Rui A. S. Lapa

Multicommutation in flow analysis. Part 1. Binary sampling: concepts, instrumentation and spectrophotometric determination of iron in plant digests

Abstract A novel strategy to handle sample and reagent introduction in continuous flow systems is proposed. Basic features such as the effect of flow cell volume, analytical pathlength and peristaltic pump pulsation on the precision of measurement were studied. The manifold was based on a set of three-way solenoid valves controlled by a microcomputer using software written in QuickBASIC. It could also control the rotation speed of the peristaltic pump. By sampling slugs of sample and reagent solutions synchronized with pulsation of the peristaltic pump, aliquots with volumes as low as 2 μl could be sampled with a relative standard deviation

Multicommutation in flow analysis: concepts, applications and trends

Multicommutation refers to flow systems designed with discrete computer-controlled commutators resulting in flow networks in which all the steps involved in sample processing can be independently implemented. The flow systems can be re-configured by the control software, presenting thus increased versatility, potential for automation and for minimization of both reagent consumption and waste generation. The main objective herein is to review the concept of multicommutation in order to permit a proper evaluation of the characteristics and potentialities of the related flow systems, to assist methodological implementation and to discuss similarities with other existing strategies. Implementation of tandem streams, controlled dilutions, wide-range determinations, sequential determinations, titrations and in-line separation/concentration are emphasized.

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

Abstract 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.

Photochemical-fluorimetric determination of folic acid in a multicommutated flow system

Abstract A flow system based on multicommutation was developed for the determination of folic acid by fluorimetry following irradiation by ultraviolet light. The photochemical reaction was carried out in a reaction coil. Insertion of time-controlled sample volumes enabled the determination of a wide range of concentrations without changing the manifold design, which makes it suitable for application to automated dissolution studies. An automated system control selects for each sample, depending on its concentration, the sampling time and therefore the most convenient sample volume. Linear calibration plots were obtained over a concentration range from 0.1 to 40.0 mg l −1 with a relative standard deviation of less than 3% and a sampling rate of about 25 samples per hour.

Development of a sequential injection analysis system for the simultaneous biosensing of glucose and ethanol in bioreactor fermentation

An on-line sequential injection analysis system using amperometric detection for the simultaneous monitoring of glucose and ethanol in fermentations media is presented. The automatic analytical procedure developed is based in a sequential injection analysis (SIA) strategy and uses catalytic reactors of oxidase enzymes immobilized on controlled-pore glass. The proposed method allows the simultaneous determination of glucose (linear range between 5 and 750 mg dm−3 with a RSD better than 1.2%) and ethanol (linear range between 0.15 and 30 mg dm−3 with a RSD better than 2.2%). It was applied to monitor both species in beer fermentation. In a way to adjust the levels of ethanol present in the broth to the characteristics of the proposed system a dialysis unit was used between fermenter and SIA manifold. The sampling rate optimized was 50 samples h−1.

Construction and evaluation of ion selective electrodes for nitrate with a summing operational amplifier. Application to tobacco analysis

In this paper, the construction and evaluation of an electrode selective to nitrate with improved sensitivity, constructed like a conventional electrode (ISE) but using an operational amplifier to sum the potentials supplied by four membranes (ESOA) is described. The two types of electrodes, without an inner reference solution, were constructed using tetraoctylammonium bromide as sensor, dibutylphthalate as solvent mediator and PVC as plastic matrix, the membranes obtained directly applied onto a conductive epoxy resin support. After the comparative evaluation of their working characteristics they were used in the determination of nitrate in different types of tobacco. The limit of detection of the direct potentiometric method developed was found to be 0.18 g kg(-1) and the precision and accuracy of the method, when applied to eight different samples of tobacco, expressed in terms of mean R.S.D. and average percentage of spike recovery was 0.6 and 100.3%, respectively. The comparison of variances showed, on all ocassions, that the results obtained by the ESOA were similar to those obtained by the conventional ISE, but with higher precision. Linear regression analysis showed good agreement (r=0.9994) between the results obtained by the developed potentiometric method and those of a spectrophotometric method based on brucine, adopted as reference method, when applied simultaneously to 32 samples of different types of tobacco.

