to determine total and inorganic Hg, using the same FI manifold. For total Hg determination, a 0.1% (w/v) KMnO4 solution was added to the FI manifold at the sample zone, followed by the addition of a 0.5% (w/v) SnCl2 solution, whereas inorganic Hg was determined by adding a 0.1% (w/v) L-cysteine solution followed by a 1.0% (w/v) SnCl2 solution to the FI system. The organic fraction was determined as the difference between total and inorganic Hg. Sample preparation, reagent consumption and parameters that can influence the FI-CVAFS performance were also evaluated. The limit of detection for this method is 3.7 ng g−1 for total Hg and 4.3 ng g−1 for inorganic Hg. The relative standard deviation for a 1.0 µg L−1 CH3Hg standard solution (n = 20) was 1.1%, and 1.3% for a 1.0 µg L–1 Hg2+ standard solution (n = 20). Accuracy was assessed by the analysis of Certified Reference Material (dogfish: DORM-2, NRCC). Recoveries of 99.1% for total Hg and 93.9% inorganic Hg were obtained. Mercury losses were not observed when sample solutions were re-analysed after a seven day period of storage at 4°C.

Sequential injections as an alternative to gradient exploitation for implementing differential kinetic analysis in a flow injection system

A novel flow-based strategy for implementing simultaneous determinations of different chemical species reacting with the same reagent(s) at different rates is proposed and applied to the spectrophotometric catalytic determination of iron and vanadium in Fe-V alloys. The method relies on the influence of Fe(II) and V(IV) on the rate of the iodide oxidation by Cr(VI) under acidic conditions; the Jones reducing agent is then needed. Three different plugs of the sample are sequentially inserted into an acidic KI reagent carrier stream, and a confluent Cr(VI) solution is added downstream. Overlap between the inserted plugs leads to a complex sample zone with several regions of maximal and minimal absorbance values. Measurements performed on these regions reveal the different degrees of reaction development and tend to be more precise. Data are treated by multivariate calibration involving the PLS algorithm. The proposed system is very simple and rugged. Two latent variables carried out ca 95% of the analytical information and the results are in agreement with ICP-OES.

Exploiting Pulsed Flows for Heating Improvement: Application to Determination of Total Reducing Sugars in Molasses and Sugar-Cane Juices

Laminar and pulsed flows typical of multi-commuted and multi-pumping flow systems, were evaluated in relation to analytical procedures carried out at high temperatures. As application, the spectrophotometric determination of total reducing sugars (TRS, hydrolyzed sucrose plus reducing sugars) in sugar-cane juice and molasses was selected. The method involves in-line hydrolysis of sucrose and alkaline degradation of the reducing sugars at about 98 o C. Better results were obtained with pulsed flows, due to the efficient radial mass transport inherent to the multi-pumping flow sys-

Zone trapping/merging zones in flow analysis: A novel approach for rapid assays involving relatively slow chemical reactions

A novel strategy for accomplishing zone trapping in flow analysis is proposed. The sample and the reagent solutions are simultaneously inserted into convergent carrier streams and the established zones merge together before reaching the detector, where the most concentrated portion of the entire sample zone is trapped. The main characteristics, potentialities and limitations of the strategy were critically evaluated in relation to an analogous flow system with zone stopping. When applied to the spectrophotometric determination of nitrite in river waters, the main figures of merit were maintained, exception made for the sampling frequency which was calculated as 189 h(-1), about 32% higher relatively to the analogous system with zone stopping. The sample inserted volume can be increased up to 1.0 mL without affecting sampling frequency and no problems with pump heating or malfunctions were noted after 8-h operation of the system. In contrast to zone stopping, only a small portion of the sample zone is halted with zone trapping, leading to these beneficial effects.

Flow Systems with MnO2-coated Open Tubular Reactors for Spectrophotometric Determination of Ascorbic Acid in Pharmaceutical Products

ABSTRACT A novel strategy for preparing polyethylene open tubular reactors coated with MnO2(s) was pioneered for flow analysis in order to minimize the inherent drawbacks of packed solid reagents. As an application, the determination of ascorbic acid in pharmaceuticals was selected. The method involved reduction of Mn(IV) by ascorbic acid, release of the formed Mn2+ ion, reaction with formaldoxime, and spectrophotometric monitoring. The influence of flow pattern was investigated by using two similar flow systems relying on constant or pulsed flow delivered by peristaltic or solenoid pumps. With pulsed flow, system versatility and ruggedness were improved, and the analytical sensitivity (0.01143 L mg−1 ascorbic acid), detection limit (0.6 mg L−1 ascorbic acid), and sampling rate (60 h−1) evaluated compared to constant flow were improved by 2.1%, 40.0%, and 6.5%, respectively. At the same time, the reagent consumption (2.8 mg per determination) and analytical precision (relative standard deviation of approximately 2.5%) were maintained. One can conclude that the mixing conditions did not limit the analyte/reagent interaction. The results were in agreement with the British Pharmacopoeia method.