Toxicity assessment of ionic liquids with Vibrio fischeri: An alternative fully automated methodology

A fully automated Vibrio fischeri methodology based on sequential injection analysis (SIA) has been developed. The methodology was based on the aspiration of 75 μL of bacteria and 50 μL of inhibitor followed by measurement of the luminescence of bacteria. The assays were conducted for contact times of 5, 15, and 30 min, by means of three mixing chambers that ensured adequate mixing conditions. The optimized methodology provided a precise control of the reaction conditions which is an asset for the analysis of a large number of samples. The developed methodology was applied to the evaluation of the impact of a set of ionic liquids (ILs) on V. fischeri and the results were compared with those provided by a conventional assay kit (Biotox(®)). The collected data evidenced the influence of different cation head groups and anion moieties on the toxicity of ILs. Generally, aromatic cations and fluorine-containing anions displayed higher impact on V. fischeri, evidenced by lower EC50. The proposed methodology was validated through statistical analysis which demonstrated a strong positive correlation (P>0.98) between assays. It is expected that the automated methodology can be tested for more classes of compounds and used as alternative to microplate based V. fischeri assay kits.

Automatic potentiometric flow titration procedure for ascorbic acid determination in pharmaceutical formulations

A flow procedure for the determination of ascorbic acid in pharmaceutical formulations exploiting potentiometric titration is described. The method is based on the reduction of IO3- by ascorbic acid and the detection was carried out employing a flow-through ion selective electrode for iodide. The flow network controlled by a microcomputer was designed to implement multicommutation for ease of operation and robustness. The titration system allowed the determination of ascorbic acid in pharmaceutical formulations with concentrations ranging from 7.5 to 15.0 mmol l(-1). No significant differences at the 95% confidence level were observed in comparison with results obtained by a manual procedure. Merit figures of results such as a relative standard deviation of 1.0% (n=6) and a reagent consumption of 21.4 mg IO3- per determination were obtained.

Construction and evaluation of ion selective electrodes for perchlorate with a summing operational amplifier: application to pyrotechnics mixtures analysis

The construction and evaluation of an electrode selective to perchlorate with improved sensitivity, constructed like a conventional electrode (ISE), but using an operational amplifier to sum the potentials supplied by four membranes (ESOA) is described. The two types of electrodes, without an inner reference solution, were constructed using octylammonium chloride as sensor, a mixture of dibutylphthalate and o-nitrophenyloctyl ether as solvent mediator and PVC as plastic matrix, the membranes being obtained by direct application onto a conductive epoxy resin support. After the comparative evaluation of their working characteristics they were used in the determination of perchlorate in propellants, fulminating powders and fireworks. On all occasions, the results obtained by the ESOA were found to be similar to those obtained by the conventional ISE, but with higher precision. The limit of detection of the direct potentiometric method developed was found to be 0.1 g kg–1, and the precision and accuracy of the method, when applied to eight different samples of pyrotechnic mixtures, expressed in terms of mean relative standard deviation and average percentage of spike recovery were 0.4% and 100.5%, respectively. Adequate agreement was found between the results obtained by the potentiometric method and the reference methods, since the calculated relative errors ranged from +0.4% to –0.5%.

Simultaneous automatic potentiometric determination of acidity, chloride and fluoride in vinegar

Abstract This paper presents an automated system capable of automatically performing the sequential determination of three species — total acidity, chloride and fluoride — in a single sample of vinegar. The microcomputer-controlled device uses common laboratory equipment and data acquisition and control software developed by the authors. This set-up also contains an electrode switch to select several ion-selective electrodes sensitive to the various species under determination. A comparison of the results obtained with this system to those obtained by conventional methods in determining the total acidity, chloride and fluoride in samples of commercial vinegar showed a fair agreement between both automatic and manual procedures.